A liquid cesium target has been developed which allows the production and separate identification of the neutron deficient isotopes of xenon. The present report describes irradiations utilizing 34 to 41 MeV protons to produce millicurie quantities of Xe-127 and Xe-129m. At higher energies, however, the target could be used without modification to produce xenon isotopes as light as 119

Blue, J. W.

Jha, S.

Leonard, R.

Sodd, V. J.

Vincent, J. S.

English

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A TARGET FOR PRODUCTION OF RADIOXENONS
by J. W. Blue and R. Leonard	 ^,s^03^t1 Z,^^^
Lewis Research Center^^°'	 ^6
S iha and V J. Sodd	 ! 	 JUN W1	 v,
Priversity of Cin! innati	 ^3 RECEIV ED
and J S, Vincent	 `'^ NASA Zn Fll uft
TRIUMF
1
TECHNICAL PAPER to be presented at the American Physical
Society Meeting
Washington, D. C. , April 26- 29, 1976
A TARGET FOR PRODUCTION OF' RAI)1OF.XFNONS
By J. W. Blue and R. Leonard
Lewis Research Center
S. Alfa and V. J. Sodd
University of Cincinnati
and
J. S. Vincent
TRIUMF
SUMMARY
A liquid cesium target has been developed which allows the production
and separate identification of the neutron deficient isotopes of xenon. The
present report describes irradiations utilizing 34 to 41 MeV protons to produce
millicurie quantities of Xe-127 and Xe-129m. At higher energies, However, the
target could be used without modification to produce xenon isotopes as light as
119.
I NTRODUCT ION
O	 Tile practical application of radioxenons in nuclear medi^ine provides
N	 the motivation of a study of the targetry appropriate fer their production
oo	 using high current proton accelerators. Xenon-123 is medically the most impor-
tant radioxenon, because it is the parent of 1-123, which is used directly in
thyroid studies and in labelling many pharmaceutical compounds. Xe-125 is sim-
ilarly valuable as the parent of 1-125, while Xe-1.27 is used directly in lung-
imaging studies. If available, Xe-129m may also prove useful for such studies.
In addition, Xe-129m is of interest in that it exhibits a Mossbauer trans-
ition. Previous Mossbauer studies (ref. l) utilized rather weak sources pro-
duced by reactor irradiation of natural xenon, followed by mass separation.
TARGET CONSTRL'CTION
In the present work a 6 cm diameter disk, 1.2 cm thick is filled with
liquid cesium. This target was irradiated with photons from the Lewis Research
Center's variable energy cyzlotron. The proton beam enters normal to one of
the flat faces of the target. For protons of the energy available Here the
target is thick, with the beam coming to rest in the target. This target .ias
several obvious advantages relative to a solid target:
1) The liquid metal does not suffer radiation damage.
2) Tile heating caused by the beam can be carried away from the target
by the evaporation of cesium.
3) The radioxenons can be removed from the target by a stream of carrier
gas so that chemical separations are unnecessary.
4) The target is indefinitely useful.
EXPERIMENTAL PROCEDURE:
The liquid cesium target has been bombarded by protons at selected ener-
gies between 28 and 40 MeV with beam currents from 2 to 5 microamperes. The
i
I2
irradiations were short (approximately 5 minutes) so that the dynamic resuollse
of the target could be studied. The radioxerions were carried from the target
by a continuous flow of helium gas (about 30 ml per minute). 'I'll(- experimental
arrangement is shown in Fig. 1. The target is connected b y 20 meters of 6.2 mm
OD tubing to two cold traps, arranged in series.
Each trap consists of a section of tubing immersed in liquid nitrogen. In
addition the second trap contained a molecular sieve. Two Ge(I.i) detectors were
used to observe the gamma radiation as radioactive products accumulated in each
trap, for a period of several hours after each run.
Anal y ses of the gamma ray spectra were carried out using; the cyclotron
computer system to fit Gaussian line shapes to the peaks after an appropriate
background subtraction. For more complex spectra, consisting of both Xe-127
and Xe-129m, (such as observed at 35.7 MeV) a pure Xe-129m spectrum (re-
corded at Ep - 32.0 MeV) was subtracted, with proper normalization, to permit a
more accurate analysis of the Xe-127 radiations. This procedure is illustrated
In Fig. 2.
RESULTS
The production of Xe-129m as indicated by the presence of a 1^6 keV line
in the gamma ray spectra was observed in all bo nhar.'.ments (27.3 MeV < F: <
40.2 MeV). Similarly, at proton energies above 33 MeV, the presence ofi'Xe-127,
as evidenced by a 203 keV gamma, was detected. Loth the Xe-129m and Xe-121
activities were observed to require more than one hour for complete removal
from the target and accumulation in the lirst trap. both activities accumulated
with the same time constant. This is shown in Fib;. 3, where counting rates for
Hie 196 keV and 203 keV gamma rays are shown as a function of time after bo.--
bardment. It was observed that this collection time could be decreased by ap-
proximately a factor of two by raising; the target temperature prior to irradi-
ation from 1001 to 2600 C.
In all irradiations, annihilation radiation was observed to accumulate
in the first trap. A positron emitter arrived at the trap promptly after the
start of the irradiation, was retained there temporarily as shown in Fig. 3, and
then appeared to diffuse into the second trap with a mean life time for reten-
tion of approximately 26 minutes. This activity was permanently trapped in the
molecular sieve, where its nuclear decay lifetime was determined to be 21 min-
utes. This activity is presumed to be due to 1 100, which arises from oxygen
impurity in the target. The reactions would be:
160(p;a,n,p)I:C
and
11C+Cs20 -► 2Cs+ 1160
1'he thick target yields of Xe-1291i. and Xe-127 are given in Fig. 4 in
microcuries/microamp-liour for a number of proton energies. These data have
been used to obtain cross sections for the reactions that yield Xe-129m and
Xe-127. The difference in yields at two successive energies is assumed to be
T	 -- -T -	
I
1	 ^	 I	 }	 !
3
1
3	 a
due to reactions taking place in a thickness of the target AX, where t.X is
the difference in range of protons at the two energies. This AX ran be con-
verted to an area] density of target nuclei, which together with the number of
Incident protons and the increase in the yield of radioxenon permits the cal-
culation of a
'
toss section. These results are shown in Fig. 5, plotted at the 	 ^+
midpoint of each energy interval.
The absolute values of the yield are estimated to have an uncertainty of
about 30. The relative uncertainty in yield from one energy to the next is
estimated to be about 107. The principle source of error is the uncertainty in
solid angle between source and detector. The location of the radioxenon frozen
on the walls of the cold trap could fluctuate several centimeters as the liquid
nitrogen level changed due to boil-oft during the experiment. The ratio of the
yield of Xe-127 to that of Xe-129m at any one energy is the most reliable num-
ber reported. The uncertainty in this value is due to the possible error in the
values used for the number of photons (196 and 203 keV) per disintegration.
For the 203 keV gamma, the value was taken to be 0.66. For the 196.7 keV gamn.a
of Xe-129m the value 0.043 was used. These are based on data given in Nuclear
Data Tables (ref. 2).
DISCUSSION
The most interesting features of the data are:
1) Both Xe-127 and Xe-129m accumulate on the target with the same timc-
constant.
2) The cross sections for production of Xe-127 and Xe-129m are compar-
able in magnitude and quite large. The yield of Xe-131m was not measured but
was observed to be at least two orders of magnitude smaller.
3) There is a considerable difference in the rate of increase of the
yield with increasing proton energy for the two isotopes.
4) The difference in the threshold for production of the two isotopes
(differing by two neutrons) is approximately 10 ..aV, even though the binding
energy for a single neutron is approximately 9 MeV.
The fact that both isotopes have the same collection time indicates that
this time is characteristic of the target and transport system, rather than the
nuclear lifetimes involved in the production. In addition, if either isotope
were produced from the beta decay of an intermediate parent, those lifetimes
would be either 32 hours or 6 hours for Cs-129 and Cs-127, respectively. The
delay in removal of the activity from the cesium target is presumed to be due to
a chemical adsorption or combination of the radioxenon with cesium. The adsorp-
tion hypothesis is obviously consistent with the decrease in collection time
which was observed at higher temperatures. If on the other hand the delay is
due to the formation of a CsXe molecule, it must be of a nature such that its
rate of dissociation increaL:es with temperature. The possibility of the forma-
tion of the CsXe molecule is supported by the fact that both Cs*2 and Xe2
molecules exist, although both species are stable only in an excited molecular
state.
The fact that the isotopes are produced directly rather than through a long-
lived precursor mans that either a (1);xn,yp) or (p;a,xn) reaction must take
4place. For Xe-127, the former reaction (x - 5, y - 2) has c^ _ -47 MeV, while
for Xe-129m (x - 3, ,, . 2) A = -31 MeV. 	 in hnth cases the proton energies em
-ployed are too low for such a reaction to be the dominant process in accounting
for cross sections as large as those observed. Consequently, one must con-
clude that alpha particle emission is an Important factor in the production of
both Xe-127 and Xe-129m, whether the alpha particle Is considered to preexist
in the nucleus or whether it is considered to form after emission of Its com-
ponents. Reactions of the type (p;a,x) have been studied (ref. 3) for med'.um A
nuclei at '39 MeV; however, x wa.; not identified in the experiment. No informa-
tion is available for the (p;u,n) and (1); , .,3n) reactions.	 A recent study
(ref. 4) of the (d, 61.i) reaction, an alpha particle pickup, showed a reduced
cross section for nuclei with A = 130. This was attributed to subshell closure.
The i,'esent study would indicate that such effects have disappeared by A
	 134.
The difference in the shape of the cross section curves for the produc-
tion of the two isotopes may be attributed in part at least to the vastly dif-
ferent natures of the final states. 	 Xe-129m, Witt; J = 1112 is lil-.ely only
formed through rather high spin intermediate states, while Xe-127, with J - 1/2
would be formed only through low spin states, with the yield curves reflecting
the density of each type of intermediate state. The large cross sections ob-
served for production of Xe-129m at 40.2 MeV is probably due to the opening of
another reaction channel. At 43 MeV it is possible for the reaction (p;3n,21))
to occur. It is still., however, not far enough above threshold for this reac-
tion to make a large contribution. A more reasonable possibility is that Ba--133
becomes the intermediate nucleus rather than Xe-130. That is, the neutron
could be emitted before the alpha particle.
CONCLUSIONS
The thick target yields of Xe-l27 and Xe-129m were measured for a num-
ber o. incident proton energies. These data were also used to obtain cross
sections.
It is concluded that a liquid cesium target is practical for routine pro-
duction of radioxenons using high current proton accelerators. The temperature
of the liquid cesium should Le at least 300 0 C in order to obtain prompt re-
moval of the xenon products from the target. This consideration would he partic-
ularly important for Xe-123 where the 2 hr half-life would give significant re-
duction in yield if allowed to decay to 1-123 which would chemically combine
with cesium and become unrecoverable.
The subtraction process of separating gamma ray spectra for analysis
should prove even more useful, at higher energies, cohere spectra will become
more complex and will arise from a greater number of radioactive isotopes.
REFERENCES
1. .1. S. Geiger, R. L. Graham and F. Brown, Can. J. Phys. 40, 1258 (1962),
2. Nuclear Data Sheets 8, 138 (1972).
3. F. E. Bertrand and R. W. Peelle, Phys. Rev. C 8, 1045 (1973).
4. F. D. Becchetti, et al., Phys. Rev. Lett. 34, 225 (1975).
i
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41
PROTON ENERGY
Figure 3. - Yield vs. energy,
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Figure 4,	 Cross section vs. proton energy.
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NASA•Low-s


A target for production of radioxenons

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