A simulated source was designed to duplicate the gamma spectrum of a uniform cylindrical 2200-watt Pu02 radioisotope thermoelectric generator containing 81% Pu-238 and 1.2 ppm Pu-236. Gamma rays from the decay of Pu-238, Am-241, Pu-239, and the 0-18(alpha,n)Ne-21 reaction were catalogued in broad energy groups. Two 46- and one 22-mc Th-228 sources provided simulation at various times in the life of the fuel capsule up to 18 years, which covers the time span of an outer planet mission. Emission from Th-228 represents the overwhelming contribution of the gamma spectrum after the first few years. The sources, in the form of 13-inch rods, were placed in a concentric hole in a cylinder of depleted uranium, which provided shielding equivalent to the self-shielding of the fuel capsule. The thickness of the U-238 cylinder (0.55cm) was determined by Monte Carlo calculations to insure that the spectrum emerging from the simulated source matched that of the fuel capsule

Reier, M.

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UNT (University of North Texas) Digital Library

Design of a Source to Simulate the Gamma-Ray Spectrum Emitted by a Radioisotope Thermoelectric Generator.

M. Reier

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DESIGN OF A SOURCE TO SIMULATE THE GAMMA-RAY SPECTRUM EMITTED BY A RADIOISOTOPE THERMOELECTRIC GENERATOR.

NASA Technical Reports Server

_E D_IGN OF A SOURCE TO SIMUIATE _E _IA-RAY SPECTRUM
_ITTED BY A RADIOISOTOPE THE_OEI_TRIC G_f_ATOR
Melvin Reier
The Jet Propulsion Laboratory
A simulated source was designed to duplicate the gamma s_tru_
of a uuifox_a cylindrical 2200-watt PuO2 RTG c_ta_ning _1_ Pu"_° and
1.2 ppm pu236. Gamma rays from the decay of pu2_, Am2_l, Pu_9,
and_he ol_(_,n)Ne21 reactic_ were cat_loKued in broad energy groups.
pu2_, which decays ultimately to Th_O, was treated separately,
since an exact duplicate can be made by using Th228. The selection
of sources involves a compromise between half life, cost, and gamma-
ray en?rgy. We have chosen BaI33(T1/2 = 7.2Y), Cs137(Tl/2 = 30y),
and Co60(TI/2 = 5.2Y) which provide _amna rays to cover %he spec_
fran about _50 kev to 1.3 Mew. Two _ and one 22 me Th22_ sources
provide simulation at various times in the life of the fuel capsule
up to 18 years, which covers the tiae span of an outer planet mission.
The emission from the The8 represents the overwhelming contribution
of the gamma spectrum after the first few years. Therefore, the
small changes in the intensity of the rest of the spectrum represent
a minor perturbation on the entire spectrum. The sources, in the
farm of 13-inch long rods are placed in a concentric hole in a
cylinder of depleted urani_n which provides shielding equivalent
t_^_he self shielding of the fuel capsule. The thickness of the
SoU_ cylinder (0.55 cm) was determined by Monte Carlo calculations
to insure that the epectru_ e=erging from the simulated source
closely _atched that of the fuel capsule.
INTRODUCTION
It is well established that RTG's fueled by
pu2380_ will be used to provide instrument l_Wer
on space missions where solar energy is inadequate.
An obvious one is the forthcoming Outer Planets
Mission.
Although the absolute n_ber of photons per
disintegration of Pu_ is small (_ 50/iO_ for
E _ 160 key), the gnmna intensity is not negligible
we ?ja_e dealing with about seventy t_atsan_ curies
_Pu_ per source. In addition, pu_-_o, which
asan ofabont to
Th , which becomes, after about 3 years, the
major source of gamma radiation. This will be
discussed in greater detail shortly.
Man_ of the instruments aboard a space
vehicle may be sensitive to this source of ganna
radiation. Long-teru exposure ma_ result in radi-
ation damage to instr_nentation and electronic
packages. Also, the increase in background result-
ing from this spurious source of radiation on
sensitive detectors must be measured. For those/
reasons the radiation field at various positions on
a spacecraft has to be mapped. The use of a fueled
Multi-Hundred Watt (Mh_g) generator for this purpose
would be wasteful. Besides, the possession of such
a source i_poses numerous safety, handling, and
accountability problems in a laboratory. In order
to avoid these difficulties, we have designed a
radioactive source which reproduces the _c_
features of the spectrum emitted by an _ within
about 20_ in the radial direction and 35_ in the
axial direction. Using this approach we are able
to represent the gasna radiation spectr_ from an
RTG for a cost of under $6000 while eliminating
the inherent problems cited above. More importantly,
the simulated source enables experiments to be
conducted with an aged radiation spectrum, thus
duplicating the conditions that wou_d exist at
various ages in the life of a mission.
SOURCE DESIGN
The basic philosoph_ of the design was to
reproduce the unattenuated spectrum and then
calculate the proper amount of shielding to
account for the self-shielding of the fuel and
subsequent attenuation of the cladding and outer
_ackets.
The first part of the problem is essentially
a bookkeeping procedure. The following sources of
radiation were included:
i. Plutoniu_-2_ decay.
2. _a_na r_ from the excited states of Ne 21
in the Ol_(_,n)Ne reaction. __.
3. Americi_-2_.decay. (_" is derived
from the decay of Pu z_).
_. Thori_-_8 decay.
Item I_ is a special (and easier) problem and
will be treated separately.
The selection of sources for Items 1-3 involves
a compromise between energy, half life, and ease of
production. We have chosen BaI33(TI/2 = 7._Y),
Cs137(T1./2 = 30Y) and coCO(T1/2 = 5.2y) to cover
the spec_r_ from about 350 key to 1.3 Mew.
Table I shows the latest available _a on the
absolute intensity of gamma rays from Pu_. A
cursory examination of this table reveals that
beyond about 160 key there is nothing substantial
until the region of 700 key is reached. Although
there are intense ic_-energy lines, they are
ignored in the source design because they are
enomously attenuated. The decay intensities are
added up and placed in broad energy groups. The
same procedure is followed fc_ the Am2_l and the
Ne21 decay. The latter two give rise to an
additiceml group around 350 key.
The simulation of the Pu_ decay is relatively
simple and can bedone very accurately. This can
be seen by an exa_atiou of Figure 1 showing _h_
decay chain of Pu_, leading ultimately to pb_O.
No g_ rays wort_ of consideration are emitted
859
https://ntrs.nasa.gov/search.jsp?R=19720010064 2020-03-17T03:46:11+00:00Z
TABLE 1
Pu 238 Absolute Gamma-RaY Intensities
photons/disintegration x 108
E (key) a Reler Lederer
(Ref. 1) et al (Ref. 2)
152.71 * 0.05 seen 1270 _ 90
200.9 ± 0.2 5 .+ 1
207.6 seen
235.9 .+ 0.3? 0.01 ± 0.005
25@.3 .+ 0.2 0.011 .+ 0.02
299.2 _ 0.2 o.07 + O.0P
706.z • 0.3 o.z4 + 0.02
7o8.42 ± 0.20 0.38 ± 0.04
742.77 _+ O.i0 6.35 + 0.21 7.6 _+ 0.7
766.39 + O.10 26.7 t 0.8 33 _+ 3
786.30 _+ O.10 4.98 _+ 0.16 4.8, O.4
805.8 Z 0.3 o.18 _+ O.02
808.25 _+ 0.15 0.999 + 0.063 l.l + O.l
851.70 + 0.i0 1.64 + 0.06 1.9 + 0.2
880.5 + 0.3 0.23 _+ 0.03
883.23 * o.10 1.27o + o.o_8 i.l _+ O.Z
90_.37 _+ 0.15 0.0"[2 + 0.021 0.i0 _+ 0.02
926.72 + 0.15 0.71 _ 0.04 0.83 t 0.08
941.9 + 0.2 0.58 _+ 0.02 0.67 _+ 0.07
9h6.0 + 0.3 o.13 .+ o.oe
1001.03 _+ O.15 i.O8+ 0.04 1.4 _+ 0.2
1041.8 + 0-3 0.20 _+ 0.02 0.28 + 0.02
i085.4 + 0. 3 0.ii _+ 0.02
" ">'1"1 (1)
 2at" Xl"l" (2)
- (3)
e"RIt + 1 . i . •"_2t + i .
1 , . e'_3t_
WN, _J
NIO is the amount of Pu a3_ present at t = 0
Table IX gives the amount of activity of each
of the isotopes for a 2200 watt fuel source at
diffe¢o_t ages of the fuel starting with 1.2 ppm
of Pu23b. The Buildup of the_a 133 source with age
is due to th_ increase of Am _ Aresulting from the
decay_ of pu2_l. The Csl37 and Co60 are used mainly
to simulate the deca_ of pu238. Their decrease
with time reflects mainly the half life of pu238
(T1/2 - 86.4y). The barium, cesium, and cobalt
will be packaged in a single capsule. In addit_ou,
we have ordered three separate capsules of Th228,
so that we will be able to reproduce the thorium
activity at fuel ages of O, 3, 5,_A 18 years.
The amount of Ba133, Cs 137, andCo °u corresponds
to t = 0 only. The error introduced by not using
different quantities of thes_ isotopes as the fuel
ages is small since the Th22_ activity overshadows
the others within a few years.
p236 _ U232 ..___1_Th228 (t .1_ Ra224 _ R220 a
2.85Y 72Y 1.91Y 3.640 52S
po 216 pb212 _ Bi212 _(367.) le 208
O.lbS lO.bR / 60M I]
OOM 8 (_ 3M
p°212 _' O.e_s3 _ pb208
FIGURE i. Plutonium-236 Decay Chain
TABLE 2
Quantity of Isotopes (Me) For Simulated Source
Year Ba 133 Cs 137 0060 Th228
0 1.3o 26.0 0.75 0
1 1.37 25.9 0.75 h.2_
5 1.58 25.3 0.73 49.9
i0 1.79 24.6 o.70 88.1
18 2.05 23.l 0.64 102
228 s r s euntil Th i eached. It then decay_ mitting
gala rays froaFo _12, Bi 212, and T120_, the latter
producing the veil-known 2.61 Mew_gamma ray in 100%
of the decays. Fortunately, Th 22_ is readil_
available and maybe purchased in the_sired
encapsulated fo_a. The buildup of Th22 is deter-
mined by solving the three linear differential
equations starting with pu236. Th_se are shown in
Eq. 1-3. The activity of the Th_ is given by the
solutiou, Eq. _.
SHIELD DESIGN
Having selected the isotopes, the next step
is to calculate ashield thickness such that the
emitted spectrum is the same as that from the real
source. The actual source is a I3 inch long
c_£inder of fuel pucks having a 2.731 cmradius,
enclosed in an 0.051 ca tantalum shell, which in
turn, is inserted concentrically in a _raphite
cylinder having a wall thickness of 3.175 ca. The
fuel is a mixture of Pu02, ThO2, and Mo. The
eoncentrntion of each element is shown in Table
III. The density of the mixture is 9.17 g/ca3.
851
TABLE3
Concentration of Elements in the Fuel
Element
68.8
Th 7.6
Mo 13.3
0 _._
Since the real fuel is composed largely of
high Z elements, depleted uranium was selected as
the shield to reproduce the self-shieldlng of the
source. The natural radioactivity of the depleted
uranium and it's impurities add insignificant-
ly to the simulated source and can be neglect-
ed. Figure 2 shows a sketch of the real and
simulated source. Monte Carlo ealemlatlc_s
were run comparing the spectrum in the radial,
axial, and 45° directions of the real source with
a 13 in. line source (the diameter of the capsules
for the simulated source is 1/4 inch). The results
in the radial direction and at 45° are very encour-
aging. Figure 3 shows a comparison between the
real and simulated source in the radial direction.
Except in the very low energy region, where the
statistics are about _0_, the agreement is within
about 20%. (The mismatch of the 600 to 700 key
peak from the csl37 and the 700 to 800 key peak
in the real case is expected and is not considered
a serious problem).
]oIJ
0.01
-- SIMULATED SOURCE
...........MHW Z200 W(th) PMC PUCK
(5 yEAR OLDFUEL)
-!
r..-.I .
i .........
i
i i iI, , Jill i t i ill[ i
o._ io
E(MeV)
'1
"4
-4
-4
,,I
IO.D
FIGURE 3. Comparison of Spectrum in the
Radial Direction from the Simulated
and Real Source
Because of the enormous self shielding, we
were not able to simulate the radiation in the
axial direction by using depleted uranium c_ a
combination of depleted uranium and a lighter
element. The best result we were able to obtain
is shown in Figure 4. This uses seven cm of
copper at the ends of the simulated source. A
compromise had to be made between the high and
low end of the spectrum. If a small medium
energy source could be placed inside the end
shield, the problem would he simplified and a
much better match could he obtained.
It is a pleasure to acknowledge the assistance
of Mr. Michael A. Dote in the computational analysis.
3
I i
I !
O 076
_GRAPHITE
_TANTALUM
--TITANIUM
DIMENSIONS IN CM
NOT TO SCALE
I
1.906 cm Ib
q 055cm
FIGURE 2. Simulated and Real Source
i0_
>-
I0!
001
-- SIMULATED SOURCE J'_I........ _,_HW 2200 W4thl PMC PUCK<5YEAR OLD FUEL1
...... .... : _
:.......""...;: •:
i .." "':
I ; ;:
...................' :':! _ i| _" :
....
I
t t ill iLiii i , i h I,l,I ,
O1 10
E!MeVI
, , , _,,
100
FIGURE 4. Comparison of Spectrum in the Axial
Direction from theSimulated and
Real Source
le
2.
REFERm_ES
M. Reier, Nucl. Sci. & Engr. (in press).
C. M. Lederer, F. Asaro, a_d I. Perlman.
Nuclear Chemistry Annual Report (1968),
uca_18667.
852


The design of a source to simulate the gamma-ray spectrum emitted by a radioisotope thermoelectric generator

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