A definitive study of the elemental abundances of nuclei over the charge range of 20 ≤ Z ≤ 100 requires a satellite mission capable of obtaining high statistics and excellent charge resolution over the full charge range. Such a mission, utilizing an electronic instrument which is an evolution of the HEA0-3 Heavy Nuclei Experiment, is described here

Binns, W. R.

Garrard, T. L.

Geer, L. Y.

Klarmann, J.

Mewaldt, R. A.

Waddington, C. J.

Caltech Authors - Main

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UHCR: A COSMIC RAV MISSION TO STUDY NUCLEI 
IN THE CHARGE RANGE FROM 20 ::;Z ::; 100. 
W.R. Binns1, T.L. Garrard2, L.Y. Geer1, J. Klarmann1, 
A.A. Mewaldt2, and C.J. Waddington3 
1. Washington University , St. Louis, MO, USA; 2. Caltech, Pasadena, CA. USA; 3. Univ. of 
Minnesota, Minneapolis, MN, USA 
ABSTRACT 
A definitive study of the elemental abundances of nuclei over the 
charge range of 20 ::; Z::; 100 requires a satellite mission capable of obtaining 
high statistics and excellent charge resolution over the full charge range. 
Such a mission, utilizing an electronic instrument which is an evolution of the 
HEA0-3 Heavy Nuclei Experiment, is described here. 
1. lntroductjon: Our present knowledge of the elemental abundances 
for the ultra heavy (UH) cosmic ray nuclei (Z ~ 30) has been obtained 
predominantly from the HEA0-3 (Binns, et al., 1989) and Ariel-6 (Fowler, et 
al., 1987) experiments which were flown circa 1980. These experiments 
measured the abundances of all even-charged elements for 30::;Z::;60. For 
elements heavier than Z=60, a combination of limited statistics and resolution 
resulted in the measurement of abundances for charge groups only. Our 
objectives for new measurements are: 1) the determination of the age of the 
heaviest cosmic rays (Z ~ 90); 2) a definitive study of neutron capture 
nucleosynthesis over the charge range 20 ::;z ::;100; 3) search for recent 
nucleosynthesis of cosmic rays (e.g. search for 96Cm; t 112= 1.6 x 1 o? yr); 4) 
study the propagation history of the entire range of charges; 5) search for 
superheavy nuclei (Z~110) which may exist in the cosmic radiation; and 6) 
study the effects of preferential acceleration over this broad charge region 
(e.g. first ionization potential effects). Achieving these goals requires the 
detection of roughly 100 times the number of particles detected by HEA0-3, 
and a charge resolution of :::;0.25 charge units (cu) over the full charge range. 
An instrument and mission concept capable of achieving these objectives is 
described below. 
2. Instrument: The instrument is modular with each module con-
sisting of two Scintillator dE/dx counters and a Cherenkov counter which 
combined give a measurement of charge and velocity, a time of flight detector 
to determine the particle trajectory sense (up/down), and a scintillating fiber 
hodoscope (Davis, et al., 1989) giving up to 4 x,y coordinates (H1-H4) for an-
gle and position corrections to the dE/dx and Cherenkov counter signals 
(Fig.1 ). The Cherenkov counter could consist of two radiators as was the case 
for HEAO, one mounted on top and the other on the bottom of a light diffusion 
box, and will be composed of Pilot-425 or Fused Silica. The scintillator 
counters (81 and S2) each have one radiator mounted on the inboard sides 
of the light diffusion boxes, thus providing a large acceptance geometry. The 
preferred scintillator material is plastic scintillator or other higher-Z scintillator. 
The light from each counter (S1, S2, C) will be viewed by -36 five inch photo-
multipliers (PMT's). Signals from 81 and 82 are also used for time of flight 
©Dublin Institute for Advanced Studi~t Provided by the NASA Astrophysics Data System 
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- , _ Unobstructed geometry,.,., ... ' ... ,_ , 
-- H
1 
-:~poo~ooqG:~ .. ~ ~ r 
1 
~~~~~~~~~~~~~~~~~~~~T~l 
Full 
85 cm 
1 
2.1 m 
Fig. 1--The UHCR instrument is symmetrical in x and y and accepts particles entering 
from above and below. The dashed lines show the "full" and "unobstructed" geometry. 
determination, especially for wide angle particles not traversing the primary 
time of flight counters (TOF1 and TOF2) on the outboard sides of the detector, 
since these particles are at wide angles and should provide sufficient 
separation to make the TOF measurement useful. Thin scintillator strips 
viewed by photomultipliers will be used to determine the TOF for particles 
traversing H1 and H4. The instrument is symmetrical and accepts particles 
from either direction. It has dimensions as shown in Fig. 1 with the radiator 
being 1.6 m square, a size very similar to that used on HEAO, thus giving a 
fairly direct means of estimating detector performance and engineering 
parameters. 
This design results in a module with a "full" geometry factor (event 
criterion 81 ·H2·C·H3·82) of 1 O m2sr, assuming that particles traverse each 
radiator at a distance >3 cm from the radiator edge. The "unobstructed" 
geometrical factor (H1 ·81 ·H2·C·H3·82·H4) for nuclei traversing the radiators 
at a distance >3 cm from the edge is 5. 7 m2sr. These geometrical factors 
include nuclei entering the instrument from both the top and bottom. 
3. The UHCR Mjssjon: The experiment would consist of 3 or 2 mod-
ules launched into an elliptical orbit such that it is outside the magnetosphere 
80% of the time. The number of particles which should be detected assuming 
solar minimum for 5 years and the full geometry factor is shown in the table 
below. The calculations assumed that nuclei had an energy of >350 MeV/n 
as they exited the Cherenkov counter. The numbers of 90 Th+92U in the table 
labeled (a) were estimated assuming Anders and Ebihara (1982) 
abundances, while those labeled (b) assumed the HEAO/Ariel (Binns et al., 
1989) abundances. For compari$on of the number of nuclei detected, the 
total number of Fe nuclei collected by HEA0-3 was 1. 7 x 1O7 particles, with 
only about half of these being of sufficiently good quality to be included in 
abundance estimates. Interactions will reduce the number of good events; 
e.g. 92U would be reduced by 40-65%, depending upon the detector 
thickness and material. The detector radiator parameters will be optimized to 
minimize interaction losses. while retainina aood charae resolution. The 
© Dublin Institute for Advanced Studies5Q3Provided by the NASA Astrophysics Data System 
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number of 90 Th+92U expected allows precise measurements of the age of 
the heaviest nuclei. For example, if the nuclei were synthesized 107 years 
ago, and we assume the detection of 200 noninteracted Th+U, the age 
determination using the 92U/90Th ratio, as derived from Blake et al. (1974), 
has a statistical precision of (1 ~\1 ) x 1 o7 years. The 92U/9sCm ratio for these 
conditions gives an independent and more precise estimate of (1±0.2) x 1 Q7 
years. If the cosmic ray age is 109 years, the 92U/90 Th ratio gives an age 
precision of ±0.3 x 109 years. For this case, the 96Cm would be decayed and 
have a zero abundance. Likewise, the 92U/94Pu ratio provides a good 
'f th t b 1 QB measure 1 e correc age 1s a out years. 
z 3-Modules 2-Modules 
26 3.3x109 2.2 x 109 
32-40 5 x 105 3.5 x 105 
41-49 9 x 104 6 x 104 
50-61 9 x 104 6 x 104 
62-73 3. x 1 Q4 2. x 104 
74-80 1.8 x 104 1.2 x 104 
81-83 5.3 x 103 3.5 x 103 
90-92 (a) 430 290 
90-92 (b) 150 100 
4. C h a r g e 
Resolution--The charge 
resolution expected for 
the Cherenkov detector 
can be scaled from that 
obtained with the HEAO 
detector which was 0.4 cu 
at 5sBa. The UHCR 
Cherenkov counter would 
have a much larger 
photocathode to box 
surface area, as well as improved PMT's, and should yield about 4 times 
more signal than HEAO which had a light collection efficiency of about 5%. 
Thus, for a radiator like that used on HEAO, a charge resolution of about 0.2 
cu in Cherenkov for relativistic nuclei up through Ba should be obtained. In 
addition, we have demonstrated Cherenkov resolution for 1 Gev/n 7gA u 
nuclei of 0.25 cu at the Bevalac (Kertzmann, 1987). Thus, based on our 
experimental work to date, we expect to be able to achieve adequate 
Cherenkov resolution at least to 79Au for particles with energy near the 
Cherenkov plateau . We anticipate that as the Cherenkov charge resolution 
degrades for particles near threshold, the dE/dx resolution will improve. 
We have recently exposed plastic scintillator counters to 47Ag and its 
interaction fragments at the Bevalac (See Paper OG 10.1.15), and a resolu-
tion of -0.25 cu was obtained. Since the S1 and S2 signals are independent, 
the two measurements should give a resolution <0.25 cu. 
At the highest charges (~90) electron capture and stripping reactions 
become important and must be carefully evaluated. There are two effects that 
must be considered. The first is a lowering of the "effective" charge as the 
particle captures electrons while traversing detector material, and the second 
is a statistical broadening in signal due to the finite number of capture and 
stripping reactions that occur for a detector of given thickness and material 
type. We have used the electron capture and stripping cross sections for 92U 
(Crawford, 1991) to calculate the effective charge for Uranium penetrating 
through carbon (plastic) and Silicon using the method described in Price et 
al. 1987. We find that the effective charae for U drops to 91.5 in Si and to 
50~ 
© Dublin Institute for Advanced Studies • Provided by the NASA Astrophysics Data System 
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91.2 in Cat 600 MeV/n. This has a negligible effect in reducing the spacing 
between, e.g., 90 Th and 92U since the effective charge of 90 Th also droops, 
though not quite as strongly. 
To estimate the significance of the statistical broadening due to elec-
tron capture and stripping, we have performed a Monte Carlo calculation 
which follows 92U nuclei through either carbon or silicon. The calculation 
includes only single electron capture and stripping reactions. In silicon the 
statistical broadening in the charge estimate is typically 0.10 cu over the full 
energy range. while in carbon the effect is about 0.23 cu at ·1200 MeV/n and 
0.18 cu at 400MeV/n. Fig. 2 shows the results of this calculation for 1000 
MeV/n. This resolution degradation is very small for a silicon detector, but is 
more significant for a carbon like detector (e.g. plastic scinti'llator). 
-soo · - soo--~~~~~~~~-
Carbon: 1 cm thick Silicon: 1 cm thick 
500 1000 MeV/n 500 1000 MeV/n 
400 400 
.! 0 c .. 
::s 300 
c 300 ::s 0 0 0 0 
200 200 
100 100 
0 01---'-~'----'-~'----'----IL--' 
90.5 91.0 91.5 92.0 90.5 91.0 91.5 92.0 
Zeff Zeff 
Fig. 2-Monte Carlo calculation of Zett distribution for U nuclei in a) plastic 
scintillator (carbon) and b) glass (silicon). 
Thus we see that the resolution obtainable with this instrument should 
be ~0.25 cu up through Pb, with only a small degradation due to electron 
capture occurring in the Th-U charge region. We plan additional tests at the 
Bevalac to study the properties of various scintillator types for the detection of 
92U group nuclei. 
5. References 
Anders, E., and M. Ebihara, (1982) GeoChim. et Cosmochim Acta, ~, 2362. 
Binns,W.B., T.L.Garrard, P.S.Gibner, M.H.lsrael, M.P. Kertzman, J. Klamiann, G.J.Newport, 
EC.Stone, and C.J.Waddington (1989) Ap. J., .a1a. 997. 
Blake, J.B. and·D.N. Schramm (1974) Astrophys. and Spa. Sci.,ail., 275. 
H.J. Crawford, 1991, Private communication. 
Davis, A.J., P.L. Hink, W.R. Binns, J.W. Epstein, J.J. Connell, M.H. Israel, J. Klarmann, V. Vylet, 
D.H. Kaplan, and S. Reucroft, (1989) Nuc. Instr. and Meth. in Phys.,~, 347. 
Fowler, P.F., B.N.F. Walker, M.B.W. Masheder, R.T. Moses, A Worley, and A.M. Gay, (1987) 
Ap.J., all, 739. 
Kertzmann, M.P. Thesis (1987), University of Minnesota. 
Price, P.B., H.Park, G.Gerbier, J. Drach, and M.Salamon, (1987) Nucl. Instr. & Meth., .621, 60. 
6. Acknowledgements: This work was supported in part by NASA grants. 
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© Dublin Institute for Advanced Studies • Provided by the NASA Astrophysics Data System 


English

UHCR: A Cosmic Ray Mission to Study Nuclei in the Charge Range From 20 ≤Z ≤100

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UHCR: A COSMIC RAV MISSION TO STUDY NUCLEI 
IN THE CHARGE RANGE FROM 20 ::;Z ::; 100. 
W.R. Binns1, T.L. Garrard2, L.Y. Geer1, J. Klarmann1, 
A.A. Mewaldt2, and C.J. Waddington3 
1. Washington University , St. Louis, MO, USA; 2. Caltech, Pasadena, CA. USA; 3. Univ. of 
Minnesota, Minneapolis, MN, USA 
ABSTRACT 
A definitive study of the elemental abundances of nuclei over the 
charge range of 20 ::; Z::; 100 requires a satellite mission capable of obtaining 
high statistics and excellent charge resolution over the full charge range. 
Such a mission, utilizing an electronic instrument which is an evolution of the 
HEA0-3 Heavy Nuclei Experiment, is described here. 
1. lntroductjon: Our present knowledge of the elemental abundances 
for the ultra heavy (UH) cosmic ray nuclei (Z ~ 30) has been obtained 
predominantly from the HEA0-3 (Binns, et al., 1989) and Ariel-6 (Fowler, et 
al., 1987) experiments which were flown circa 1980. These experiments 
measured the abundances of all even-charged elements for 30::;Z::;60. For 
elements heavier than Z=60, a combination of limited statistics and resolution 
resulted in the measurement of abundances for charge groups only. Our 
objectives for new measurements are: 1) the determination of the age of the 
heaviest cosmic rays (Z ~ 90); 2) a definitive study of neutron capture 
nucleosynthesis over the charge range 20 ::;z ::;100; 3) search for recent 
nucleosynthesis of cosmic rays (e.g. search for 96Cm; t 112= 1.6 x 1 o? yr); 4) 
study the propagation history of the entire range of charges; 5) search for 
superheavy nuclei (Z~110) which may exist in the cosmic radiation; and 6) 
study the effects of preferential acceleration over this broad charge region 
(e.g. first ionization potential effects). Achieving these goals requires the 
detection of roughly 100 times the number of particles detected by HEA0-3, 
and a charge resolution of :::;0.25 charge units (cu) over the full charge range. 
An instrument and mission concept capable of achieving these objectives is 
described below. 
2. Instrument: The instrument is modular with each module con-
sisting of two Scintillator dE/dx counters and a Cherenkov counter which 
combined give a measurement of charge and velocity, a time of flight detector 
to determine the particle trajectory sense (up/down), and a scintillating fiber 
hodoscope (Davis, et al., 1989) giving up to 4 x,y coordinates (H1-H4) for an-
gle and position corrections to the dE/dx and Cherenkov counter signals 
(Fig.1 ). The Cherenkov counter could consist of two radiators as was the case 
for HEAO, one mounted on top and the other on the bottom of a light diffusion 
box, and will be composed of Pilot-425 or Fused Silica. The scintillator 
counters (81 and S2) each have one radiator mounted on the inboard sides 
of the light diffusion boxes, thus providing a large acceptance geometry. The 
preferred scintillator material is plastic scintillator or other higher-Z scintillator. 
The light from each counter (S1, S2, C) will be viewed by -36 five inch photo-
multipliers (PMT's). Signals from 81 and 82 are also used for time of flight 
©Dublin Institute for Advanced Studi~t Provided by the NASA Astrophysics Data System 
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OG l 0. l . l 4P 
- , _ Unobstructed geometry,.,., ... ' ... 
,_ , 
-- H
1 
-:~poo~ooqG:~ .. ~ ~ r 1 
~~~~~~~~~~~~~~~~~~~~T~l 
Full 
85 cm 
1 
2.1 m 
Fig. 1--The UHCR instrument is symmetrical in x and y and accepts particles entering 
from above and below. The dashed lines show the "full" and "unobstructed" geometry. 
determination, especially for wide angle particles not traversing the primary 
time of flight counters (TOF1 and TOF2) on the outboard sides of the detector, 
since these particles are at wide angles and should provide sufficient 
separation to make the TOF measurement useful. Thin scintillator strips 
viewed by photomultipliers will be used to determine the TOF for particles 
traversing H1 and H4. The instrument is symmetrical and accepts particles 
from either direction. It has dimensions as shown in Fig. 1 with the radiator 
being 1.6 m square, a size very similar to that used on HEAO, thus giving a 
fairly direct means of estimating detector performance and engineering 
parameters. 
This design results in a module with a "full" geometry factor (event 
criterion 81 ·H2·C·H3·82) of 1 O m2sr, assuming that particles traverse each 
radiator at a distance >3 cm from the radiator edge. The "unobstructed" 
geometrical factor (H1 ·81 ·H2·C·H3·82·H4) for nuclei traversing the radiators 
at a distance >3 cm from the edge is 5. 7 m2sr. These geometrical factors 
include nuclei entering the instrument from both the top and bottom. 
3. The UHCR Mjssjon: The experiment would consist of 3 or 2 mod-
ules launched into an elliptical orbit such that it is outside the magnetosphere 
80% of the time. The number of particles which should be detected assuming 
solar minimum for 5 years and the full geometry factor is shown in the table 
below. The calculations assumed that nuclei had an energy of >350 MeV/n 
as they exited the Cherenkov counter. The numbers of 90 Th+92U in the table 
labeled (a) were estimated assuming Anders and Ebihara (1982) 
abundances, while those labeled (b) assumed the HEAO/Ariel (Binns et al., 
1989) abundances. For compari$on of the number of nuclei detected, the 
total number of Fe nuclei collected by HEA0-3 was 1. 7 x 1O7 particles, with 
only about half of these being of sufficiently good quality to be included in 
abundance estimates. Interactions will reduce the number of good events; 
e.g. 92U would be reduced by 40-65%, depending upon the detector 
thickness and material. The detector radiator parameters will be optimized to 
minimize interaction losses. while retainina aood charae resolution. The 
© Dublin Institute for Advanced Studies5Q3Provided by the NASA Astrophysics Data System 
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number of 90 Th+92U expected allows precise measurements of the age of 
the heaviest nuclei. For example, if the nuclei were synthesized 107 years 
ago, and we assume the detection of 200 noninteracted Th+U, the age 
determination using the 92U/90Th ratio, as derived from Blake et al. (1974), 
has a statistical precision of (1 ~\1 ) x 1 o7 years. The 92U/9sCm ratio for these 
conditions gives an independent and more precise estimate of (1±0.2) x 1 Q7 
years. If the cosmic ray age is 109 years, the 92U/90 Th ratio gives an age 
precision of ±0.3 x 109 years. For this case, the 96Cm would be decayed and 
have a zero abundance. Likewise, the 92U/94Pu ratio provides a good 
'f th t b 1 QB measure 1 e correc age 1s a out years. 
z 3-Modules 2-Modules 
26 3.3x109 2.2 x 109 
32-40 5 x 105 3.5 x 105 
41-49 9 x 104 6 x 104 
50-61 9 x 104 6 x 104 
62-73 3. x 1 Q4 2. x 104 
74-80 1.8 x 104 1.2 x 104 
81-83 5.3 x 103 3.5 x 103 
90-92 (a) 430 290 
90-92 (b) 150 100 
4. C h a r g e 
Resolution--The charge 
resolution expected for 
the Cherenkov detector 
can be scaled from that 
obtained with the HEAO 
detector which was 0.4 cu 
at 5sBa. The UHCR 
Cherenkov counter would 
have a much larger 
photocathode to box 
surface area, as well as improved PMT's, and should yield about 4 times 
more signal than HEAO which had a light collection efficiency of about 5%. 
Thus, for a radiator like that used on HEAO, a charge resolution of about 0.2 
cu in Cherenkov for relativistic nuclei up through Ba should be obtained. In 
addition, we have demonstrated Cherenkov resolution for 1 Gev/n 7gA u 
nuclei of 0.25 cu at the Bevalac (Kertzmann, 1987). Thus, based on our 
experimental work to date, we expect to be able to achieve adequate 
Cherenkov resolution at least to 79Au for particles with energy near the 
Cherenkov plateau . We anticipate that as the Cherenkov charge resolution 
degrades for particles near threshold, the dE/dx resolution will improve. 
We have recently exposed plastic scintillator counters to 47Ag and its 
interaction fragments at the Bevalac (See Paper OG 10.1.15), and a resolu-
tion of -0.25 cu was obtained. Since the S1 and S2 signals are independent, 
the two measurements should give a resolution <0.25 cu. 
At the highest charges (~90) electron capture and stripping reactions 
become important and must be carefully evaluated. There are two effects that 
must be considered. The first is a lowering of the "effective" charge as the 
particle captures electrons while traversing detector material, and the second 
is a statistical broadening in signal due to the finite number of capture and 
stripping reactions that occur for a detector of given thickness and material 
type. We have used the electron capture and stripping cross sections for 92U 
(Crawford, 1991) to calculate the effective charge for Uranium penetrating 
through carbon (plastic) and Silicon using the method described in Price et 
al. 1987. We find that the effective charae for U drops to 91.5 in Si and to 
50~ 
© Dublin Institute for Advanced Studies • Provided by the NASA Astrophysics Data System 
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91.2 in Cat 600 MeV/n. This has a negligible effect in reducing the spacing 
between, e.g., 90 Th and 92U since the effective charge of 90 Th also droops, 
though not quite as strongly. 
To estimate the significance of the statistical broadening due to elec-
tron capture and stripping, we have performed a Monte Carlo calculation 
which follows 92U nuclei through either carbon or silicon. The calculation 
includes only single electron capture and stripping reactions. In silicon the 
statistical broadening in the charge estimate is typically 0.10 cu over the full 
energy range. while in carbon the effect is about 0.23 cu at ·1200 MeV/n and 
0.18 cu at 400MeV/n. Fig. 2 shows the results of this calculation for 1000 
MeV/n. This resolution degradation is very small for a silicon detector, but is 
more significant for a carbon like detector (e.g. plastic scinti'llator). 
-soo · - soo--~~~~~~~~-
Carbon: 1 cm thick Silicon: 1 cm thick 
500 1000 MeV/n 500 1000 MeV/n 
400 400 
.! 0 
c .. 
::s 300 c 300 ::s 0 0 0 0 
200 200 
100 100 
0 01---'-~'----'-~'----'----IL--' 
90.5 91.0 91.5 92.0 90.5 91.0 91.5 92.0 
Zeff Zeff 
Fig. 2-Monte Carlo calculation of Zett distribution for U nuclei in a) plastic 
scintillator (carbon) and b) glass (silicon). 
Thus we see that the resolution obtainable with this instrument should 
be ~0.25 cu up through Pb, with only a small degradation due to electron 
capture occurring in the Th-U charge region. We plan additional tests at the 
Bevalac to study the properties of various scintillator types for the detection of 
92U group nuclei. 
5. References 
Anders, E., and M. Ebihara, (1982) GeoChim. et Cosmochim Acta, ~, 2362. 
Binns,W.B., T.L.Garrard, P.S.Gibner, M.H.lsrael, M.P. Kertzman, J. Klamiann, G.J.Newport, 
EC.Stone, and C.J.Waddington (1989) Ap. J., .a1a. 997. 
Blake, J.B. and·D.N. Schramm (1974) Astrophys. and Spa. Sci.,ail., 275. 
H.J. Crawford, 1991, Private communication. 
Davis, A.J., P.L. Hink, W.R. Binns, J.W. Epstein, J.J. Connell, M.H. Israel, J. Klarmann, V. Vylet, 
D.H. Kaplan, and S. Reucroft, (1989) Nuc. Instr. and Meth. in Phys.,~, 347. 
Fowler, P.F., B.N.F. Walker, M.B.W. Masheder, R.T. Moses, A Worley, and A.M. Gay, (1987) 
Ap.J., all, 739. 
Kertzmann, M.P. Thesis (1987), University of Minnesota. 
Price, P.B., H.Park, G.Gerbier, J. Drach, and M.Salamon, (1987) Nucl. Instr. & Meth., .621, 60. 
6. Acknowledgements: This work was supported in part by NASA grants. 
!) 10 
© Dublin Institute for Advanced Studies • Provided by the NASA Astrophysics Data System 


http://authors.library.caltech.edu/50239/1/1991-20.pdf

UHCR: A Cosmic Ray Mission to Study Nuclei in the Charge Range From 20 ≤Z ≤100

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