Multiplicities of quasi-continuum γ-rays have been measured for the 160Gd(α, xnγ) reactions as a function of bombarding energy and for the 4n exit channel also as a function of γ-ray energy. Increase of beam energy causes initially an increase of the energy and angular momentum of the entry line in the residual nucleus until the neutron “drip line” is reached. This can be understood in terms of competing compound and precompound processes

de Voigt, MJA

Ockels, WJ

Sujkowski, Z

Physics Letters B

University of Groningen Research Database

  
 University of Groningen
Compound and precompound γ-ray entry lines from measured multiplicities and energies in
α-induced reactions
Ockels, WJ; de Voigt, MJA; Sujkowski, Z
Published in:
Physics Letters B
DOI:
10.1016/0370-2693(78)90470-7
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
1978
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Ockels, WJ., de Voigt, MJA., & Sujkowski, Z. (1978). Compound and precompound γ-ray entry lines from
measured multiplicities and energies in α-induced reactions. Physics Letters B, 78(4), 401-404.
https://doi.org/10.1016/0370-2693(78)90470-7
Copyright
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Download date: 22-05-2019
Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 
COMPOUND AND PRECOMPOUND 3`-RAY ENTRY LINES FROM MEASURED 
MULTIPLICITIES AND ENERGIES IN ix-INDUCED REACTIONS 
W.J. OCKELS, M.J.A. DE VOIGT and Z. SUJKOWSKI 1 
Kemfysisch Versneller Instituut, Groningen, The Netherlands 
Received 14 July 1978 
Multiplicities of quasi-continuum y-rays have been measured for the 160 Gd (c~, xn'r) reactions as a function of bombard- 
ing energy and for the 4n exit channel also as a function of-r-ray energy. Increase of beam energy causes initially an in- 
crease of the energy and angular momentum of the entry line in the residual nucleus until the neutron "drip line" is reached. 
This can be understood in terms of competing compound and precompound processes. 
The entry line in the excitation energy versus angu- 
lar momentum plane at which the quasi-continuum 3`- 
ray cascades originate can be deduced from multiplic- 
ity measurements. Quasi-continuum "),-ray spectra re 
observed in a and heavy-ion (H.I.) induced reactions. 
Their gross structure has been established by measured 
3`-ray intensities [1] and multiplicities [2] which indi- 
cated the existence of two main components: a high- 
energy exponential tail and a low-energy bump. It has 
been shown earlier [3] for the 58Ni(160, xpT) reac- 
tions that the energies of the entry lines can be re- 
trieved from the product of the average 3`-ray energy 
and the average multiplicity. 
In this letter we report on a determination of the 
entry line as a function of bombarding energy for the 
reaction 160Gd(c~, 4nT)160Dy. The energy and angular 
momentum of the entry line are seen to increase with 
beam energy until the neutron "drip line" is reached. 
We thus located the drip line in the residual nucleus 
which is the neutron instability line with zero binding 
energy for the neutrons to decay. Multiplicities are al- 
so reported as a function of bombarding energy for 
16°Gd(a, xn) reactions with x = 4°8.  
The reactions 160Gd(a,xn3`)were induced by 40-  
110 MeV a-particles from the Groningen 280 cm AVF- 
cyclotron. The ")'-ray cascades were detected with a set- 
up consisting of a 110 cm3Ge(Li) detector in combina- 
1 I.B.J., Swicrk, Poland. 
tion with 16 "3 X 2" diam. NaI (T1) crystals in lead 
shieldings at a distance of 15 cm from the target. The 
full information was stored event by event on magnetic 
tape. The 0 to 6 fold coincidence Ge(Li) energy spec- 
tra, the Ge(Li) time spectrum and the 1-fold coinci- 
dence energy spectra of the 16 NaI detectors were si- 
multaneously updated in the computer memory during 
the experiment. Using a new and faster converging algo- 
rithm [4] the multiplicity shape parameters were de- 
duced from the measured multifold coincidence prob- 
abilities. These were obtained from the 0 to 6 fold co- 
incidence Ge (Li) spectra for all relevant 3`-ray transi- 
tions in the various final Dy isotopes. Multiplicities 
were also deduced from the same set data with differ- 
ent lower thresholds introduced off-line in the NaI 
energy spectra. The efficiency of the NaI detectors 
used in the extraction of the multiplicity was corrected 
for the cut off effects of the thresholds uch as the 
Compton efficiency for 3`-ray energies above the thresh- 
olds. Subtraction of multiplicities obtained with differ- 
ent thresholds yielded multiplicities within various 333- 
ray energy intervals. The related 7-ray energies were 
deduced from peeled and efficiency (thresholds in- 
cluded) corrected NaI spectra. 
Fig. 1 shows the systematic behaviour of the aver- 
age multiplicity (./14) and multiplicity width a M as a 
function of bombarding energy. All exit channels but 
especially the (a, 4n) and (c~, 6n) reactions how a steep 
increase of the multiplicity with increasing bombarding 
401 
Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 
6 
4 
2 
14 
12 
10 
- 16°GO (a, xn) i64-XDy 
L ,,< 
~ o0~, 5n ] 
/ / : , '  ll a ,6n  
8 T7 / ¢' ~,7r~ 
t I T /  * A o~,8n 
40 60 80 tO0 
Ea(MeV) 
Fig. 1. Average multiplicities (~/) and widths 8M determined 
for various reaction channels (top) which are cross-section 
averaged over all relevant (g.s.b.) transitions. The error bars 
are taken from the strongest transition i  each reaction. The 
corresponding reaction cross sections (bottom) are determined 
from previous [5] and the present experimental data. 
energy up to about 10 MeV above the energy where 
the cross section of that reaction reaches its maximum. 
At still higher beam energies the multiplicities and 
widths at least for the 4n and 6n exit channels remain 
approximately constant. A similar behaviour of the 
multiplicity has also been observed [6] previously for 
(c~, xn) reactions on neutron deficient Sm isotopes. 
This saturation of the multiplicity for the individual 
reaction channels is apparently in contrast with a pre- 
viously suggested [7] linear relationship between the 
average multiplicity .and the average initial angular 
momentum brought into the compound nucleus by 
protons and H.I. projectiles. However, such a relation- 
ship can also be derived from the present data if one 
considers the multiplicity of the total reaction, titus 
taking the cross-section averaged multiplicity of all 
exit channels as a function of input angular momentum. 
The contrasting behaviour of the multiplicity of the 
total reaction and of the individual exit channels indi- 
cates that the reaction mechanism should be considered 
in the interpretation of the relation between T-ray mul- 
tiplicity and angular momentum. 
The main result of the present data concerns the re- 
lation between the multiplicity and the T-ray energy 
deduced for the reaction 160Gd(a, 4nT)160Dy in the 
range ofEc~ = 40-70  MeV. From the total multiplicity 
for each g.s.b, member up to jTr = 14 + "side-feeding" 
multiplicities were deduced for T-rays within various 
energy intervals. These multiplicities were averaged 
with the intensities of the corresponding g.s.b, transi- 
tions as weight factors and the results are presented in 
fig. 2. The side-feeding T-rays appear to have significant- 
ly higher energies than those of the g.s.b, and their 
energy averaged multiplicities increase with beam 
energy in the region of 40-50  MeV. From fig. 2 it is 
seen that mainly the high-energy (E 7 > 1400 keV) T- 
rays are responsible for this multiplicity increase. 
The T-ray entry excitation energies have been re- 
trieved for each g.s.b, member by adding the partial 
50 ~- 160Gd(~,4n) 160Dy j 
.- E IkeVl 
4.0 ~ ) 1400 ! 
50 i 
2.0 
~ ~ . 600-1000 
I .O ~ . " ~ ,~ 900_1400 t 
[ 
. . . . . . . .  . . . . . . .  200-  0.0- i l -- 
! 
i 
i i J J i 
40 50 60 70 
Ea(MeV) 
Fig. 2. Side-feeding multiplicities which are cross-section 
averaged over the g.s.b, transitions for different y-ray energy 
intervals, indicated on the right. 
402 
Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 
products of the side-feeding multiplicities and corre- 
sponding average nergies within the energy bins indi- 
cated in fig. 2. These results can be compared with 
those obtained with the more commonly [3] em- 
ployed method of taking the product of the total aver- 
age multiplicity and average 3'-ray energy (ff'r ~ 930 
keV at Ec~ = 49 MeV). We find that the entry lines ob- 
tained with the two different methods agree within 
10%. This agreement indicates that possible correla- 
tions between 7-ray multiplicity and "r-ray energy are 
of minor importance. This also implies that all quasi- 
continuum cascades have about the same 3,-ray multi- 
plicity and energy distributions. 
The corresponding angular momenta of the "x-ray 
entry lines have been determined by assuming that the 
~,-rays carry away on the average two units of angular 
momentum for E'r < 600 keV (yrast transitions), one 
unit for E'r = 600-1000 keV (mixed E1 and E2 transi- 
tions) and half a unit for E'r > 1000 keV (mixed 
stretched and non-stretched E1 transitions). These as- 
sumptions are based on the measured angular distribu- 
tions of the low-energy (/;'r < 600 keV) component 
which were found to be consistent with stretched E2 
transitions while those of the high-energy exponential 
tail were nearly isotropic. This finding has also been 
reported earlier [1] for H.I. induced reactions. Combi- 
nation of this information with the recently established 
[8] predominant E1 character of the high-energy 7-ray 
component for the same reaction and residual nucleus 
indicates a mixed stretched and non-stretched E1 char- 
acter of this radiation. The deduced entry lines and 
the schematic ")'-ray decay for the reaction 160Gd(a, 
4n3,)160Dy are presented in fig. 3. The measured widths 
of the multiplicity distributions ( ee fig. 1) are proba- 
bly related to a spreading of the entry points in the ex- 
citation energy versus angular momentum plane. The 
entry lines thus merely represent the centroids of en- 
try clouds. Therefore the relatively small variations in 
the positions of the entry lines seen in fig. 3 in the 
beam energy range ofE,~ = 50-70  MeV may be of mi- 
nor significance. It should also be stated that reason- 
able changes in the above spin assumptions do not af- 
fect the positions of the entry lines significantly. 
The data can be interpreted in terms of competing 
compound and precompound processes. The reactions 
appear to be dominated by compound nucleus proces- 
ses in the region where the 7-ray multiplicity and the 
energy of the entry line increase with beam energy. 
15 L 160Gd(a,4n) 160Dy 
/ , /  ¢,.49 ov 4 
IO ,~ ' "¢" 
/ / / - 
,," /k /' / - 
"- i  ,,' ,/ 
L ;'~S /' / .~12*  "* 
oV_  I , , ,  , i , ,  
0 I0 I(~) 20 
Fig. 3. Energy-spin entry lines or centroids of entry clouds of 
quasi-continuum "},-ray cascades following evaporation ofneu- 
trons at various bombarding energies. Energy values were ob- 
tained from average multiplicities and "r-ray energies and cor- 
responding spin values by assuming certain spin releases per 
"r-ray s transition (see text). M' and E'r represent average quan- 
tities of the "r-ray cascades connecting entry lines and g.s.b. 
members. The upper numbers are averaged over the results ob- 
tained at 49, 56 and 65 MeV. The upper four entry lines locate 
the neutron drip line. 
Additional energy and angular momentum brought in- 
to the system are largely carried away by the 7-ray cas- 
cades. As is seen from fig. 1 this is the case up to beam 
energies of about 10 MeV above the energy at which 
the cross section for that reaction channel peaks. At 
even higher beam energies the emission of the next 
neutron becomes ignificant and a saturation of multi- 
plicity and excitation energy occurs which is retained 
up to the highest applied beam energy. Competing pre- 
compound processes become then important in which 
neutrons carry off most of the additional angular mo- 
mentum and energy. The entry lines remain located 
near the neutron drip line for all higher beam energies. 
This drip line is found about 8 MeV above and nearly 
parallel to the yrast line. This value is very close to the 
neutron binding energy (E n = -8 .6  MeV) in 160Dy. 
403 
Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 
Additional evidence for the importance of precom- 
pound processes has recently been obtained [9] from 
measured neutron energy spectra in Dy(a, xnT) Er re- 
actions and from measured multiplicities in a and H.I. 
induced reactions [10]. The present data, particularly 
for the (a, 4n) and (a, 6n) reactions, in turn indicate 
that in the region where precompound processes are 
expected to dominate the system after neutron evap- 
oration is left with excitation energy and angular mo- 
mentum which are nearly independent of bombarding 
energy. 
The authors like to acknowledge gratefully Dr. 
A. Zglinski and J. Mooibroek for assistance in part of 
the experiments and analysis and Prof. R.H. Siernssen 
for his continuous interest and fruitful suggestions. 
This work was performed as part of the research pro- 
gram of the "Stichting voor Fundamenteel Onderzoek 
der Materie" (FOM) with financial support from the 
"Nederlandse Organisatie voor Zuiver Wetenschappelijk 
Onderzoek" (ZWO). 
References 
[1] R.S. Simon et al., Phys. Rev. Lett. 36 (1976) 359. 
[2] J.O. Newton et al., Phys. Rev. Lett. 38 (1977) 810. 
[3] J.J. Simpson et al., Nucl. Phys. A287 (1977) 362. 
[4] W.J. Ockels, Z. Physik, to be published. 
[5] D. Chmielewska, Z. Sujkowski, J.F.W. Jansen, W.J. Ockels 
and M.J.A. de Voigt, Nukleonika, to be published. 
[6] M. Ogawa et al., Z. Physik 284 (1978) 271. 
[7] P.O. Tj6m et al., Phys. Rev. Lett. 33 (1974) 593. 
[8] S.J. Feenstra et al., Phys. Lett. 69B (1977) 403. 
[9] H. Ejiri et al., Proc. Int. Conf. on Nuclear structure, 
Tokyo, ed. T. Marimori (Academic Printing Co., Ltd., 
Japan, 1977) p. 729. 
[10] D.G. Sarantites, L. Westerberg, R.A. Dayras, M.L. Halbert, 
D.C. Hensley and J.H. Barker, Phys. Rev. C17 (1978) 601. 
404 


Compound and precompound γ-ray entry lines from measured multiplicities and energies in α-induced reactions

Multiplicities of quasi-continuum γ-rays have been measured for the 160Gd(α, xnγ) reactions as a function of bombarding energy and for the 4n exit channel also as a function of γ-ray energy. Increase of beam energy causes initially an increase of the energy and angular momentum of the entry line in the residual nucleus until the neutron “drip line” is reached. This can be understood in terms of competing compound and precompound processes.<br/

University of Groningen

University of GroningenCompound and precompound γ-ray entry lines from measured multiplicities and energies inα-induced reactionsOckels, WJ; de Voigt, MJA; Sujkowski, ZPublished in:Physics Letters BDOI:10.1016/0370-2693(78)90470-7IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1978Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Ockels, WJ., de Voigt, MJA., & Sujkowski, Z. (1978). Compound and precompound γ-ray entry lines frommeasured multiplicities and energies in α-induced reactions. Physics Letters B, 78(4), 401-404.https://doi.org/10.1016/0370-2693(78)90470-7CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 30-12-2025Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 COMPOUND AND PRECOMPOUND 3`-RAY ENTRY LINES FROM MEASURED MULTIPLICITIES AND ENERGIES IN ix-INDUCED REACTIONS W.J. OCKELS, M.J.A. DE VOIGT and Z. SUJKOWSKI 1 Kemfysisch Versneller Instituut, Groningen, The Netherlands Received 14 July 1978 Multiplicities of quasi-continuum y-rays have been measured for the 160 Gd (c~, xn'r) reactions as a function of bombard- ing energy and for the 4n exit channel also as a function of-r-ray energy. Increase of beam energy causes initially an in- crease of the energy and angular momentum of the entry line in the residual nucleus until the neutron "drip line" is reached. This can be understood in terms of competing compound and precompound processes. The entry line in the excitat ion energy versus angu- lar momentum plane at which the quasi-continuum 3`- ray cascades originate can be deduced from multiplic- ity measurements. Quasi-continuum "),-ray spectra are observed in a and heavy-ion (H.I.) induced reactions. Their gross structure has been established by measured 3`-ray intensities [1] and multiplicities [2] which indi- cated the existence of  two main components:  a high- energy exponential  tail and a low-energy bump. It has been shown earlier [3] for the 58Ni(160, xpT) reac- tions that  the energies of  the entry lines can be re- trieved from the product  of  the average 3`-ray energy and the average multiplicity.  In this let ter  we report  on a determination of  the entry line as a function of  bombarding energy for the reaction 160Gd(c~, 4nT)160Dy. The energy and angular momentum of  the entry line are seen to increase with beam energy until the neutron "drip line" is reached. We thus located the drip line in the residual nucleus which is the neutron instability line with zero binding energy for the neutrons to decay. Multiplicities are al- so reported as a function of  bombarding energy for 16°Gd(a, xn)  reactions with x = 4 °8 .  The reactions 160Gd(a,xn3`)were induced by 4 0 -  110 MeV a-particles from the Groningen 280 cm AVF- cyclotron. The ")'-ray cascades were detected with a set- up consisting of  a 110 cm3Ge(Li)  detector in combina- 1 I.B.J., Swicrk, Poland. tion with 16 "3 X 2" diam. NaI (T1) crystals in lead shieldings at a distance of  15 cm from the target. The full information was stored event by event on magnetic tape. The 0 to 6 fold coincidence Ge(Li) energy spec- tra, the Ge(Li) time spectrum and the 1-fold coinci- dence energy spectra of  the 16 NaI detectors were si- multaneously updated in the computer  memory during the experiment.  Using a new and faster converging algo- rithm [4] the multiplicity shape parameters were de- duced from the measured multifold coincidence prob- abilities. These were obtained from the 0 to 6 fo ld  co- incidence Ge (Li) spectra for all relevant 3`-ray transi- tions in the various final Dy isotopes. Multiplicities were also deduced from the same set data with differ- ent lower thresholds introduced off-line in the NaI energy spectra. The efficiency of  the NaI detectors used in the extraction of  the multiplici ty was corrected for the cut off  effects of  the thresholds such as the Compton efficiency for 3`-ray energies above the thresh- olds. Subtraction of  multiplicities obtained with differ- ent thresholds yielded multiplicities within various 333- ray energy intervals. The related 7-ray energies were deduced from peeled and efficiency (thresholds in- cluded) corrected NaI spectra. Fig. 1 shows the systematic behaviour of  the aver- age multiplicity (./14) and multiplicity width a M as a function of bombarding energy. All exit channels but especially the (a, 4n) and (c~, 6n) reactions show a steep increase of  the multiplicity with increasing bombarding 401 Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 6 4 2 14 12 10 - 16°GO (a, xn) i64-XDy L ,,< ~ o0~, 5n ] / / : , '  ll a , 6 n  8 T7 / ¢' ~ , 7 r ~  t I T /  * A o~,8n 40 60 80 tO0 Ea(MeV) Fig. 1. Average multiplicities (~/) and widths 8M determined for various reaction channels (top) which are cross-section averaged over all relevant (g.s.b.) transitions. The error bars are taken from the strongest transition in each reaction. The corresponding reaction cross sections (bottom) are determined from previous [5] and the present experimental data. energy up to about 10 MeV above the energy where the cross section of that reaction reaches its maximum. At still higher beam energies the multiplicities and widths at least for the 4n and 6n exit channels remain approximately constant. A similar behaviour of  the multiplicity has also been observed [6] previously for (c~, xn) reactions on neutron deficient Sm isotopes. This saturation of  the multiplicity for the individual reaction channels is apparently in contrast with a pre- viously suggested [7] linear relationship between the average multiplicity .and the average initial angular momentum brought into the compound nucleus by protons and H.I. projectiles. However, such a relation- ship can also be derived from the present data if one considers the multiplicity of  the total  reaction, titus taking the cross-section averaged multiplicity of all exit channels as a function of input angular momentum. The contrasting behaviour of the multiplicity of the total reaction and of the individual exit channels indi- cates that the reaction mechanism should be considered in the interpretation of the relation between T-ray mul- tiplicity and angular momentum. The main result of the present data concerns the re- lation between the multiplicity and the T-ray energy deduced for the reaction 160Gd(a, 4nT)160Dy in the range ofEc~ = 4 0 - 7 0  MeV. From the total multiplicity for each g.s.b, member up to jTr = 14 + "side-feeding" multiplicities were deduced for T-rays within various energy intervals. These multiplicities were averaged with the intensities of the corresponding g.s.b, transi- tions as weight factors and the results are presented in fig. 2. The side-feeding T-rays appear to have significant- ly higher energies than those of  the g.s.b, and their energy averaged multiplicities increase with beam energy in the region of 4 0 - 5 0  MeV. From fig. 2 it is seen that  mainly the high-energy (E 7 > 1400 keV) T- rays are responsible for this multiplicity increase. The T-ray entry excitation energies have been re- trieved for each g.s.b, member by adding the partial 50 ~- 160Gd(~,4n) 160Dy j .- E IkeVl 4.0 ~ ) 1400 ! 50 i 2.0 ~ ~ . 6 0 0 - 1 0 0 0  I .O  ~ . " ~ ,~ 9 0 0 _ 1 4 0 0  t [ . . . . . . . .  . . . . . . .  2 0 0 -  0.0- i l -- ! i i i J J i 40 50 60 70 Ea(MeV) Fig. 2. Side-feeding multiplicities which are cross-section averaged over the g.s.b, transitions for different y-ray energy intervals, indicated on the right. 402 Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 products of  the side-feeding multiplicities and corre- sponding average energies within the energy bins indi- cated in fig. 2. These results can be compared with those obtained with the more commonly [3] em- ployed method of  taking the product  of  the total aver- age multiplicity and average 3'-ray energy (ff'r ~ 930 keV at Ec~ = 49 MeV). We find that the entry lines ob- tained with the two different methods agree within 10%. This agreement indicates that possible correla- tions between 7-ray multiplicity and "r-ray energy are of minor importance. This also implies that all quasi- continuum cascades have about the same 3,-ray multi- plicity and energy distributions. The corresponding angular momenta  of  the "x-ray entry lines have been determined by assuming that the ~,-rays carry away on the average two units of  angular momentum for E'r < 600 keV (yrast transitions), one unit for E'r = 6 0 0 - 1 0 0 0  keV (mixed E1 and E2 transi- tions) and half a unit for E'r > 1000 keV (mixed stretched and non-stretched E1 transitions). These as- sumptions are based on the measured angular distribu- tions of the low-energy (/;'r < 600 keV) component  which were found to be consistent with stretched E2 transitions while those of the high-energy exponential  tail were nearly isotropic. This finding has also been reported earlier [1] for H.I. induced reactions. Combi- nation of  this information with the recently established [8] predominant  E1 character of  the high-energy 7-ray component  for the same reaction and residual nucleus indicates a mixed stretched and non-stretched E1 char- acter of  this radiation. The deduced entry lines and the schematic ")'-ray decay for the reaction 160Gd(a, 4n3,)160Dy are presented in fig. 3. The measured widths of the multiplici ty distributions (see fig. 1) are proba- bly related to a spreading of  the entry points in the ex- citation energy versus angular momentum plane. The entry lines thus merely represent the centroids of  en- try clouds. Therefore the relatively small variations in the positions of  the entry lines seen in fig. 3 in the beam energy range ofE,~ = 5 0 - 7 0  MeV may be of  mi- nor significance. It should also be stated that reason- able changes in the above spin assumptions do not af- fect the positions of  the entry lines significantly. The data can be interpreted in terms of  competing compound and precompound processes. The reactions appear to be dominated by compound nucleus proces- ses in the region where the 7-ray multiplici ty and the energy of  the entry line increase with beam energy. 15 L 160Gd(a,4n) 160Dy / , /  ¢,.49 ov 4 IO ,~ ' "¢" / / / - ,," /k /' / - " - i  ,,' ,/ L ;'~S /' / . ~ 1 2 *  "* oV_  I , , ,  , i , ,  0 I0 I(~) 20 Fig. 3. Energy-spin entry lines or centroids of entry clouds of quasi-continuum "},-ray cascades following evaporation of neu- trons at various bombarding energies. Energy values were ob- tained from average multiplicities and "r-ray energies and cor- responding spin values by assuming certain spin releases per "r-ray s transition (see text). M' and E'r represent average quan- tities of the "r-ray cascades connecting entry lines and g.s.b. members. The upper numbers are averaged over the results ob- tained at 49, 56 and 65 MeV. The upper four entry lines locate the neutron drip line. Additional energy and angular momentum brought in- to the system are largely carried away by the 7-ray cas- cades. As is seen from fig. 1 this is the case up to beam energies of  about 10 MeV above the energy at which the cross section for that reaction channel peaks. At even higher beam energies the emission of  the next neutron becomes significant and a saturation of  multi- plicity and excitation energy occurs which is retained up to the highest applied beam energy. Competing pre- compound processes become then important  in which neutrons carry off most of  the additional angular mo- mentum and energy. The entry lines remain located near the neutron drip line for all higher beam energies. This drip line is found about 8 MeV above and nearly parallel to the yrast line. This value is very close to the neutron binding energy (E n = - 8 . 6  MeV) in 160Dy. 403 Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 Additional evidence for the importance of  precom- pound processes has recently been obtained [9] from measured neutron energy spectra in Dy(a ,  xnT) Er re- actions and from measured multiplicities in a and H.I. induced reactions [10]. The present data, particularly for the (a, 4n) and (a, 6n) reactions, in turn indicate that in the region where precompound processes are expected to dominate the system after neutron evap- oration is left with excitation energy and angular mo- mentum which are nearly independent of  bombarding energy. The authors like to acknowledge gratefully Dr. A. Zglinski and J. Mooibroek for assistance in part of the experiments and analysis and Prof. R.H. Siernssen for his continuous interest and fruitful suggestions. This work was performed as part of  the research pro- gram of the "Stichting voor Fundamenteel  Onderzoek der Materie" (FOM) with financial support from the "Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek" (ZWO). References [1] R.S. Simon et al., Phys. Rev. Lett. 36 (1976) 359. [2] J.O. Newton et al., Phys. Rev. Lett. 38 (1977) 810. [3] J.J. Simpson et al., Nucl. Phys. A287 (1977) 362. [4] W.J. Ockels, Z. Physik, to be published. [5] D. Chmielewska, Z. Sujkowski, J.F.W. Jansen, W.J. Ockels and M.J.A. de Voigt, Nukleonika, to be published. [6] M. Ogawa et al., Z. Physik 284 (1978) 271. [7] P.O. Tj6m et al., Phys. Rev. Lett. 33 (1974) 593. [8] S.J. Feenstra et al., Phys. Lett. 69B (1977) 403. [9] H. Ejiri et al., Proc. Int. Conf. on Nuclear structure, Tokyo, ed. T. Marimori (Academic Printing Co., Ltd., Japan, 1977) p. 729. [10] D.G. Sarantites, L. Westerberg, R.A. Dayras, M.L. Halbert, D.C. Hensley and J.H. Barker, Phys. Rev. C17 (1978) 601. 404 

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University of GroningenCompound and precompound γ-ray entry lines from measured multiplicities and energies inα-induced reactionsOckels, WJ; de Voigt, MJA; Sujkowski, ZPublished in:Physics Letters BDOI:10.1016/0370-2693(78)90470-7IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1978Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Ockels, WJ., de Voigt, MJA., & Sujkowski, Z. (1978). Compound and precompound γ-ray entry lines frommeasured multiplicities and energies in α-induced reactions. Physics Letters B, 78(4), 401-404.https://doi.org/10.1016/0370-2693(78)90470-7CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 24-12-2025Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 COMPOUND AND PRECOMPOUND 3`-RAY ENTRY LINES FROM MEASURED MULTIPLICITIES AND ENERGIES IN ix-INDUCED REACTIONS W.J. OCKELS, M.J.A. DE VOIGT and Z. SUJKOWSKI 1 Kemfysisch Versneller Instituut, Groningen, The Netherlands Received 14 July 1978 Multiplicities of quasi-continuum y-rays have been measured for the 160 Gd (c~, xn'r) reactions as a function of bombard- ing energy and for the 4n exit channel also as a function of-r-ray energy. Increase of beam energy causes initially an in- crease of the energy and angular momentum of the entry line in the residual nucleus until the neutron "drip line" is reached. This can be understood in terms of competing compound and precompound processes. The entry line in the excitat ion energy versus angu- lar momentum plane at which the quasi-continuum 3`- ray cascades originate can be deduced from multiplic- ity measurements. Quasi-continuum "),-ray spectra are observed in a and heavy-ion (H.I.) induced reactions. Their gross structure has been established by measured 3`-ray intensities [1] and multiplicities [2] which indi- cated the existence of  two main components:  a high- energy exponential  tail and a low-energy bump. It has been shown earlier [3] for the 58Ni(160, xpT) reac- tions that  the energies of  the entry lines can be re- trieved from the product  of  the average 3`-ray energy and the average multiplicity.  In this let ter  we report  on a determination of  the entry line as a function of  bombarding energy for the reaction 160Gd(c~, 4nT)160Dy. The energy and angular momentum of  the entry line are seen to increase with beam energy until the neutron "drip line" is reached. We thus located the drip line in the residual nucleus which is the neutron instability line with zero binding energy for the neutrons to decay. Multiplicities are al- so reported as a function of  bombarding energy for 16°Gd(a, xn)  reactions with x = 4 °8 .  The reactions 160Gd(a,xn3`)were induced by 4 0 -  110 MeV a-particles from the Groningen 280 cm AVF- cyclotron. The ")'-ray cascades were detected with a set- up consisting of  a 110 cm3Ge(Li)  detector in combina- 1 I.B.J., Swicrk, Poland. tion with 16 "3 X 2" diam. NaI (T1) crystals in lead shieldings at a distance of  15 cm from the target. The full information was stored event by event on magnetic tape. The 0 to 6 fold coincidence Ge(Li) energy spec- tra, the Ge(Li) time spectrum and the 1-fold coinci- dence energy spectra of  the 16 NaI detectors were si- multaneously updated in the computer  memory during the experiment.  Using a new and faster converging algo- rithm [4] the multiplicity shape parameters were de- duced from the measured multifold coincidence prob- abilities. These were obtained from the 0 to 6 fo ld  co- incidence Ge (Li) spectra for all relevant 3`-ray transi- tions in the various final Dy isotopes. Multiplicities were also deduced from the same set data with differ- ent lower thresholds introduced off-line in the NaI energy spectra. The efficiency of  the NaI detectors used in the extraction of  the multiplici ty was corrected for the cut off  effects of  the thresholds such as the Compton efficiency for 3`-ray energies above the thresh- olds. Subtraction of  multiplicities obtained with differ- ent thresholds yielded multiplicities within various 333- ray energy intervals. The related 7-ray energies were deduced from peeled and efficiency (thresholds in- cluded) corrected NaI spectra. Fig. 1 shows the systematic behaviour of  the aver- age multiplicity (./14) and multiplicity width a M as a function of bombarding energy. All exit channels but especially the (a, 4n) and (c~, 6n) reactions show a steep increase of  the multiplicity with increasing bombarding 401 Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 6 4 2 14 12 10 - 16°GO (a, xn) i64-XDy L ,,< ~ o0~, 5n ] / / : , '  ll a , 6 n  8 T7 / ¢' ~ , 7 r ~  t I T /  * A o~,8n 40 60 80 tO0 Ea(MeV) Fig. 1. Average multiplicities (~/) and widths 8M determined for various reaction channels (top) which are cross-section averaged over all relevant (g.s.b.) transitions. The error bars are taken from the strongest transition in each reaction. The corresponding reaction cross sections (bottom) are determined from previous [5] and the present experimental data. energy up to about 10 MeV above the energy where the cross section of that reaction reaches its maximum. At still higher beam energies the multiplicities and widths at least for the 4n and 6n exit channels remain approximately constant. A similar behaviour of  the multiplicity has also been observed [6] previously for (c~, xn) reactions on neutron deficient Sm isotopes. This saturation of  the multiplicity for the individual reaction channels is apparently in contrast with a pre- viously suggested [7] linear relationship between the average multiplicity .and the average initial angular momentum brought into the compound nucleus by protons and H.I. projectiles. However, such a relation- ship can also be derived from the present data if one considers the multiplicity of  the total  reaction, titus taking the cross-section averaged multiplicity of all exit channels as a function of input angular momentum. The contrasting behaviour of the multiplicity of the total reaction and of the individual exit channels indi- cates that the reaction mechanism should be considered in the interpretation of the relation between T-ray mul- tiplicity and angular momentum. The main result of the present data concerns the re- lation between the multiplicity and the T-ray energy deduced for the reaction 160Gd(a, 4nT)160Dy in the range ofEc~ = 4 0 - 7 0  MeV. From the total multiplicity for each g.s.b, member up to jTr = 14 + "side-feeding" multiplicities were deduced for T-rays within various energy intervals. These multiplicities were averaged with the intensities of the corresponding g.s.b, transi- tions as weight factors and the results are presented in fig. 2. The side-feeding T-rays appear to have significant- ly higher energies than those of  the g.s.b, and their energy averaged multiplicities increase with beam energy in the region of 4 0 - 5 0  MeV. From fig. 2 it is seen that  mainly the high-energy (E 7 > 1400 keV) T- rays are responsible for this multiplicity increase. The T-ray entry excitation energies have been re- trieved for each g.s.b, member by adding the partial 50 ~- 160Gd(~,4n) 160Dy j .- E IkeVl 4.0 ~ ) 1400 ! 50 i 2.0 ~ ~ . 6 0 0 - 1 0 0 0  I .O  ~ . " ~ ,~ 9 0 0 _ 1 4 0 0  t [ . . . . . . . .  . . . . . . .  2 0 0 -  0.0- i l -- ! i i i J J i 40 50 60 70 Ea(MeV) Fig. 2. Side-feeding multiplicities which are cross-section averaged over the g.s.b, transitions for different y-ray energy intervals, indicated on the right. 402 Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 products of  the side-feeding multiplicities and corre- sponding average energies within the energy bins indi- cated in fig. 2. These results can be compared with those obtained with the more commonly [3] em- ployed method of  taking the product  of  the total aver- age multiplicity and average 3'-ray energy (ff'r ~ 930 keV at Ec~ = 49 MeV). We find that the entry lines ob- tained with the two different methods agree within 10%. This agreement indicates that possible correla- tions between 7-ray multiplicity and "r-ray energy are of minor importance. This also implies that all quasi- continuum cascades have about the same 3,-ray multi- plicity and energy distributions. The corresponding angular momenta  of  the "x-ray entry lines have been determined by assuming that the ~,-rays carry away on the average two units of  angular momentum for E'r < 600 keV (yrast transitions), one unit for E'r = 6 0 0 - 1 0 0 0  keV (mixed E1 and E2 transi- tions) and half a unit for E'r > 1000 keV (mixed stretched and non-stretched E1 transitions). These as- sumptions are based on the measured angular distribu- tions of the low-energy (/;'r < 600 keV) component  which were found to be consistent with stretched E2 transitions while those of the high-energy exponential  tail were nearly isotropic. This finding has also been reported earlier [1] for H.I. induced reactions. Combi- nation of  this information with the recently established [8] predominant  E1 character of  the high-energy 7-ray component  for the same reaction and residual nucleus indicates a mixed stretched and non-stretched E1 char- acter of  this radiation. The deduced entry lines and the schematic ")'-ray decay for the reaction 160Gd(a, 4n3,)160Dy are presented in fig. 3. The measured widths of the multiplici ty distributions (see fig. 1) are proba- bly related to a spreading of  the entry points in the ex- citation energy versus angular momentum plane. The entry lines thus merely represent the centroids of  en- try clouds. Therefore the relatively small variations in the positions of  the entry lines seen in fig. 3 in the beam energy range ofE,~ = 5 0 - 7 0  MeV may be of  mi- nor significance. It should also be stated that reason- able changes in the above spin assumptions do not af- fect the positions of  the entry lines significantly. The data can be interpreted in terms of  competing compound and precompound processes. The reactions appear to be dominated by compound nucleus proces- ses in the region where the 7-ray multiplici ty and the energy of  the entry line increase with beam energy. 15 L 160Gd(a,4n) 160Dy / , /  ¢,.49 ov 4 IO ,~ ' "¢" / / / - ,," /k /' / - " - i  ,,' ,/ L ;'~S /' / . ~ 1 2 *  "* oV_  I , , ,  , i , ,  0 I0 I(~) 20 Fig. 3. Energy-spin entry lines or centroids of entry clouds of quasi-continuum "},-ray cascades following evaporation of neu- trons at various bombarding energies. Energy values were ob- tained from average multiplicities and "r-ray energies and cor- responding spin values by assuming certain spin releases per "r-ray s transition (see text). M' and E'r represent average quan- tities of the "r-ray cascades connecting entry lines and g.s.b. members. The upper numbers are averaged over the results ob- tained at 49, 56 and 65 MeV. The upper four entry lines locate the neutron drip line. Additional energy and angular momentum brought in- to the system are largely carried away by the 7-ray cas- cades. As is seen from fig. 1 this is the case up to beam energies of  about 10 MeV above the energy at which the cross section for that reaction channel peaks. At even higher beam energies the emission of  the next neutron becomes significant and a saturation of  multi- plicity and excitation energy occurs which is retained up to the highest applied beam energy. Competing pre- compound processes become then important  in which neutrons carry off most of  the additional angular mo- mentum and energy. The entry lines remain located near the neutron drip line for all higher beam energies. This drip line is found about 8 MeV above and nearly parallel to the yrast line. This value is very close to the neutron binding energy (E n = - 8 . 6  MeV) in 160Dy. 403 Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 Additional evidence for the importance of  precom- pound processes has recently been obtained [9] from measured neutron energy spectra in Dy(a ,  xnT) Er re- actions and from measured multiplicities in a and H.I. induced reactions [10]. The present data, particularly for the (a, 4n) and (a, 6n) reactions, in turn indicate that in the region where precompound processes are expected to dominate the system after neutron evap- oration is left with excitation energy and angular mo- mentum which are nearly independent of  bombarding energy. The authors like to acknowledge gratefully Dr. A. Zglinski and J. Mooibroek for assistance in part of the experiments and analysis and Prof. R.H. Siernssen for his continuous interest and fruitful suggestions. This work was performed as part of  the research pro- gram of the "Stichting voor Fundamenteel  Onderzoek der Materie" (FOM) with financial support from the "Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek" (ZWO). References [1] R.S. Simon et al., Phys. Rev. Lett. 36 (1976) 359. [2] J.O. Newton et al., Phys. Rev. Lett. 38 (1977) 810. [3] J.J. Simpson et al., Nucl. Phys. A287 (1977) 362. [4] W.J. Ockels, Z. Physik, to be published. [5] D. Chmielewska, Z. Sujkowski, J.F.W. Jansen, W.J. Ockels and M.J.A. de Voigt, Nukleonika, to be published. [6] M. Ogawa et al., Z. Physik 284 (1978) 271. [7] P.O. Tj6m et al., Phys. Rev. Lett. 33 (1974) 593. [8] S.J. Feenstra et al., Phys. Lett. 69B (1977) 403. [9] H. Ejiri et al., Proc. Int. Conf. on Nuclear structure, Tokyo, ed. T. Marimori (Academic Printing Co., Ltd., Japan, 1977) p. 729. [10] D.G. Sarantites, L. Westerberg, R.A. Dayras, M.L. Halbert, D.C. Hensley and J.H. Barker, Phys. Rev. C17 (1978) 601. 404 

   University of GroningenCompound and precompound γ-ray entry lines from measured multiplicities and energies inα-induced reactionsOckels, WJ; de Voigt, MJA; Sujkowski, ZPublished in:Physics Letters BDOI:10.1016/0370-2693(78)90470-7IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1978Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Ockels, WJ., de Voigt, MJA., & Sujkowski, Z. (1978). Compound and precompound γ-ray entry lines frommeasured multiplicities and energies in α-induced reactions. Physics Letters B, 78(4), 401-404.https://doi.org/10.1016/0370-2693(78)90470-7CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 COMPOUND AND PRECOMPOUND 3`-RAY ENTRY LINES FROM MEASURED MULTIPLICITIES AND ENERGIES IN ix-INDUCED REACTIONS W.J. OCKELS, M.J.A. DE VOIGT and Z. SUJKOWSKI 1 Kemfysisch Versneller Instituut, Groningen, The Netherlands Received 14 July 1978 Multiplicities of quasi-continuum y-rays have been measured for the 160 Gd (c~, xn'r) reactions as a function of bombard- ing energy and for the 4n exit channel also as a function of-r-ray energy. Increase of beam energy causes initially an in- crease of the energy and angular momentum of the entry line in the residual nucleus until the neutron "drip line" is reached. This can be understood in terms of competing compound and precompound processes. The entry line in the excitation energy versus angu- lar momentum plane at which the quasi-continuum 3`- ray cascades originate can be deduced from multiplic- ity measurements. Quasi-continuum "),-ray spectra re observed in a and heavy-ion (H.I.) induced reactions. Their gross structure has been established by measured 3`-ray intensities [1] and multiplicities [2] which indi- cated the existence of two main components: a high- energy exponential tail and a low-energy bump. It has been shown earlier [3] for the 58Ni(160, xpT) reac- tions that the energies of the entry lines can be re- trieved from the product of the average 3`-ray energy and the average multiplicity. In this letter we report on a determination of the entry line as a function of bombarding energy for the reaction 160Gd(c~, 4nT)160Dy. The energy and angular momentum of the entry line are seen to increase with beam energy until the neutron "drip line" is reached. We thus located the drip line in the residual nucleus which is the neutron instability line with zero binding energy for the neutrons to decay. Multiplicities are al- so reported as a function of bombarding energy for 16°Gd(a, xn) reactions with x = 4°8.  The reactions 160Gd(a,xn3`)were induced by 40-  110 MeV a-particles from the Groningen 280 cm AVF- cyclotron. The ")'-ray cascades were detected with a set- up consisting of a 110 cm3Ge(Li) detector in combina- 1 I.B.J., Swicrk, Poland. tion with 16 "3 X 2" diam. NaI (T1) crystals in lead shieldings at a distance of 15 cm from the target. The full information was stored event by event on magnetic tape. The 0 to 6 fold coincidence Ge(Li) energy spec- tra, the Ge(Li) time spectrum and the 1-fold coinci- dence energy spectra of the 16 NaI detectors were si- multaneously updated in the computer memory during the experiment. Using a new and faster converging algo- rithm [4] the multiplicity shape parameters were de- duced from the measured multifold coincidence prob- abilities. These were obtained from the 0 to 6 fold co- incidence Ge (Li) spectra for all relevant 3`-ray transi- tions in the various final Dy isotopes. Multiplicities were also deduced from the same set data with differ- ent lower thresholds introduced off-line in the NaI energy spectra. The efficiency of the NaI detectors used in the extraction of the multiplicity was corrected for the cut off effects of the thresholds uch as the Compton efficiency for 3`-ray energies above the thresh- olds. Subtraction of multiplicities obtained with differ- ent thresholds yielded multiplicities within various 333- ray energy intervals. The related 7-ray energies were deduced from peeled and efficiency (thresholds in- cluded) corrected NaI spectra. Fig. 1 shows the systematic behaviour of the aver- age multiplicity (./14) and multiplicity width a M as a function of bombarding energy. All exit channels but especially the (a, 4n) and (c~, 6n) reactions how a steep increase of the multiplicity with increasing bombarding 401 Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 6 4 2 14 12 10 - 16°GO (a, xn) i64-XDy L ,,< ~ o0~, 5n ] / / : , '  ll a ,6n  8 T7 / ¢' ~,7r~ t I T /  * A o~,8n 40 60 80 tO0 Ea(MeV) Fig. 1. Average multiplicities (~/) and widths 8M determined for various reaction channels (top) which are cross-section averaged over all relevant (g.s.b.) transitions. The error bars are taken from the strongest transition i  each reaction. The corresponding reaction cross sections (bottom) are determined from previous [5] and the present experimental data. energy up to about 10 MeV above the energy where the cross section of that reaction reaches its maximum. At still higher beam energies the multiplicities and widths at least for the 4n and 6n exit channels remain approximately constant. A similar behaviour of the multiplicity has also been observed [6] previously for (c~, xn) reactions on neutron deficient Sm isotopes. This saturation of the multiplicity for the individual reaction channels is apparently in contrast with a pre- viously suggested [7] linear relationship between the average multiplicity .and the average initial angular momentum brought into the compound nucleus by protons and H.I. projectiles. However, such a relation- ship can also be derived from the present data if one considers the multiplicity of the total reaction, titus taking the cross-section averaged multiplicity of all exit channels as a function of input angular momentum. The contrasting behaviour of the multiplicity of the total reaction and of the individual exit channels indi- cates that the reaction mechanism should be considered in the interpretation of the relation between T-ray mul- tiplicity and angular momentum. The main result of the present data concerns the re- lation between the multiplicity and the T-ray energy deduced for the reaction 160Gd(a, 4nT)160Dy in the range ofEc~ = 40-70  MeV. From the total multiplicity for each g.s.b, member up to jTr = 14 + "side-feeding" multiplicities were deduced for T-rays within various energy intervals. These multiplicities were averaged with the intensities of the corresponding g.s.b, transi- tions as weight factors and the results are presented in fig. 2. The side-feeding T-rays appear to have significant- ly higher energies than those of the g.s.b, and their energy averaged multiplicities increase with beam energy in the region of 40-50  MeV. From fig. 2 it is seen that mainly the high-energy (E 7 > 1400 keV) T- rays are responsible for this multiplicity increase. The T-ray entry excitation energies have been re- trieved for each g.s.b, member by adding the partial 50 ~- 160Gd(~,4n) 160Dy j .- E IkeVl 4.0 ~ ) 1400 ! 50 i 2.0 ~ ~ . 600-1000 I .O ~ . " ~ ,~ 900_1400 t [ . . . . . . . .  . . . . . . .  200-  0.0- i l -- ! i i i J J i 40 50 60 70 Ea(MeV) Fig. 2. Side-feeding multiplicities which are cross-section averaged over the g.s.b, transitions for different y-ray energy intervals, indicated on the right. 402 Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 products of the side-feeding multiplicities and corre- sponding average nergies within the energy bins indi- cated in fig. 2. These results can be compared with those obtained with the more commonly [3] em- ployed method of taking the product of the total aver- age multiplicity and average 3'-ray energy (ff'r ~ 930 keV at Ec~ = 49 MeV). We find that the entry lines ob- tained with the two different methods agree within 10%. This agreement indicates that possible correla- tions between 7-ray multiplicity and "r-ray energy are of minor importance. This also implies that all quasi- continuum cascades have about the same 3,-ray multi- plicity and energy distributions. The corresponding angular momenta of the "x-ray entry lines have been determined by assuming that the ~,-rays carry away on the average two units of angular momentum for E'r < 600 keV (yrast transitions), one unit for E'r = 600-1000 keV (mixed E1 and E2 transi- tions) and half a unit for E'r > 1000 keV (mixed stretched and non-stretched E1 transitions). These as- sumptions are based on the measured angular distribu- tions of the low-energy (/;'r < 600 keV) component which were found to be consistent with stretched E2 transitions while those of the high-energy exponential tail were nearly isotropic. This finding has also been reported earlier [1] for H.I. induced reactions. Combi- nation of this information with the recently established [8] predominant E1 character of the high-energy 7-ray component for the same reaction and residual nucleus indicates a mixed stretched and non-stretched E1 char- acter of this radiation. The deduced entry lines and the schematic ")'-ray decay for the reaction 160Gd(a, 4n3,)160Dy are presented in fig. 3. The measured widths of the multiplicity distributions ( ee fig. 1) are proba- bly related to a spreading of the entry points in the ex- citation energy versus angular momentum plane. The entry lines thus merely represent the centroids of en- try clouds. Therefore the relatively small variations in the positions of the entry lines seen in fig. 3 in the beam energy range ofE,~ = 50-70  MeV may be of mi- nor significance. It should also be stated that reason- able changes in the above spin assumptions do not af- fect the positions of the entry lines significantly. The data can be interpreted in terms of competing compound and precompound processes. The reactions appear to be dominated by compound nucleus proces- ses in the region where the 7-ray multiplicity and the energy of the entry line increase with beam energy. 15 L 160Gd(a,4n) 160Dy / , /  ¢,.49 ov 4 IO ,~ ' "¢" / / / - ,," /k /' / - "- i  ,,' ,/ L ;'~S /' / .~12*  "* oV_  I , , ,  , i , ,  0 I0 I(~) 20 Fig. 3. Energy-spin entry lines or centroids of entry clouds of quasi-continuum "},-ray cascades following evaporation ofneu- trons at various bombarding energies. Energy values were ob- tained from average multiplicities and "r-ray energies and cor- responding spin values by assuming certain spin releases per "r-ray s transition (see text). M' and E'r represent average quan- tities of the "r-ray cascades connecting entry lines and g.s.b. members. The upper numbers are averaged over the results ob- tained at 49, 56 and 65 MeV. The upper four entry lines locate the neutron drip line. Additional energy and angular momentum brought in- to the system are largely carried away by the 7-ray cas- cades. As is seen from fig. 1 this is the case up to beam energies of about 10 MeV above the energy at which the cross section for that reaction channel peaks. At even higher beam energies the emission of the next neutron becomes ignificant and a saturation of multi- plicity and excitation energy occurs which is retained up to the highest applied beam energy. Competing pre- compound processes become then important in which neutrons carry off most of the additional angular mo- mentum and energy. The entry lines remain located near the neutron drip line for all higher beam energies. This drip line is found about 8 MeV above and nearly parallel to the yrast line. This value is very close to the neutron binding energy (E n = -8 .6  MeV) in 160Dy. 403 Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 Additional evidence for the importance of precom- pound processes has recently been obtained [9] from measured neutron energy spectra in Dy(a, xnT) Er re- actions and from measured multiplicities in a and H.I. induced reactions [10]. The present data, particularly for the (a, 4n) and (a, 6n) reactions, in turn indicate that in the region where precompound processes are expected to dominate the system after neutron evap- oration is left with excitation energy and angular mo- mentum which are nearly independent of bombarding energy. The authors like to acknowledge gratefully Dr. A. Zglinski and J. Mooibroek for assistance in part of the experiments and analysis and Prof. R.H. Siernssen for his continuous interest and fruitful suggestions. This work was performed as part of the research pro- gram of the "Stichting voor Fundamenteel Onderzoek der Materie" (FOM) with financial support from the "Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek" (ZWO). References [1] R.S. Simon et al., Phys. Rev. Lett. 36 (1976) 359. [2] J.O. Newton et al., Phys. Rev. Lett. 38 (1977) 810. [3] J.J. Simpson et al., Nucl. Phys. A287 (1977) 362. [4] W.J. Ockels, Z. Physik, to be published. [5] D. Chmielewska, Z. Sujkowski, J.F.W. Jansen, W.J. Ockels and M.J.A. de Voigt, Nukleonika, to be published. [6] M. Ogawa et al., Z. Physik 284 (1978) 271. [7] P.O. Tj6m et al., Phys. Rev. Lett. 33 (1974) 593. [8] S.J. Feenstra et al., Phys. Lett. 69B (1977) 403. [9] H. Ejiri et al., Proc. Int. Conf. on Nuclear structure, Tokyo, ed. T. Marimori (Academic Printing Co., Ltd., Japan, 1977) p. 729. [10] D.G. Sarantites, L. Westerberg, R.A. Dayras, M.L. Halbert, D.C. Hensley and J.H. Barker, Phys. Rev. C17 (1978) 601. 404 

NARCIS 

   University of GroningenCompound and precompound γ-ray entry lines from measured multiplicities and energies inα-induced reactionsOckels, WJ; de Voigt, MJA; Sujkowski, ZPublished in:Physics Letters BDOI:10.1016/0370-2693(78)90470-7IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1978Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Ockels, WJ., de Voigt, MJA., & Sujkowski, Z. (1978). Compound and precompound γ-ray entry lines frommeasured multiplicities and energies in α-induced reactions. Physics Letters B, 78(4), 401-404.https://doi.org/10.1016/0370-2693(78)90470-7CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 COMPOUND AND PRECOMPOUND 3`-RAY ENTRY LINES FROM MEASURED MULTIPLICITIES AND ENERGIES IN ix-INDUCED REACTIONS W.J. OCKELS, M.J.A. DE VOIGT and Z. SUJKOWSKI 1 Kemfysisch Versneller Instituut, Groningen, The Netherlands Received 14 July 1978 Multiplicities of quasi-continuum y-rays have been measured for the 160 Gd (c~, xn'r) reactions as a function of bombard- ing energy and for the 4n exit channel also as a function of-r-ray energy. Increase of beam energy causes initially an in- crease of the energy and angular momentum of the entry line in the residual nucleus until the neutron "drip line" is reached. This can be understood in terms of competing compound and precompound processes. The entry line in the excitat ion energy versus angu- lar momentum plane at which the quasi-continuum 3`- ray cascades originate can be deduced from multiplic- ity measurements. Quasi-continuum "),-ray spectra are observed in a and heavy-ion (H.I.) induced reactions. Their gross structure has been established by measured 3`-ray intensities [1] and multiplicities [2] which indi- cated the existence of  two main components:  a high- energy exponential  tail and a low-energy bump. It has been shown earlier [3] for the 58Ni(160, xpT) reac- tions that  the energies of  the entry lines can be re- trieved from the product  of  the average 3`-ray energy and the average multiplicity.  In this let ter  we report  on a determination of  the entry line as a function of  bombarding energy for the reaction 160Gd(c~, 4nT)160Dy. The energy and angular momentum of  the entry line are seen to increase with beam energy until the neutron "drip line" is reached. We thus located the drip line in the residual nucleus which is the neutron instability line with zero binding energy for the neutrons to decay. Multiplicities are al- so reported as a function of  bombarding energy for 16°Gd(a, xn)  reactions with x = 4 °8 .  The reactions 160Gd(a,xn3`)were induced by 4 0 -  110 MeV a-particles from the Groningen 280 cm AVF- cyclotron. The ")'-ray cascades were detected with a set- up consisting of  a 110 cm3Ge(Li)  detector in combina- 1 I.B.J., Swicrk, Poland. tion with 16 "3 X 2" diam. NaI (T1) crystals in lead shieldings at a distance of  15 cm from the target. The full information was stored event by event on magnetic tape. The 0 to 6 fold coincidence Ge(Li) energy spec- tra, the Ge(Li) time spectrum and the 1-fold coinci- dence energy spectra of  the 16 NaI detectors were si- multaneously updated in the computer  memory during the experiment.  Using a new and faster converging algo- rithm [4] the multiplicity shape parameters were de- duced from the measured multifold coincidence prob- abilities. These were obtained from the 0 to 6 fo ld  co- incidence Ge (Li) spectra for all relevant 3`-ray transi- tions in the various final Dy isotopes. Multiplicities were also deduced from the same set data with differ- ent lower thresholds introduced off-line in the NaI energy spectra. The efficiency of  the NaI detectors used in the extraction of  the multiplici ty was corrected for the cut off  effects of  the thresholds such as the Compton efficiency for 3`-ray energies above the thresh- olds. Subtraction of  multiplicities obtained with differ- ent thresholds yielded multiplicities within various 333- ray energy intervals. The related 7-ray energies were deduced from peeled and efficiency (thresholds in- cluded) corrected NaI spectra. Fig. 1 shows the systematic behaviour of  the aver- age multiplicity (./14) and multiplicity width a M as a function of bombarding energy. All exit channels but especially the (a, 4n) and (c~, 6n) reactions show a steep increase of  the multiplicity with increasing bombarding 401 Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 6 4 2 14 12 10 - 16°GO (a, xn) i64-XDy L ,,< ~ o0~, 5n ] / / : , '  ll a , 6 n  8 T7 / ¢' ~ , 7 r ~  t I T /  * A o~,8n 40 60 80 tO0 Ea(MeV) Fig. 1. Average multiplicities (~/) and widths 8M determined for various reaction channels (top) which are cross-section averaged over all relevant (g.s.b.) transitions. The error bars are taken from the strongest transition in each reaction. The corresponding reaction cross sections (bottom) are determined from previous [5] and the present experimental data. energy up to about 10 MeV above the energy where the cross section of that reaction reaches its maximum. At still higher beam energies the multiplicities and widths at least for the 4n and 6n exit channels remain approximately constant. A similar behaviour of  the multiplicity has also been observed [6] previously for (c~, xn) reactions on neutron deficient Sm isotopes. This saturation of  the multiplicity for the individual reaction channels is apparently in contrast with a pre- viously suggested [7] linear relationship between the average multiplicity .and the average initial angular momentum brought into the compound nucleus by protons and H.I. projectiles. However, such a relation- ship can also be derived from the present data if one considers the multiplicity of  the total  reaction, titus taking the cross-section averaged multiplicity of all exit channels as a function of input angular momentum. The contrasting behaviour of the multiplicity of the total reaction and of the individual exit channels indi- cates that the reaction mechanism should be considered in the interpretation of the relation between T-ray mul- tiplicity and angular momentum. The main result of the present data concerns the re- lation between the multiplicity and the T-ray energy deduced for the reaction 160Gd(a, 4nT)160Dy in the range ofEc~ = 4 0 - 7 0  MeV. From the total multiplicity for each g.s.b, member up to jTr = 14 + "side-feeding" multiplicities were deduced for T-rays within various energy intervals. These multiplicities were averaged with the intensities of the corresponding g.s.b, transi- tions as weight factors and the results are presented in fig. 2. The side-feeding T-rays appear to have significant- ly higher energies than those of  the g.s.b, and their energy averaged multiplicities increase with beam energy in the region of 4 0 - 5 0  MeV. From fig. 2 it is seen that  mainly the high-energy (E 7 > 1400 keV) T- rays are responsible for this multiplicity increase. The T-ray entry excitation energies have been re- trieved for each g.s.b, member by adding the partial 50 ~- 160Gd(~,4n) 160Dy j .- E IkeVl 4.0 ~ ) 1400 ! 50 i 2.0 ~ ~ . 6 0 0 - 1 0 0 0  I .O  ~ . " ~ ,~ 9 0 0 _ 1 4 0 0  t [ . . . . . . . .  . . . . . . .  2 0 0 -  0.0- i l -- ! i i i J J i 40 50 60 70 Ea(MeV) Fig. 2. Side-feeding multiplicities which are cross-section averaged over the g.s.b, transitions for different y-ray energy intervals, indicated on the right. 402 Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 products of  the side-feeding multiplicities and corre- sponding average energies within the energy bins indi- cated in fig. 2. These results can be compared with those obtained with the more commonly [3] em- ployed method of  taking the product  of  the total aver- age multiplicity and average 3'-ray energy (ff'r ~ 930 keV at Ec~ = 49 MeV). We find that the entry lines ob- tained with the two different methods agree within 10%. This agreement indicates that possible correla- tions between 7-ray multiplicity and "r-ray energy are of minor importance. This also implies that all quasi- continuum cascades have about the same 3,-ray multi- plicity and energy distributions. The corresponding angular momenta  of  the "x-ray entry lines have been determined by assuming that the ~,-rays carry away on the average two units of  angular momentum for E'r < 600 keV (yrast transitions), one unit for E'r = 6 0 0 - 1 0 0 0  keV (mixed E1 and E2 transi- tions) and half a unit for E'r > 1000 keV (mixed stretched and non-stretched E1 transitions). These as- sumptions are based on the measured angular distribu- tions of the low-energy (/;'r < 600 keV) component  which were found to be consistent with stretched E2 transitions while those of the high-energy exponential  tail were nearly isotropic. This finding has also been reported earlier [1] for H.I. induced reactions. Combi- nation of  this information with the recently established [8] predominant  E1 character of  the high-energy 7-ray component  for the same reaction and residual nucleus indicates a mixed stretched and non-stretched E1 char- acter of  this radiation. The deduced entry lines and the schematic ")'-ray decay for the reaction 160Gd(a, 4n3,)160Dy are presented in fig. 3. The measured widths of the multiplici ty distributions (see fig. 1) are proba- bly related to a spreading of  the entry points in the ex- citation energy versus angular momentum plane. The entry lines thus merely represent the centroids of  en- try clouds. Therefore the relatively small variations in the positions of  the entry lines seen in fig. 3 in the beam energy range ofE,~ = 5 0 - 7 0  MeV may be of  mi- nor significance. It should also be stated that reason- able changes in the above spin assumptions do not af- fect the positions of  the entry lines significantly. The data can be interpreted in terms of  competing compound and precompound processes. The reactions appear to be dominated by compound nucleus proces- ses in the region where the 7-ray multiplici ty and the energy of  the entry line increase with beam energy. 15 L 160Gd(a,4n) 160Dy / , /  ¢,.49 ov 4 IO ,~ ' "¢" / / / - ,," /k /' / - " - i  ,,' ,/ L ;'~S /' / . ~ 1 2 *  "* oV_  I , , ,  , i , ,  0 I0 I(~) 20 Fig. 3. Energy-spin entry lines or centroids of entry clouds of quasi-continuum "},-ray cascades following evaporation of neu- trons at various bombarding energies. Energy values were ob- tained from average multiplicities and "r-ray energies and cor- responding spin values by assuming certain spin releases per "r-ray s transition (see text). M' and E'r represent average quan- tities of the "r-ray cascades connecting entry lines and g.s.b. members. The upper numbers are averaged over the results ob- tained at 49, 56 and 65 MeV. The upper four entry lines locate the neutron drip line. Additional energy and angular momentum brought in- to the system are largely carried away by the 7-ray cas- cades. As is seen from fig. 1 this is the case up to beam energies of  about 10 MeV above the energy at which the cross section for that reaction channel peaks. At even higher beam energies the emission of  the next neutron becomes significant and a saturation of  multi- plicity and excitation energy occurs which is retained up to the highest applied beam energy. Competing pre- compound processes become then important  in which neutrons carry off most of  the additional angular mo- mentum and energy. The entry lines remain located near the neutron drip line for all higher beam energies. This drip line is found about 8 MeV above and nearly parallel to the yrast line. This value is very close to the neutron binding energy (E n = - 8 . 6  MeV) in 160Dy. 403 Volume 78B, number 4 PHYSICS LETTERS 9 October 1978 Additional evidence for the importance of  precom- pound processes has recently been obtained [9] from measured neutron energy spectra in Dy(a ,  xnT) Er re- actions and from measured multiplicities in a and H.I. induced reactions [10]. The present data, particularly for the (a, 4n) and (a, 6n) reactions, in turn indicate that in the region where precompound processes are expected to dominate the system after neutron evap- oration is left with excitation energy and angular mo- mentum which are nearly independent of  bombarding energy. The authors like to acknowledge gratefully Dr. A. Zglinski and J. Mooibroek for assistance in part of the experiments and analysis and Prof. R.H. Siernssen for his continuous interest and fruitful suggestions. This work was performed as part of  the research pro- gram of the "Stichting voor Fundamenteel  Onderzoek der Materie" (FOM) with financial support from the "Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek" (ZWO). References [1] R.S. Simon et al., Phys. Rev. Lett. 36 (1976) 359. [2] J.O. Newton et al., Phys. Rev. Lett. 38 (1977) 810. [3] J.J. Simpson et al., Nucl. Phys. A287 (1977) 362. [4] W.J. Ockels, Z. Physik, to be published. [5] D. Chmielewska, Z. Sujkowski, J.F.W. Jansen, W.J. Ockels and M.J.A. de Voigt, Nukleonika, to be published. [6] M. Ogawa et al., Z. Physik 284 (1978) 271. [7] P.O. Tj6m et al., Phys. Rev. Lett. 33 (1974) 593. [8] S.J. Feenstra et al., Phys. Lett. 69B (1977) 403. [9] H. Ejiri et al., Proc. Int. Conf. on Nuclear structure, Tokyo, ed. T. Marimori (Academic Printing Co., Ltd., Japan, 1977) p. 729. [10] D.G. Sarantites, L. Westerberg, R.A. Dayras, M.L. Halbert, D.C. Hensley and J.H. Barker, Phys. Rev. C17 (1978) 601. 404 

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Compound and precompound γ-ray entry lines from measured multiplicities and energies in α-induced reactions

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