Angular distributions of the 14–17 MeV γ rays from the 7Li(p,γ) reaction at Ep=450, 402, and 80 keV were measured at 0°≤θlab≤135°, using a BGO detector and a 28-μg/cm2 LiF target. The angular distributions at Ep=450 and 402 keV agree with the previous results by Mainsbridge; at Ep=80 keV the ground-state transition is anisotropic on the order of 20%, confirming recent results by Chasteler et al

Brune, C. R.

Hahn, K. I.

Kavanagh, R. W.

English

Caltech Authors - Main

PHYSICAL REVIEW C MARCH 1996VOLUME 53, NUMBER 3
0556-2813/9Angular distributions of g rays from the 7Lip,g reaction at low energies
K. I. Hahn, C. R. Brune, and R. W. Kavanagh
W. K. Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, California 91125
~Received 1 November 1995!
Angular distributions of the 14–17 MeV g rays from the 7Li~p ,g) reaction at Ep5450, 402, and 80 keV
were measured at 0°<u lab<135°, using a BGO detector and a 28-mg/cm2 LiF target. The angular distribu-
tions at Ep5450 and 402 keV agree with the previous results by Mainsbridge; at Ep580 keV the ground-state
transition is anisotropic on the order of 20%, confirming recent results by Chasteler et al.
PACS number~s!: 25.40.Lw, 27.20.1nI. INTRODUCTION
A recent report @1# of significant g-ray anisotropy and
analyzing power in the 7Li~p ,g) reaction at Ep<80 keV
deduced a considerable p-wave contribution ~18–95 %! and
a lower extrapolated zero-energy S factor than previously
reported. Chasteler et al. suggested a similar effect for the
p-wave contribution to the 7Be(p ,g) reaction at low ener-
gies, which reduces the S(E) factor considerably and thus
decreases the solar-model flux of high-energy solar neutri-
nos. This suggestion has been criticized by Barker @2# in a
detailed theoretical analysis.
The anisotropy of the 7Li~p ,g) reaction measured by
Chasteler et al. is ;20%, in disagreement with the previous
experiment by Cecil et al. @3#, who reported isotropy within
10%. Mainsbridge @4# measured angular distributions at
higher energies, 200<Ep<1100 keV. Our results for angular
distributions of the 7Li(p ,g) reaction measured at three pro-
ton energies are discussed in the following sections.
II. EXPERIMENTAL SETUP
Beams with intensities of 10–30 mA (H31) for Ep580
keV and 1–2 mA ~protons! for both Ep5402 keV and
Ep5450 keV were produced by the Caltech Pelletron accel-
erator. The beams were rastered magnetically to sweep over
the area of a 6-mm 3 17-mm aperture ;1 m before the
target. We used the same detectors and the target holder de-
veloped for a previous experiment @5# as shown in Fig. 1.
The g rays from the 7Li(p ,g) reaction were measured with
a 5.08-cm-length 3 5.08-cm-diam BGO detector and a 7.62-
cm-length 3 7.62-cm-diam NaI detector. The front face of
the BGO detector was positioned at 7.62 cm from the center
of the target for the Ep5402 and Ep5450 keV runs, and
3.81 cm for the Ep580 keV run. The BGO detector was
rotated in the horizontal plane between 0° and 135°, with a
15° interval, with respect to the incident beam. The NaI de-
tector positioned at 3.81 cm was used to normalize the BGO
yields. For the 80-keV run, a 2.54-cm-thick and
63.5358.4-cm2-area plastic scintillator placed above both
detectors was used to veto cosmic rays in order to reduce
background.
A 28-mg/cm2 target of natLiF ~92.5% 7Li!, evaporated
onto a 0.81-mm-thick Cu backing, was mounted at 45° with
respect to the incident beam and was water cooled. The536/53~3!/1273~5!/$10.00‘‘positive’’ side ~as labeled in Fig. 1! was used for runs with
the BGO detector at u<90°, whereas the ‘‘negative’’ side
was used for runs at u>90°, in order to avoid the large
corrections due to the absorbers at certain angles, for ex-
ample, u51135° and u5245°. The two 90° runs were
used to normalize other angles. Using the S factors for the
7Li(p ,g) 8Be reaction measured by Zahnow et al. @6#, the
cross-section-weighted energies within the target are
Eeff5442 and 396 keV for Ep5450 and 402 keV, respec-
tively. At Ep580 keV, the proton energy loss in the target
was 21 keV, and Eeff573 keV ~the same as for the thick Li
target used by @1#!.
Because we depend exclusively on Monte Carlo calcula-
tions for the ~relative! g-ray detection efficiencies at various
angles, it is important to test the program, EGS4 @7#, in our
asymmetric-absorption geometry. Accurate dimensions and
elemental composition of the target and detector system were
determined as input for the program. In a previous test @5#
using the 6.13-MeV g rays from 19F(p ,ag) 16O, the abso-
lute efficiency from EGS4 was found accurate to within 6%.
FIG. 1. Schematic of the experimental setup, showing the target
and some of the positions of the BGO detector. The NaI detector
~not shown here! was positioned at ‘‘negative’’ 90° and ‘‘positive’’
90° for BGO runs at ‘‘positive’’ angles and ‘‘negative’’ angles, re-
spectively.1273 © 1996 The American Physical Society
1274 53K. I. HAHN, C. R. BRUNE, AND R. W. KAVANAGHHere, to test the angular dependence, we produced a 24Na
radioactive source of 2.75-MeV g rays that accurately simu-
lated the geometry used for 7Li(p ,g).
III. ANGULAR DISTRIBUTION OF 24Na g RAYS
A 1-mg/cm2-thick Na 2WO4 target on a 0.81-mm-thick
Cu backing was bombarded for 50 min by a 2.1-MeV deu-
teron beam of 1.0–1.5 mA, rastered and collimated as for our
7Li(p ,g) experiment. From the 23Na(d ,p) reaction, 24Na is
produced in the target, distributed according to the rastering,
and emitting g rays isotropically. A thin Au layer, evaporated
onto the surface, prevented loss of 24Na during the produc-
tion run. After a 1-day wait for the activity to decay to a
suitable level, spectra from the BGO ~at 7.62 cm! were taken
at various angles ~between 0° and 135° with a 15° interval!.
Figure 2 shows the experimental data ~triangles! of the BGO
detector at 290° and the corresponding Monte Carlo spec-
trum ~histogram!, which was convolved with energy resolu-
tion of 5% to fit the shapes of the experimental spectra. The
relative raw and corrected g yields, normalized to the 90°
FIG. 2. The experimental ~triangles! and the Monte Carlo ~his-
tograms! spectra for the 2.75-MeV g’s from 24Na are compared.
The energy spectrum from the Monte Carlo simulation was smeared
with ;5% energy resolution to match the experimental data of the
BGO detector.
FIG. 3. The relative yields normalized to the 90° yields are
plotted for the raw experimental data, and the corrected data using
EGS4, for the 24Na source.yields, are plotted in Fig. 2. In view of the 45° target holder,
and the beam pipe at backward angles, the raw yields be-
haved as expected; for example, for 0°<u<90°, the raw
yields are expected to be maximum at 45° and symmetric
about the 45° line, and for 90°<u<135°, the raw yields
decrease when the detector angle increases.
The yield at the ‘‘positive’’ 90°, as indicated in Fig. 1,
was used to normalize the yields at ,90°, and the yield at
the ‘‘negative’’ 90° was used to normalize the yields at
.90°. The relative yields have then to be corrected for the
angle-dependent absorption losses. For example, due to ab-
sorptions of the Cu backing, the stainless-steel ~SS! target
holder, and cooling water, the loss at u lab50° is larger than
at u lab5135°, where there is only the SS beam pipe
~0.318-cm wall thickness!.
The raw g yields are corrected for the half-life of 24Na,
t1/2514.96 h, but not for different absorption coefficients and
detector angles due to the finite size of the BGO detector.
These latter corrections were calculated using EGS4, with re-
sults as shown in Fig. 3. The errors of the corrected yields
include the statistical errors of the total counts in the Monte
Carlo calculations and the experimental g counts, and 1.5%
systematic errors, which are mainly due to the uncertainty in
the detector distance. The angular distribution of the cor-
rected yields is isotropic within the experimental errors, giv-
ing confidence in the use of the Monte Carlo simulations for
the 7Li(p ,g) case.
FIG. 4. The top spectrum shows the ~background-subtracted!
experimental data of the BGO detector for the 7Li(p ,g) reaction at
Eeff5442 keV, u590°. The middle and bottom spectra are the
Monte Carlo results. The ratio g0 /(g01g1)50.7460.03.
53 1275ANGULAR DISTRIBUTIONS OF g RAYS FROM THE . . .FIG. 6. The relative yields normalized to the 90° yields are
plotted for the 7Li(p ,g) reaction at Eeff5442 keV. The solid lines
are fits to our data ~points! and the dotted lines are from Mains-
bridge.
FIG. 5. The top spectrum shows the ~background-subtracted!
experimental data of the BGO detector for the 7Li(p ,g) reaction at
Eeff5396 keV, u590°. The middle and bottom spectra are the
Monte Carlo results. g0 /(g01g1)50.4160.03.IV. EXPERIMENTAL PROCEDURES AND RESULTS
Our main objective was to measure the angular distribu-
tion of the g rays from the 7Li(p ,g) reaction at Ep<80 keV,
because the 20% anisotropy measured in the recent experi-
ment @1#, which plays an important role in determining the
p-wave contribution, is in disagreement with the previous
experiment @3#. We also measured angular distributions at the
higher energies, 442 and 396 keV, in order to compare with
Mainsbridge.
As for the 24Na case, we used the EGS4 code to calculate
the efficiencies at various angles as well as the effective
angles due to finite detector size. For the forward angles, the
main absorbers ~the Cu backing, water, the target holder! are
symmetric about 45°. Our EGS4 results confirm this, as the
calculated absorptions at 0°, 15°, and 30°, agree with those
at 90°, 75°, and 60°, respectively, within 3% statistical er-
rors.
In the Monte Carlo calculations, for each proton energy,
one million g0 rays and one million g1 rays, from the 7Li
(p ,g0) 8Be~g.s.! and 7Li(p ,g1) 8Be*~3.04-MeV! reactions,
respectively, on the 6-mm 3 17-mm target area were gener-
ated isotropically. The width of the 3.04-MeV state in 8Be,
Gc.m.51500620 keV @8#, exceeds the detector resolution,
and we used the shape and width of the state from @9# for the
g1 calculations. The good statistics for the g rays observed at
the 442-keV resonance enabled us to study the shape of the
spectrum in the BGO detector and to compare with the
Monte Carlo simulations used to separate the g0’s from the
g1’s. The g-ray energies in the laboratory system were cor-
rected for recoil energy and Doppler shift and their energies
deposited in the BGO crystal were determined.
A typical spectrum of the g rays from the 7Li(p ,g) reac-
tion at Eeff5442 keV is shown in the top of Fig. 4. The
FIG. 7. The relative yields normalized to the 90° yields are
plotted for the 7Li(p ,g) reaction at Eeff5396 keV. The solid lines
are fits to our data ~points!. The dotted lines are from Mainsbridge.
1276 53K. I. HAHN, C. R. BRUNE, AND R. W. KAVANAGHTABLE I. Values of the Legendre coefficients of the fits for Figs. 5, 7, and 9.
Eeff g01g1 g0
~keV! A0 A1 A2 A0 A1 A2
442 1.00360.007 0.03860.015 0.01060.018 0.99860.009 0.07960.019 20.00260.024
396 0.97860.011 20.19160.021 20.03360.035 0.98360.019 20.23260.036 0.02260.057
73 1.00160.012 20.08660.022 20.00560.031 1.08460.021 20.17960.043Monte Carlo results shown in the middle spectrum of Fig. 4
accurately predict the shape and peak positions of the experi-
mental data shown in the top spectrum. The bottom spectrum
shows the Monte Carlo simulation for g0 and g1 separately,
and indicates that g0 events dominate above 16 MeV. Simi-
larly, Fig. 5 shows the experimental data and the Monte
Carlo results for the 7Li(p ,g) reaction at Eeff5396 keV.
The solid lines in Figs. 6 and 7 are least-squares fits by an
expansion in Legendre polynomials of the form,
A01A1P1(cosu)1A2P2(cosu), to our experimental ~ EGS4
corrected! results for 0°<u<135°. Higher-order terms were
not required to fit the data. The values of the coefficients of
the angular distributions in Figs. 6 and 7 are shown in Table
I. The results for the angular distributions at Eeff5442 keV
and Eeff5396 keV are in reasonable agreement with the re-
sults by Mainsbridge @4#, shown as dotted lines.
Figure 8 shows the g spectrum from the 7Li(p ,g) reac-
tion at Eeff573 keV. Errors in Fig. 9 are largely due to the
FIG. 8. The top spectrum shows the ~background-subtracted!
experimental data of the BGO detector for the 7Li(p ,g) reaction at
Eeff573 keV, u590°. The middle and bottom spectra are the
Monte Carlo results. g0 /(g01g1)50.2960.03.statistical errors of the total g counts as well as 1.5% sys-
tematic errors for the uncertainties in the detector position.
The solid lines in Fig. 9 are the results of least-squares fits to
the experimental data. The coefficients are listed in Table I.
Our fit to the g0 angular distribution, which indicates
;20% anisotropy, agrees well with the results of Chasteler
et al.
V. CONCLUSION
Our results for the angular distributions of the 7Li(p ,g)
reaction at Eeff5442 keV and Eeff5396 keV agree with the
previous results by Mainsbridge within errors. They also
agree well with the forward-backward asymmetries,
I(0°)/I(150°), recently reported by Zahnow et al. @6# We
have measured angular distributions of both g0 and the sum,
g01g1 , at Eeff573 keV. Our results for g0 at Eeff573 keV
are in good agreement with the results by Chasteler et al.,
but disagree with the isotropy reported by Cecil et al. Barker
FIG. 9. The relative yields normalized to the 90° yields are
plotted for our results (g01g1 and g0) and Chasteler et al. (g0) for
the 7Li(p ,g) at Eeff573 keV. The solid lines are fits to our results.
The dotted line in the bottom spectrum is the fit to our results,
normalized to unity average.
53 1277ANGULAR DISTRIBUTIONS OF g RAYS FROM THE . . .@2# has shown that acceptable theoretical fits to the low-
energy angular-distribution and analyzing-power data @1# can
be obtained by attributing the p-wave M1 contributions to
the tails of the higher resonances at Ep5441 and 1030 keV.ACKNOWLEDGMENT
This work was supported in part by the National Science
Foundation Grant No. PHY94-20470.@1# R. M. Chasteler, H. R. Weller, D. R. Tilley, and R. M. Prior,
Phys. Rev. Lett. 72, 3949 ~1994!.
@2# F. C. Barker, Aust. J. Phys. 48, 813 ~1995!.
@3# F. E. Cecil, D. Ferg, H. Liu, J. C. Scorby, and J. A. McNeil,
Nucl. Phys. A539, 75 ~1992!.
@4# B. Mainsbridge, Nucl. Phys. 21, 1 ~1960!.
@5# K. I. Hahn, C. R. Brune, and R. W. Kavanagh, Phys. Rev. C51, 1624 ~1995!.
@6# D. Zahnow, C. Angulo, C. Rolfs, S. Schmidt, W. H. Schulte,
and E. Somorjai, Z. Phys. A 351, 107 ~1995!.
@7# W. R. Nelson, H. Hirayama, and D. O. Rogers, SLAC Report
No. 265, 1985.
@8# F. Ajzenberg-Selove, Nucl. Phys. A490, 1 ~1988!.
@9# F. C. Barker, Aust. J. Phys. 22, 293 ~1969!.


Angular distributions of γ rays from the 7Li(p,γ) reaction at low energies

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Angular distributions of γ rays from the 7Li(p,γ) reaction at low energies

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