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Evaluation of neutron inelastic scattering for radioisotope production
Appl. Radiat. Isot. Vol. 48, No. 4, pp. 441-446, 1997 © 1997ElsevierScienceLtd
Pergamon
PII: S0969-8043(96)00284-9
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Evaluation of Neutron Inelastic Scattering for Radioisotope Production S. M I R Z A D E H .1, F. F. K N A P P J R l, C. W. A L E X A N D E R 2 a n d L. F. M A U S N E R 3 ~Nuclear Medicine Group, Health Sciences Research Division, qsotope Technology Section, Chemical Technology Division, Oak Ridge National Laboratory (ORNL), P.O. Box 2008, Oak Ridge, TN 37831-6229, U.S.A. and 3Medical Department, Brookhaven National Laboratory (BNL), Upton, NY 11973, U.S.A.
(Received 10 July 1996; in revisedform 4 September 1996) A systematic study of the production of 117mSn, H~Sn and ~95mptin the hydraulic tube facility (HT) of the ORNL High Flux Isotope Reactor (HFIR) has demonstrated that in all three cases the yieldsfrom neutron inelastic scattering (*Z[n,n'] *mz) were higher than yields obtained from neutron capture reactions (~A OZ[n,y]Amz). The corresponding fission-averaged cross-sections were 222 + 16, 168 _+ 12 and 287 + 20 rob, respectively.The relative gains in the specific activity were 1.4 for 195raPt, 3.3 for mmSnand 4.4 for H9'*Sn.The larger gain for ng'*Sncould be attributed to the relatively lower excitation energy (89.5 keV) of this metastable nucleus. As a part of these studies, the thermal neutron capture cross-sections and resonance integrals for the above reactions and the cross-sections for the [n,2n] reactions leading to the above nuclides were also evaluated. © 1997 Elsevier Science Ltd. All rights reserved
Introduction The production of metastable nuclei such as H7mSn, ~95mpt (for biomedical applications, Atkins et al., 1993, 1995, Willins and Sgouros, 1995, Mirzadeh and Packard, 1995) and ng~Sn (for M6ssbauer spectroscopy, Dickson and Berry, 1986) via neutron radiative capture reactions are characterized by small neutron cross-sections and, hence, low production rates. Metastable nuclei typically have excitation energies on the order of 100 keV and large differences in angular momentum from ground states (most metastable nuclei have high angular momentum). An alternative route for producing these types of metastable nuclei is through neutron inelastic scattering where the cross-section of the AZ[n,n'] ~ Z reaction is, in some cases, substantially higher than the cross-section for the CA-,Z[n,y]~Z route. As has been shown for the case of "7mSn (Mausner et al., 1985), the magnitude of gain in the cross-section may compensate for the relatively lower fast neutron flux from a well-moderated fission spectrum, En >__Qtn~l. Note that the excitation energy of metastable nuclei will represent the threshold for inelastic scattering. Large research reactors, such as H F I R t and HFBR:~, *To whom all correspondence should be addressed. tHFIR: High Flux Isotope Reactor, ORNL. :~HFBR: High Flux Beam Reactor, Brookhaven National Laboratory (BNL).
with significant epithermal and fast neutron fluxes are well suited for these types of reactions. In this paper, we report the results of a systematic study of the production of nTmSn, "9~Sn and t95"Pt in the hydraulic tube irradiation facility (HT) of the H F I R via both radiative neutron capture reactions and inelastic scattering. These studies also include an assessment of the contribution of the epithermal neutrons to overall reaction rates. Cross-sections of the [n,2n] reactions leading to the above radionuclides were also evaluated.
Experimental Irradiation facility The ORNL HFIR is a compact, flux trap type reactor with facilities for performing a wide variety of irradiations. The trap at the center of the core is filled with a bundle of AI targets holding primarily transuranium targets which are accessible only during refueling. The hydraulic tube facility (HT) is a single tube which passes through the bundle and exits the reactor. A train of nine vertically stacked capsules, or rabbits, can be sent in and out of the reactor on demand while the reactor is operating. Position 5 of the hydraulic tube (HT#5) is centered at the core midplane and has the highest neutron flux. The train of capsules is driven by hydraulic pressure and the capsules are cooled by water at a velocity of
441
442
S. Mirzadeh et al.
~ 3 m s - ~ (flow of ~ 24 L m i n - 1). The HT is principally used for the production of medical radioisotopes. Additional information with regard to the HT and other H F I R irradiation facilities can be found elsewhere (Mahmood et al., 1995; Mirzadeh et al., 1992).
Materials and Methods For cross-section measurements, samples ( ~ 10 mg) of highly enriched isotopes of Sn (as SnO2) and metallic Pt together with flux monitoring wires were irradiated for 1 h in HT-HFIR. The isotopic compositions of enriched Sn and Pt samples (from ORNL) are given in Table 1. Targets were sealed in 3 x 25 mm thin wall ( ~ 0.3 ram) Suprasil quartz tubes (Heraeus, Duluth, GA). To assess the contribution from epithermal neutrons, samples wrapped in Cd filters (0.26 mm) were also simultaneously irradiated. To minimize the Cd depletion, the reactor power level was maintained at 9 M W ( ~ 10%) during irradiation. The flux monitors for thermal and epithermal neutrons were alloys of Au/A1, Ag/AI and Co/AI (Reactor Experiments, Inc., San Carlos, CA). The monitors for fast neutrons (E, > 1 MeV) were high purity metal wires including A1, Cu, Fe, Ni and Ti (ASAR, Johnson Mattey, Seabrook, NH). After irradiation, the quartz sample tubes were soaked in conc. HNO3 for a few minutes, then thoroughly rinsed with water in order to remove any surface contamination on the quartz tubes. After an appropriate cooling time, the activities of these sealed sources were measured at a 30 cm distance from a well-calibrated solid-state HPGe-7-ray detector (50 cm 3, E G & G Ortec, Oak Ridge, TN). The detector was coupled to a PC-based MCA (Canberra Industries, Inc., Meriden, CT). The 23.8 keV T-ray of HgmSn was measured in a l-cm-thick Ge detector (EG&G Ortec, Oak Ridge, TN). All activities were followed for several half-lives and the activities at the end of bombardment (EOB) were extrapolated by the CLSQ code (Cummings, 1962). When multiple v-rays were used for assay, their intensifies were used as weighing factors in calculation of the average activity
values. When not explicitly stated, the relevant nuclear data were taken from Lederer et al. (1978) and Reus and Westmeier (1983). The thermal and epithermal cross-sections were from Mughabghab e t al. (1981) and Mughabghab (1981). For large-scale production of"TraSh, targets were in the form of metallic "TSn powder or small pieces (the 84.23% enriched material was from ORNL, and 92.2% was Russian made and obtained from Chemotrade, Diiesseldrof, Germany). The targets were sealed under a slight negative pressure of He in the quartz ampules (Suprasil, 6.0 x 45 mm). The sealed quartz ampules were then placed in the welded AI rabbits (6.7 x 51 mm) for irradiation. As mentioned earlier, the AI rabbits were cooled to ~ 60°C by water at a flow of ~ 24 L m - t during irradiation. In large scale production, Sn targets were found molten at the time of discharge from reactor due to the rather low melting point of Sn metal (232°C) and insufficient heat conduction between the quartz ampoule and the A1 rabbit. The quartz ampoule, however, remained intact in all irradiations. Backfilling the AI rabbit with ~ 1 atmosphere of He prior to welding the rabbits did not prevent melting of the Sn metal. The quartz tubes containing the irradiated targets were sent to BNL for processing and radiopharmaceutical formulation.
Results and Discussion The neutron spectrum at the HT#5 of H F I R was measured using refined activation techniques, crosssection data bases, and calculation methods. The measurements included a number of Cd-flltered and unfiltered flux monitors exhibiting different responses at different neutron energy intervals. The reactor power level was maintained at ~ 10% during these studies. A summary of the spectrum-averaged fluxes over selected energy ranges, relevant to the nuclear reactions discussed here, is given in Table 2. Detailed information with regard to neutron flux measurements and neutron spectra unfolding can be found elsewhere (Mahmood e t al., 1995). The results from a systematic study of the p r o d u c t i o n o f llTmSn, "gmSn and 19Smptin the HT of the
Table 1. Isotopic compositions of enriched Sn and Pt targets Sample
Composition (at. %) Mass No.
"~Sn "~Sn "SSn 119Srl mSn
116
117
118
95.60 ± 0.05 2.54 ± 0.04 0.30 + 0.02 0.65 ± 0.05 0.20 ± 0.01
1.63 + 0.02 84.23 + 0.15 0.49 ± 0.02 0.63 + 0.05 0.12 + 0.01
1.48 ± 0.02 9.42 ± 0.10 97.07 ± 0.20 3.01 ± 0.05 0.49 ± 0.01
194
195
196
Others
97.41 + 0.03 1.16 + 0.01 0.63 + 0.01
1.99 + 0.02 97.28 ± 0.03 1.57 ± 0.02
0.52 + 0.01 1.47 + 0.02 97.51 ± 0.02
0.08 0.09 0.30
0.27 2.03 0.92 89.8 0.36
119
120
Others
+ + ± ± +
0.63 ± 0.02 0.26 ± 0.02 1.11 + 0.02 5.34 ± 0.05 98.57 ± 0.02
0.39 0.32 0.13 0.55 0.22
0.02 0.04 0.03 0.10 0.01
Mass No. ~4Pt JgSPt ~Pt
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Evaluation of neutron inelastic scattering for radioisotope production Table 2. Averaged neutron fluxes at the HT#5 of the HFIR at operating power level
of 9 MW Neutron energy < 0.5 eV (thermal) 0.5 eV < E, ~ 100 keV (epithermal) > 100 keV > 300 keV > 1 MeV
Neutron flux (n s-tcm 2)
0,h/O'
(2.65 -+ 0.15) × l0 i4
1.0
(1.06 -+ 0.21) x 10~J " (9.42 5: 1.41) x 10~3 (8.29 -+ 1.24) x 10~3 (4.87 5- 0.78) × 10~3
25 2.81 3.20 5.43
"Per unit In E (lethargy), E is the neutron energy in MeV (Stoughton and Halperin, 1959).
H F I R a r e p r e s e n t e d in T a b l e 3. I n all t h r e e cases, t h e yields f r o m t h e [n,n'] r e a c t i o n s were h i g h e r t h a n t h o s e o b t a i n e d f r o m t h e [n,v] r e a c t i o n s . T h e relative g a i n s in t h e specific a c t i v i t y o f t h e u n a l t e r e d t a r g e t s w e r e 1.4, f o r 195rapt, 3.3 f o r 117mSn a n d 4.4 f o r Hg~sn. T h e l a r g e r g a i n for "9~Sn c o u l d be a t t r i b u t e d to t h e relatively l o w e r e x c i t a t i o n e n e r g y (89.5 k e V ) o f this m e t a s t a b l e n u c l e u s . T h e e x c i t a t i o n e n e r g i e s for H7mSn a n d t95mpt a r e 314.6 a n d 259.2 keV, respectively. A s s h o w n , c o n t r i b u t i o n s f r o m t h e [n,2n] r e a c t i o n s to t h e o v e r a l l yields w e r e i n s i g n i f i c a n t . Since t h e t h r e s h o l d s f o r t h e s e inelastic s c a t t e r i n g r e a c t i o n s a r e well a b o v e t h e c a d m i u m c u t o f f , C d filters w o u l d n o t be e x p e c t e d to h a v e a n y effect o n t h e yield o f t h e s e r e a c t i o n s . T h e slightly h i g h e r yield o f J95mpt f r o m t h e C d - f i l t e r e d 19spt s a m p l e m a y be a t t r i b u t e d to d i m i n i s h e d b u r n - u p o f this n u c l i d e d u e to c a p t u r e o f t h e r m a l n e u t r o n s b y t h e C d filters ( T a b l e 3). W h e n e x t r a p o l a t i n g f r o m a 1 h i r r a d i a t i o n at a 1 0 % p o w e r level to a o n e - c y c l e i r r a d i a t i o n (21 d) at
a 85 M W p o w e r level, t h e e x p e c t e d specific activities o f HgmSn a n d ~9~Sn a r e 5.2 x 102 a n d 3.1 x 102 M B q / m g (14 a n d 8.5 m C i / m g ) , respectively. A s i m i l a r e x t r a p o l a t i o n r e s u l t s in a n a p p a r e n t specific activity o f 6.3 × 102 M B q / m g (17.1 m C i / m g ) at s a t u r a t i o n f o r 195raPt. I n this case, t h e v e r y l a r g e b u r n - u p c r o s s - s e c t i o n o f ~gSmpt (1.3 X 104 b, H o e s c h e l e e t al., 1982) yields a specific activity w h i c h is s i g n i f i c a n t l y lower. N e v e r t h e l e s s , it is e x p e c t e d t h e g a i n o f 1.4 o b t a i n e d b y t h e u s e o f inelastic r e a c t i o n will be m a i n t a i n e d f o r large-scale p r o d u c t i o n . T h e f i s s i o n - a v e r a g e d c r o s s - s e c t i o n s for t h e individu a l r e a c t i o n s , try, l e a d i n g to 117mSn, "gmsn a n d 195raPt were c a l c u l a t e d by s o l v i n g sets o f t h r e e s i m u l t a n e o u s e q u a t i o n s for e a c h r a d i o i s o t o p e . These three e q u a t i o n s w h i c h a r e d e f n e d b y s u b s c r i p t j refer to t h e t h r e e t y p e s o f t a r g e t s u s e d in e a c h case. F o r e x a m p l e , in t h e c a s e o f "TSn, j = 1 refers to t a r g e t e n r i c h e d in l~rSn, j = 2 refers to t a r g e t e n r i c h e d in llTSn, a n d j = 3 refers to t a r g e t e n r i c h e d in "SSn. L e t s u b s c r i p t i = 1,
Table 3. Production of tl7mSn, ~t9~Snand 195mptvia [n,~], [n,n'] and [n,2n] reactions Nuclear reaction
Saturation yield at EOB (Bq/mg)
Cross-sections (mb) This work
"+Snln,y]
U F
"TSn[n,n']
U F
"SSn[n,2n]
U F
"sSn[n,y]
U F
~gSn[n,n']
U F
12°Sn[n,2nl
U F
~uPt[n,y]
U F
mPt[n,n']
U F
~9~Pt[n,2n]
U F
llTmSn(t,;2 = 14.0 d, I = 11/2 + , Q = 314.6 keV) 2.60 x 107 a,~ (1.95 -+ 0.20) x l& 1.97 x 107 10 (3.50 5: 0.53) x 102 a~h 5.8 -+ 1.2 8.59 × 107 (2.25 + 0.23) × 103 8.39 x 103 (2.19 __.0.22) × 102 Av. (2.22 -+ 1.16) × 103 4.46 x 105 (1.5 + 0.2) x 10 -t 5.43 × 105 (3.9 __.0.6) x 10 t Av. (2.7 -+ t.2) × 10 tiTmSn(/i/2 = 293 d, I = 11/2 + , Q = 89.5 keV) 1.78 x 107 a,fr (1.32 5: 0.20) x 10L 7.86 x 106 I0 (1.44 5: 0.22) x 102 a~h 7.4 -+ 2.0 7.86 × 107 (1.63 + 0.16) x 102 7.41 x 103 (1.73 ± 0.17) x 102 Av. (1.68 + 0.12) x 103 (4 -+ 1) x 103 --(2 -+ 1) × 103 --195Pt (tx/2 = 4.02 d, I = 13/2 + , Q = 259.2 keV) 5.11 x 107 a~r (6.27 + 0.63) x 10~ 1.89 x 107 I0 (5.26 -+ 0.79) x 103 a,h (4.2 -+ 0.8) × l0 t 7.21 × 107 (2.74 + 0.27) x 103 7.70 × 107 (3.00 -+ 0.30) × 103 Av. (2.87 5: 0.20) x 103 1.93 x 106 5.4 -4- 1.1 1.33 x 103 2.0 5:0.4 Av. 3.7 ± 0.7
Ref.
Reported (1.98 + 0.12) x l0 t (4.90 _+ 1.60) × 102 6--+2 (r) (1.76 _ 0.14) x 102
1 1 2 1
1.0 × 10 ~
I
(1.0 -+ 0.6 x 10)~
2
(3.1 -i- 0.1) x 103 (9.8 + 1.1) x 10~
2 2
T~.= 1 h at 9 MW power level, U: unfil~red, F: Cd filtered. With the exception to the mSn[n,2n] reaction, the uncertainties in the saturation yields are less then + 10%. Ref. 1: Mausner et al. (1985) and Ref. 2: Mughabghab et aL 0981),
444
S. Mirzadeh et al.
2 and 3 refer to the In,),], [n,n'] and [n,2n] reactions, respectively. Then ~a~:x, = k.b,
(i = 1,3 and j = 1,3)
where a 0 denotes the mass fraction of each target isotope divided by the average atomic mass of enriched target isotope. The variable xi is the product of the neutron flux, ~b, (at an appropriate neutron energy range), and the cross-section for appropriate reaction, a,. The parameter b, is the saturation activity of a given radioisotope (column 3 of Table 3, dps/mg of target), and k is a proportionality constant. In the case of neutron capture reaction, a, represents the effective cross-section, O'en= Crth+ (1/r)'lo where a,h is the thermal cross-section, I0 is the resonance integral, and r = q~th/~b~(q~this the thermal neutron flux and ~b,p~is the epithermal neutron flux per unit In E, lethargy, see Stoughton and Halperin, 1959). For a filtered sample, ai is equal to I0. The corresponding cross-sections for the inelastic neutron scattering reactions for the production of H7mSn, HgmSn and 19smpt are 222 + 16, 168 + 12 and 287 + 20 mb, respectively (Table 3). A value of 176 + 14 mb for the cross-section of "TSn[n,n'] HTmSn reaction obtained at H F B R was reported earlier (Mausner et al., 1985). Our measured cross-sections for neutron capture reactions leading to ll7raSn, ~'~mSnand 195raPt, together with reported values are also given in Table 3. In the case of H6Sn[n,7]"TmSn reaction, the measured thermal neutron capture cross-section (O'th) and resonance integral (I0) were 5.8_+ 1.2 and 350_+53mb, respectively, in contrast to reported values of 6 _+ 2 and 490 _+ 160 mb. For "SSn[n,~'] "gmSn reaction, the aegis 13.2 _+ 2.0 mb which is within the upper limit of the reported value of 10 _+ 6 mb. The measured thermal and resonance integral for this reaction are 7.4 _+ 2.0 and 144 _+ 22 mb, respectively. The measured a,h for J95pt[n,~,] 195raPtis 42 + 1 rob, lower than the reported value by about a factor of 2. Our measured I0 for this reaction at (5.3 -+ 0.8) x 102 mb is, however, ~ 6 fold lower than reported value. As demonstrated for these three cases, in the determination of the neutron capture cross-sections contribution from inelastic neutron scattering to overall reactions are significant and appropriate corrections are required even when highly enriched target isotopes are used. Consequently, our measured cross-sections for capture reactions are lower than reported values. The cross-sections for [n,2n] reactions leading to HTmSn, "~mSn and 19Smpt nuclides are also given in Table 3. These cross-sections are in the range of 0.3-4 mb and, therefore, contributions from these routes to the overall reaction rates are rather small (in order of a few percent).
With the exception of the ~2°Sn[n,2n] reaction, the uncertainties in the saturation yields given in Table 3 are less then + 10%. This includes uncertainty due to counting statistics and detector efficiency. The large errors ( ~ 25% for unfiltered and ~ 50% for the filtered samples) in the saturation yields of Hgmsn, produced from the ~2°Sn[n,2n] reaction, were primarily due to counting statistics. The uncertainties of the listed thermal cross-sections and resonance integrals include a 5% error in the thermal-to-epithermal ratio, as the three flux monitors used have varying sensitivity to epithermal neutrons Co < Ag < Au. The uncertainties of the listed cross-sections for the [n,n'] and [n,2n] reactions represent the larger values of either systematic error or the deviation from the arithmetic means of the unfiltered and Cd-filtered samples.
Large Scale Production Phase II of the clinical trials of 117mSn as Sn(IV)-DTPA for bone palliation has been completed. A larger phase III trial will commence late in 1996. In collaboration with BNL, large-scale production of ll7mSn via the inelastic scattering reaction is currently under evaluation at the HFIR. Within the past year, 11 targets ranging in mass from 4 to 100 mg and enrichment of 87-92% were each irradiated at the position 5 of H T - H F I R for one reactor cycle (21-27 d) (Table 4). For the 21-day cycle, the average yield was (2.76 + 0.51) × 102 MBq/mg of Sn (7.5 + 1.4 mCi/mg). The average yield at saturation was (5.07 + 0.41) x 102 MBq/mg of "TSn (13.7 + 1.1 mCi/mg). The yield from largescale production is only slightly lower than the 5.2 × 102 MBq/mg value extrapolated from short irradiations at a 10% power level. These results indicate that the burn-up cross-section of Ix7mSnis not significant. The saturation yield of HTmSn for a 27 d cycle was somewhat higher, 6.48 x 102 MBq/mg of JlTSn. This enhancement most likely resulted from favorable rearrangements of other experiments in the core of the reactor resulting in an absolute increase in the epithermal neutron flux. For a one-cycle irradiation and EOB, the radionuclidic impurities in 117mSn samples include 115 d N3Sn (0.10%), 2.7 d 122Sb (0.22%), 2.7 y ~25Sb (0.03%) and 12.4 d ~26Sb (0.03%). These values represent the average of nine runs using 84.23% enriched tl7mSn. In three runs with 92% enriched target, the impurities were "3Sn (0.03%), x22Sb (0.13%), ~25Sb(0.05%) and ~26Sb(0.01%). The more highly enriched target produces a more pure product as expected. Antimony-125 is primarily produced from the 13- -decay of 9.5 m 125mSn. This isotope can also capture a neutron to form J26Sb. The ~22Sb is likely produced from neutron capture of stable 12tSb formed from the 13--decay of ~2~Sn. Two other potential contaminants are 27 h J2tSn (a pure 13emitter and not detectable with "/-ray spectroscopy),
445
Evaluation of neutron inelastic scattering for radioisotope production Table 4. Summary of large scale production of "TSn in the ORNL-HFIR Yield Run No.
1
2 3 4 5 6~ 7 8 9 10 II
Target mass (rag)
Enrichment (%)
4.2 10.1 26.0 67.4 83.0 100.5 100.5 86.3
84.23 84.23 84.23 84.23 84.23 84.23 84.23 84.23
68.64 79.19 83.15
92.2 92.2 92.2
HT Level
T~, (h)
21-d cycle 5 500.0 6 228.0' 5 492.5 5 404.0 5 476.0 5 452.1 5 468.7 5 474.7 Average 27-d cycle 5 611.2 5 604.6 5 550.4 Average
EOB (MBq/mg of Sn)
Saturation (MBq/mg of nTSn)
2.59 x 102 1.67 x 102 3.11 x 102 2.63 x 102 3.03 × 102 3.25 × 102 3.15 x 102 2.66 × 102 (2.76 + 0.51) x 102
4.77 × 102 6.11 x 102b 5.81 x 102 5.55 x 102 5.77 x 10: 6.36 x 102 6.03 × 102 5.07 x 10: (5.07 x 0.41) x l02
4.18 x 102 4.5 × 102 4.11 x 102 (4.26 + 0.22) x l02
6.33 x 10: 6.85 x 10: 6.29 x 10: (6.48 _ 0.30) x 102
"Not included in the average. bCorrected by a factor of 1.16 for flux difference. °Encapsulated in the AI rabbit tube under He.
and 9.64 d ~25Sn(also a parent of t25Sb). The level of 121Sn at 7 days post EOB is estimated at ~ 0.1%. No J25Sn was detected in any run and its level was estimated to be < 0.01%. As described in Experimental, large Sn targets melt during irradiation due to insufficient heat transfer. There is concern about excessive internal pressure in very large targets. The vapor pressure of Sn is given as log (p, atm) = 6.036-15710 (T, K)-~ from room temperature to the melting point (232°C), and as 5.262-15332(T, K) -~, from the melting point to 2129°C (b.p. of Sn is 2603°C) (Lide, 1993). The gamma heating rate in the HT#5, which is located in the midplane of the core, is estimated to be in excess of 32 W/g. The vapor pressure of Sn at 700°C (just above the melting point of A1) is only 4 x 1 0 - " atm. At 2000°C (assuming such a temperature can be reached within the quartz ampule) the vapor pressure of Sn is ~ 4 x 10- 2 atm, resulting in a 4% increase in the internal pressure of the quartz ampule. This small increase in the internal pressure of the target, due to the vapor pressure of Sn, is within the safety margin. In c o n c l u s i o n , it was s h o w n t h a t c r o s s - s e c t i o n s for the p r o d u c t i o n o f ~t7mSn, ngmSn a n d ~9s'pt via n e u t r o n inelastic s c a t t e r i n g r e a c t i o n s were s u b s t a n t i a l l y h i g h e r t h a n the c o r r e s p o n d i n g n e u t r o n c a p t u r e r e a c t i o n s l e a d i n g to the s a m e nuclides. In all t h r e e cases e x a m i n e d , t h e m a g n i t u d e o f gain in c r o s s - s e c t i o n c o m p e n s a t e d for the l o w e r n e u t r o n flux w i t h E, > 100 keV, resulting in overall e n h a n c e m e n t in the specific activity o f these t h r e e r a d i o n u c l i d e s . I n s u p p o r t o f P h a s e III clinical trials, tlTraSn is n o w routinely p r o d u c e d at the h y d r a u l i c t u b e i r r a d i a t i o n facility o f H F I R via the n e u t r o n s c a t t e r i n g reaction. Finally, these studies h a v e s h o w n t h a t the 'lTSn[n,n'] HTmSn a n d tgsPt[n,n'] 19smpt r e a c t i o n s s h o u l d
be considered as neutron flux monitoring reactions for the 0.1-1 M e V region o f t h e r e a c t o r n e u t r o n spectrum.
Acknowledgements--The ORNL Reactor Division, and in particular Mr G. Hirtz for his assistance related to the irradiation scheduling, are acknowledged with appreciation. This research was supported in part by the Office of Health and Environmental Research, U.S. Department of Energy, under contract DE-AC05-960R22464, with Lockheed Martin Energy Research Corporation.
References Atkins H. L., Mausner L. F., Srivastava S. C., Meinken G. E., Strub R. F., Cabahug C. J., Weber D. A., Wong C. T. C., Sacker D. F., Madajewicz S., Park T. I. and Meek A. G. (1993) Biodistribution of Sn-ll7m(DTPA) for palliative therapy of painful osseous metastases. J. Radiol. 186, 279. Atkins H. L., Mausner L. F., Srivastava S. C., Meinken G. E., Cabahug C. J. and D'Alessandro T. D. (1995) Tin-117m( + 4)-DTPA for palliative of pain from osseous metastases. J. Nucl. Med. 38, 725. Cummings, J. B., (1962) National Academy of Sciences-National Research Council, Nuclear Science Series, Report No. NAS-NS-3107. Dickson, D. P. E. and Berry, F. J., Editors (1986) M6ssbauer Spectroscopy. Cambridge University Press, Cambridge, U.K., pp. 21~i9. Hoeschele J. D., Butler T., Roberts J. and Guyer C. (1982) Analysis and refinement of the microscale synthesis of the '95=Pt-labeled antitumor drug, cis-DDP. Radiochimica Acta 31, 27. Lederer, C. M. and Shirley, V. S., Editors (1978), Table of Isotopes, 7th edn. Wiley, New York. Lide, D. R., Editor in Chief (1993) Handbook of Chemistry and Physics, 73th edn. CRC Press, London. Mahmood, S. T., Mirzadeh, S., Farrell, K., Pace, J. V. and Oliver, B. M. (1995) Neutron dosimetry of the HFIR hydraulic facility. ORNL/TM-12831. Mausner, L. F., Mirzadeh, S. and Ward, T. E. (1985) Nuclear data for production of nT=Sn for biomedical application. In Proceedings of the International Conference on Nuclear Data for Basic and Applied Science, Santa Fe, NM, 1985, pp. 733-737. Mirzadeh, S. and Packard, A. B. (1995), Synthesis of AZT-Pt(terpy)--a potential compound for radiotherapy of AIDS. Proceedings of Symposium on Advances in Bioconjugate Chemistry, 210th ACS National Meeting, Chicago, IL, 1995.
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Mirzadeh, S., Schenter, R. E., Callahan, A. P. and Knapp, F. F. (1992) Production capabilities in U.S. nuclear reactors for medical radioisotopes, ORNL/TM-12010. Mughabghab, S. F. (1981) Neutron Cross Sections, Vol. 1, Part B; Z = 61-100. Academic Press, New York. Mughabghab, S. F., Divadeenam, M. and Holden, N. E. (1981) Neutron Cross Sections, Vol. 1, Part A; Z = 1-60. Academic Press, New York. Reus, U. and Westmeier, W. (1983) Catalog of Gamma-rays
from Radioactive Decay, At. Data and Nucl. Data Tables. Parts I and II, p. 29. Stoughton R. W. and Halperin J. (1959) Heavy nuclide cross sections of particular interest to thermal reactor operation: conventions, measurements and preferred values. Nucl. Sci. Engng 2, 100. Willins, J. D. and Sgouros, (1995) Modeling analysis of Pt-195m for targeting individual blood-born cells in adjuvant radioimmunotherapy. J. Nucl. Med. 36, 315.
Pergamon
PII: S0969-8043(96)00284-9
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Evaluation of Neutron Inelastic Scattering for Radioisotope Production S. M I R Z A D E H .1, F. F. K N A P P J R l, C. W. A L E X A N D E R 2 a n d L. F. M A U S N E R 3 ~Nuclear Medicine Group, Health Sciences Research Division, qsotope Technology Section, Chemical Technology Division, Oak Ridge National Laboratory (ORNL), P.O. Box 2008, Oak Ridge, TN 37831-6229, U.S.A. and 3Medical Department, Brookhaven National Laboratory (BNL), Upton, NY 11973, U.S.A.
(Received 10 July 1996; in revisedform 4 September 1996) A systematic study of the production of 117mSn, H~Sn and ~95mptin the hydraulic tube facility (HT) of the ORNL High Flux Isotope Reactor (HFIR) has demonstrated that in all three cases the yieldsfrom neutron inelastic scattering (*Z[n,n'] *mz) were higher than yields obtained from neutron capture reactions (~A OZ[n,y]Amz). The corresponding fission-averaged cross-sections were 222 + 16, 168 _+ 12 and 287 + 20 rob, respectively.The relative gains in the specific activity were 1.4 for 195raPt, 3.3 for mmSnand 4.4 for H9'*Sn.The larger gain for ng'*Sncould be attributed to the relatively lower excitation energy (89.5 keV) of this metastable nucleus. As a part of these studies, the thermal neutron capture cross-sections and resonance integrals for the above reactions and the cross-sections for the [n,2n] reactions leading to the above nuclides were also evaluated. © 1997 Elsevier Science Ltd. All rights reserved
Introduction The production of metastable nuclei such as H7mSn, ~95mpt (for biomedical applications, Atkins et al., 1993, 1995, Willins and Sgouros, 1995, Mirzadeh and Packard, 1995) and ng~Sn (for M6ssbauer spectroscopy, Dickson and Berry, 1986) via neutron radiative capture reactions are characterized by small neutron cross-sections and, hence, low production rates. Metastable nuclei typically have excitation energies on the order of 100 keV and large differences in angular momentum from ground states (most metastable nuclei have high angular momentum). An alternative route for producing these types of metastable nuclei is through neutron inelastic scattering where the cross-section of the AZ[n,n'] ~ Z reaction is, in some cases, substantially higher than the cross-section for the CA-,Z[n,y]~Z route. As has been shown for the case of "7mSn (Mausner et al., 1985), the magnitude of gain in the cross-section may compensate for the relatively lower fast neutron flux from a well-moderated fission spectrum, En >__Qtn~l. Note that the excitation energy of metastable nuclei will represent the threshold for inelastic scattering. Large research reactors, such as H F I R t and HFBR:~, *To whom all correspondence should be addressed. tHFIR: High Flux Isotope Reactor, ORNL. :~HFBR: High Flux Beam Reactor, Brookhaven National Laboratory (BNL).
with significant epithermal and fast neutron fluxes are well suited for these types of reactions. In this paper, we report the results of a systematic study of the production of nTmSn, "9~Sn and t95"Pt in the hydraulic tube irradiation facility (HT) of the H F I R via both radiative neutron capture reactions and inelastic scattering. These studies also include an assessment of the contribution of the epithermal neutrons to overall reaction rates. Cross-sections of the [n,2n] reactions leading to the above radionuclides were also evaluated.
Experimental Irradiation facility The ORNL HFIR is a compact, flux trap type reactor with facilities for performing a wide variety of irradiations. The trap at the center of the core is filled with a bundle of AI targets holding primarily transuranium targets which are accessible only during refueling. The hydraulic tube facility (HT) is a single tube which passes through the bundle and exits the reactor. A train of nine vertically stacked capsules, or rabbits, can be sent in and out of the reactor on demand while the reactor is operating. Position 5 of the hydraulic tube (HT#5) is centered at the core midplane and has the highest neutron flux. The train of capsules is driven by hydraulic pressure and the capsules are cooled by water at a velocity of
441
442
S. Mirzadeh et al.
~ 3 m s - ~ (flow of ~ 24 L m i n - 1). The HT is principally used for the production of medical radioisotopes. Additional information with regard to the HT and other H F I R irradiation facilities can be found elsewhere (Mahmood et al., 1995; Mirzadeh et al., 1992).
Materials and Methods For cross-section measurements, samples ( ~ 10 mg) of highly enriched isotopes of Sn (as SnO2) and metallic Pt together with flux monitoring wires were irradiated for 1 h in HT-HFIR. The isotopic compositions of enriched Sn and Pt samples (from ORNL) are given in Table 1. Targets were sealed in 3 x 25 mm thin wall ( ~ 0.3 ram) Suprasil quartz tubes (Heraeus, Duluth, GA). To assess the contribution from epithermal neutrons, samples wrapped in Cd filters (0.26 mm) were also simultaneously irradiated. To minimize the Cd depletion, the reactor power level was maintained at 9 M W ( ~ 10%) during irradiation. The flux monitors for thermal and epithermal neutrons were alloys of Au/A1, Ag/AI and Co/AI (Reactor Experiments, Inc., San Carlos, CA). The monitors for fast neutrons (E, > 1 MeV) were high purity metal wires including A1, Cu, Fe, Ni and Ti (ASAR, Johnson Mattey, Seabrook, NH). After irradiation, the quartz sample tubes were soaked in conc. HNO3 for a few minutes, then thoroughly rinsed with water in order to remove any surface contamination on the quartz tubes. After an appropriate cooling time, the activities of these sealed sources were measured at a 30 cm distance from a well-calibrated solid-state HPGe-7-ray detector (50 cm 3, E G & G Ortec, Oak Ridge, TN). The detector was coupled to a PC-based MCA (Canberra Industries, Inc., Meriden, CT). The 23.8 keV T-ray of HgmSn was measured in a l-cm-thick Ge detector (EG&G Ortec, Oak Ridge, TN). All activities were followed for several half-lives and the activities at the end of bombardment (EOB) were extrapolated by the CLSQ code (Cummings, 1962). When multiple v-rays were used for assay, their intensifies were used as weighing factors in calculation of the average activity
values. When not explicitly stated, the relevant nuclear data were taken from Lederer et al. (1978) and Reus and Westmeier (1983). The thermal and epithermal cross-sections were from Mughabghab e t al. (1981) and Mughabghab (1981). For large-scale production of"TraSh, targets were in the form of metallic "TSn powder or small pieces (the 84.23% enriched material was from ORNL, and 92.2% was Russian made and obtained from Chemotrade, Diiesseldrof, Germany). The targets were sealed under a slight negative pressure of He in the quartz ampules (Suprasil, 6.0 x 45 mm). The sealed quartz ampules were then placed in the welded AI rabbits (6.7 x 51 mm) for irradiation. As mentioned earlier, the AI rabbits were cooled to ~ 60°C by water at a flow of ~ 24 L m - t during irradiation. In large scale production, Sn targets were found molten at the time of discharge from reactor due to the rather low melting point of Sn metal (232°C) and insufficient heat conduction between the quartz ampoule and the A1 rabbit. The quartz ampoule, however, remained intact in all irradiations. Backfilling the AI rabbit with ~ 1 atmosphere of He prior to welding the rabbits did not prevent melting of the Sn metal. The quartz tubes containing the irradiated targets were sent to BNL for processing and radiopharmaceutical formulation.
Results and Discussion The neutron spectrum at the HT#5 of H F I R was measured using refined activation techniques, crosssection data bases, and calculation methods. The measurements included a number of Cd-flltered and unfiltered flux monitors exhibiting different responses at different neutron energy intervals. The reactor power level was maintained at ~ 10% during these studies. A summary of the spectrum-averaged fluxes over selected energy ranges, relevant to the nuclear reactions discussed here, is given in Table 2. Detailed information with regard to neutron flux measurements and neutron spectra unfolding can be found elsewhere (Mahmood e t al., 1995). The results from a systematic study of the p r o d u c t i o n o f llTmSn, "gmSn and 19Smptin the HT of the
Table 1. Isotopic compositions of enriched Sn and Pt targets Sample
Composition (at. %) Mass No.
"~Sn "~Sn "SSn 119Srl mSn
116
117
118
95.60 ± 0.05 2.54 ± 0.04 0.30 + 0.02 0.65 ± 0.05 0.20 ± 0.01
1.63 + 0.02 84.23 + 0.15 0.49 ± 0.02 0.63 + 0.05 0.12 + 0.01
1.48 ± 0.02 9.42 ± 0.10 97.07 ± 0.20 3.01 ± 0.05 0.49 ± 0.01
194
195
196
Others
97.41 + 0.03 1.16 + 0.01 0.63 + 0.01
1.99 + 0.02 97.28 ± 0.03 1.57 ± 0.02
0.52 + 0.01 1.47 + 0.02 97.51 ± 0.02
0.08 0.09 0.30
0.27 2.03 0.92 89.8 0.36
119
120
Others
+ + ± ± +
0.63 ± 0.02 0.26 ± 0.02 1.11 + 0.02 5.34 ± 0.05 98.57 ± 0.02
0.39 0.32 0.13 0.55 0.22
0.02 0.04 0.03 0.10 0.01
Mass No. ~4Pt JgSPt ~Pt
443
Evaluation of neutron inelastic scattering for radioisotope production Table 2. Averaged neutron fluxes at the HT#5 of the HFIR at operating power level
of 9 MW Neutron energy < 0.5 eV (thermal) 0.5 eV < E, ~ 100 keV (epithermal) > 100 keV > 300 keV > 1 MeV
Neutron flux (n s-tcm 2)
0,h/O'
(2.65 -+ 0.15) × l0 i4
1.0
(1.06 -+ 0.21) x 10~J " (9.42 5: 1.41) x 10~3 (8.29 -+ 1.24) x 10~3 (4.87 5- 0.78) × 10~3
25 2.81 3.20 5.43
"Per unit In E (lethargy), E is the neutron energy in MeV (Stoughton and Halperin, 1959).
H F I R a r e p r e s e n t e d in T a b l e 3. I n all t h r e e cases, t h e yields f r o m t h e [n,n'] r e a c t i o n s were h i g h e r t h a n t h o s e o b t a i n e d f r o m t h e [n,v] r e a c t i o n s . T h e relative g a i n s in t h e specific a c t i v i t y o f t h e u n a l t e r e d t a r g e t s w e r e 1.4, f o r 195rapt, 3.3 f o r 117mSn a n d 4.4 f o r Hg~sn. T h e l a r g e r g a i n for "9~Sn c o u l d be a t t r i b u t e d to t h e relatively l o w e r e x c i t a t i o n e n e r g y (89.5 k e V ) o f this m e t a s t a b l e n u c l e u s . T h e e x c i t a t i o n e n e r g i e s for H7mSn a n d t95mpt a r e 314.6 a n d 259.2 keV, respectively. A s s h o w n , c o n t r i b u t i o n s f r o m t h e [n,2n] r e a c t i o n s to t h e o v e r a l l yields w e r e i n s i g n i f i c a n t . Since t h e t h r e s h o l d s f o r t h e s e inelastic s c a t t e r i n g r e a c t i o n s a r e well a b o v e t h e c a d m i u m c u t o f f , C d filters w o u l d n o t be e x p e c t e d to h a v e a n y effect o n t h e yield o f t h e s e r e a c t i o n s . T h e slightly h i g h e r yield o f J95mpt f r o m t h e C d - f i l t e r e d 19spt s a m p l e m a y be a t t r i b u t e d to d i m i n i s h e d b u r n - u p o f this n u c l i d e d u e to c a p t u r e o f t h e r m a l n e u t r o n s b y t h e C d filters ( T a b l e 3). W h e n e x t r a p o l a t i n g f r o m a 1 h i r r a d i a t i o n at a 1 0 % p o w e r level to a o n e - c y c l e i r r a d i a t i o n (21 d) at
a 85 M W p o w e r level, t h e e x p e c t e d specific activities o f HgmSn a n d ~9~Sn a r e 5.2 x 102 a n d 3.1 x 102 M B q / m g (14 a n d 8.5 m C i / m g ) , respectively. A s i m i l a r e x t r a p o l a t i o n r e s u l t s in a n a p p a r e n t specific activity o f 6.3 × 102 M B q / m g (17.1 m C i / m g ) at s a t u r a t i o n f o r 195raPt. I n this case, t h e v e r y l a r g e b u r n - u p c r o s s - s e c t i o n o f ~gSmpt (1.3 X 104 b, H o e s c h e l e e t al., 1982) yields a specific activity w h i c h is s i g n i f i c a n t l y lower. N e v e r t h e l e s s , it is e x p e c t e d t h e g a i n o f 1.4 o b t a i n e d b y t h e u s e o f inelastic r e a c t i o n will be m a i n t a i n e d f o r large-scale p r o d u c t i o n . T h e f i s s i o n - a v e r a g e d c r o s s - s e c t i o n s for t h e individu a l r e a c t i o n s , try, l e a d i n g to 117mSn, "gmsn a n d 195raPt were c a l c u l a t e d by s o l v i n g sets o f t h r e e s i m u l t a n e o u s e q u a t i o n s for e a c h r a d i o i s o t o p e . These three e q u a t i o n s w h i c h a r e d e f n e d b y s u b s c r i p t j refer to t h e t h r e e t y p e s o f t a r g e t s u s e d in e a c h case. F o r e x a m p l e , in t h e c a s e o f "TSn, j = 1 refers to t a r g e t e n r i c h e d in l~rSn, j = 2 refers to t a r g e t e n r i c h e d in llTSn, a n d j = 3 refers to t a r g e t e n r i c h e d in "SSn. L e t s u b s c r i p t i = 1,
Table 3. Production of tl7mSn, ~t9~Snand 195mptvia [n,~], [n,n'] and [n,2n] reactions Nuclear reaction
Saturation yield at EOB (Bq/mg)
Cross-sections (mb) This work
"+Snln,y]
U F
"TSn[n,n']
U F
"SSn[n,2n]
U F
"sSn[n,y]
U F
~gSn[n,n']
U F
12°Sn[n,2nl
U F
~uPt[n,y]
U F
mPt[n,n']
U F
~9~Pt[n,2n]
U F
llTmSn(t,;2 = 14.0 d, I = 11/2 + , Q = 314.6 keV) 2.60 x 107 a,~ (1.95 -+ 0.20) x l& 1.97 x 107 10 (3.50 5: 0.53) x 102 a~h 5.8 -+ 1.2 8.59 × 107 (2.25 + 0.23) × 103 8.39 x 103 (2.19 __.0.22) × 102 Av. (2.22 -+ 1.16) × 103 4.46 x 105 (1.5 + 0.2) x 10 -t 5.43 × 105 (3.9 __.0.6) x 10 t Av. (2.7 -+ t.2) × 10 tiTmSn(/i/2 = 293 d, I = 11/2 + , Q = 89.5 keV) 1.78 x 107 a,fr (1.32 5: 0.20) x 10L 7.86 x 106 I0 (1.44 5: 0.22) x 102 a~h 7.4 -+ 2.0 7.86 × 107 (1.63 + 0.16) x 102 7.41 x 103 (1.73 ± 0.17) x 102 Av. (1.68 + 0.12) x 103 (4 -+ 1) x 103 --(2 -+ 1) × 103 --195Pt (tx/2 = 4.02 d, I = 13/2 + , Q = 259.2 keV) 5.11 x 107 a~r (6.27 + 0.63) x 10~ 1.89 x 107 I0 (5.26 -+ 0.79) x 103 a,h (4.2 -+ 0.8) × l0 t 7.21 × 107 (2.74 + 0.27) x 103 7.70 × 107 (3.00 -+ 0.30) × 103 Av. (2.87 5: 0.20) x 103 1.93 x 106 5.4 -4- 1.1 1.33 x 103 2.0 5:0.4 Av. 3.7 ± 0.7
Ref.
Reported (1.98 + 0.12) x l0 t (4.90 _+ 1.60) × 102 6--+2 (r) (1.76 _ 0.14) x 102
1 1 2 1
1.0 × 10 ~
I
(1.0 -+ 0.6 x 10)~
2
(3.1 -i- 0.1) x 103 (9.8 + 1.1) x 10~
2 2
T~.= 1 h at 9 MW power level, U: unfil~red, F: Cd filtered. With the exception to the mSn[n,2n] reaction, the uncertainties in the saturation yields are less then + 10%. Ref. 1: Mausner et al. (1985) and Ref. 2: Mughabghab et aL 0981),
444
S. Mirzadeh et al.
2 and 3 refer to the In,),], [n,n'] and [n,2n] reactions, respectively. Then ~a~:x, = k.b,
(i = 1,3 and j = 1,3)
where a 0 denotes the mass fraction of each target isotope divided by the average atomic mass of enriched target isotope. The variable xi is the product of the neutron flux, ~b, (at an appropriate neutron energy range), and the cross-section for appropriate reaction, a,. The parameter b, is the saturation activity of a given radioisotope (column 3 of Table 3, dps/mg of target), and k is a proportionality constant. In the case of neutron capture reaction, a, represents the effective cross-section, O'en= Crth+ (1/r)'lo where a,h is the thermal cross-section, I0 is the resonance integral, and r = q~th/~b~(q~this the thermal neutron flux and ~b,p~is the epithermal neutron flux per unit In E, lethargy, see Stoughton and Halperin, 1959). For a filtered sample, ai is equal to I0. The corresponding cross-sections for the inelastic neutron scattering reactions for the production of H7mSn, HgmSn and 19smpt are 222 + 16, 168 + 12 and 287 + 20 mb, respectively (Table 3). A value of 176 + 14 mb for the cross-section of "TSn[n,n'] HTmSn reaction obtained at H F B R was reported earlier (Mausner et al., 1985). Our measured cross-sections for neutron capture reactions leading to ll7raSn, ~'~mSnand 195raPt, together with reported values are also given in Table 3. In the case of H6Sn[n,7]"TmSn reaction, the measured thermal neutron capture cross-section (O'th) and resonance integral (I0) were 5.8_+ 1.2 and 350_+53mb, respectively, in contrast to reported values of 6 _+ 2 and 490 _+ 160 mb. For "SSn[n,~'] "gmSn reaction, the aegis 13.2 _+ 2.0 mb which is within the upper limit of the reported value of 10 _+ 6 mb. The measured thermal and resonance integral for this reaction are 7.4 _+ 2.0 and 144 _+ 22 mb, respectively. The measured a,h for J95pt[n,~,] 195raPtis 42 + 1 rob, lower than the reported value by about a factor of 2. Our measured I0 for this reaction at (5.3 -+ 0.8) x 102 mb is, however, ~ 6 fold lower than reported value. As demonstrated for these three cases, in the determination of the neutron capture cross-sections contribution from inelastic neutron scattering to overall reactions are significant and appropriate corrections are required even when highly enriched target isotopes are used. Consequently, our measured cross-sections for capture reactions are lower than reported values. The cross-sections for [n,2n] reactions leading to HTmSn, "~mSn and 19Smpt nuclides are also given in Table 3. These cross-sections are in the range of 0.3-4 mb and, therefore, contributions from these routes to the overall reaction rates are rather small (in order of a few percent).
With the exception of the ~2°Sn[n,2n] reaction, the uncertainties in the saturation yields given in Table 3 are less then + 10%. This includes uncertainty due to counting statistics and detector efficiency. The large errors ( ~ 25% for unfiltered and ~ 50% for the filtered samples) in the saturation yields of Hgmsn, produced from the ~2°Sn[n,2n] reaction, were primarily due to counting statistics. The uncertainties of the listed thermal cross-sections and resonance integrals include a 5% error in the thermal-to-epithermal ratio, as the three flux monitors used have varying sensitivity to epithermal neutrons Co < Ag < Au. The uncertainties of the listed cross-sections for the [n,n'] and [n,2n] reactions represent the larger values of either systematic error or the deviation from the arithmetic means of the unfiltered and Cd-filtered samples.
Large Scale Production Phase II of the clinical trials of 117mSn as Sn(IV)-DTPA for bone palliation has been completed. A larger phase III trial will commence late in 1996. In collaboration with BNL, large-scale production of ll7mSn via the inelastic scattering reaction is currently under evaluation at the HFIR. Within the past year, 11 targets ranging in mass from 4 to 100 mg and enrichment of 87-92% were each irradiated at the position 5 of H T - H F I R for one reactor cycle (21-27 d) (Table 4). For the 21-day cycle, the average yield was (2.76 + 0.51) × 102 MBq/mg of Sn (7.5 + 1.4 mCi/mg). The average yield at saturation was (5.07 + 0.41) x 102 MBq/mg of "TSn (13.7 + 1.1 mCi/mg). The yield from largescale production is only slightly lower than the 5.2 × 102 MBq/mg value extrapolated from short irradiations at a 10% power level. These results indicate that the burn-up cross-section of Ix7mSnis not significant. The saturation yield of HTmSn for a 27 d cycle was somewhat higher, 6.48 x 102 MBq/mg of JlTSn. This enhancement most likely resulted from favorable rearrangements of other experiments in the core of the reactor resulting in an absolute increase in the epithermal neutron flux. For a one-cycle irradiation and EOB, the radionuclidic impurities in 117mSn samples include 115 d N3Sn (0.10%), 2.7 d 122Sb (0.22%), 2.7 y ~25Sb (0.03%) and 12.4 d ~26Sb (0.03%). These values represent the average of nine runs using 84.23% enriched tl7mSn. In three runs with 92% enriched target, the impurities were "3Sn (0.03%), x22Sb (0.13%), ~25Sb(0.05%) and ~26Sb(0.01%). The more highly enriched target produces a more pure product as expected. Antimony-125 is primarily produced from the 13- -decay of 9.5 m 125mSn. This isotope can also capture a neutron to form J26Sb. The ~22Sb is likely produced from neutron capture of stable 12tSb formed from the 13--decay of ~2~Sn. Two other potential contaminants are 27 h J2tSn (a pure 13emitter and not detectable with "/-ray spectroscopy),
445
Evaluation of neutron inelastic scattering for radioisotope production Table 4. Summary of large scale production of "TSn in the ORNL-HFIR Yield Run No.
1
2 3 4 5 6~ 7 8 9 10 II
Target mass (rag)
Enrichment (%)
4.2 10.1 26.0 67.4 83.0 100.5 100.5 86.3
84.23 84.23 84.23 84.23 84.23 84.23 84.23 84.23
68.64 79.19 83.15
92.2 92.2 92.2
HT Level
T~, (h)
21-d cycle 5 500.0 6 228.0' 5 492.5 5 404.0 5 476.0 5 452.1 5 468.7 5 474.7 Average 27-d cycle 5 611.2 5 604.6 5 550.4 Average
EOB (MBq/mg of Sn)
Saturation (MBq/mg of nTSn)
2.59 x 102 1.67 x 102 3.11 x 102 2.63 x 102 3.03 × 102 3.25 × 102 3.15 x 102 2.66 × 102 (2.76 + 0.51) x 102
4.77 × 102 6.11 x 102b 5.81 x 102 5.55 x 102 5.77 x 10: 6.36 x 102 6.03 × 102 5.07 x 10: (5.07 x 0.41) x l02
4.18 x 102 4.5 × 102 4.11 x 102 (4.26 + 0.22) x l02
6.33 x 10: 6.85 x 10: 6.29 x 10: (6.48 _ 0.30) x 102
"Not included in the average. bCorrected by a factor of 1.16 for flux difference. °Encapsulated in the AI rabbit tube under He.
and 9.64 d ~25Sn(also a parent of t25Sb). The level of 121Sn at 7 days post EOB is estimated at ~ 0.1%. No J25Sn was detected in any run and its level was estimated to be < 0.01%. As described in Experimental, large Sn targets melt during irradiation due to insufficient heat transfer. There is concern about excessive internal pressure in very large targets. The vapor pressure of Sn is given as log (p, atm) = 6.036-15710 (T, K)-~ from room temperature to the melting point (232°C), and as 5.262-15332(T, K) -~, from the melting point to 2129°C (b.p. of Sn is 2603°C) (Lide, 1993). The gamma heating rate in the HT#5, which is located in the midplane of the core, is estimated to be in excess of 32 W/g. The vapor pressure of Sn at 700°C (just above the melting point of A1) is only 4 x 1 0 - " atm. At 2000°C (assuming such a temperature can be reached within the quartz ampule) the vapor pressure of Sn is ~ 4 x 10- 2 atm, resulting in a 4% increase in the internal pressure of the quartz ampule. This small increase in the internal pressure of the target, due to the vapor pressure of Sn, is within the safety margin. In c o n c l u s i o n , it was s h o w n t h a t c r o s s - s e c t i o n s for the p r o d u c t i o n o f ~t7mSn, ngmSn a n d ~9s'pt via n e u t r o n inelastic s c a t t e r i n g r e a c t i o n s were s u b s t a n t i a l l y h i g h e r t h a n the c o r r e s p o n d i n g n e u t r o n c a p t u r e r e a c t i o n s l e a d i n g to the s a m e nuclides. In all t h r e e cases e x a m i n e d , t h e m a g n i t u d e o f gain in c r o s s - s e c t i o n c o m p e n s a t e d for the l o w e r n e u t r o n flux w i t h E, > 100 keV, resulting in overall e n h a n c e m e n t in the specific activity o f these t h r e e r a d i o n u c l i d e s . I n s u p p o r t o f P h a s e III clinical trials, tlTraSn is n o w routinely p r o d u c e d at the h y d r a u l i c t u b e i r r a d i a t i o n facility o f H F I R via the n e u t r o n s c a t t e r i n g reaction. Finally, these studies h a v e s h o w n t h a t the 'lTSn[n,n'] HTmSn a n d tgsPt[n,n'] 19smpt r e a c t i o n s s h o u l d
be considered as neutron flux monitoring reactions for the 0.1-1 M e V region o f t h e r e a c t o r n e u t r o n spectrum.
Acknowledgements--The ORNL Reactor Division, and in particular Mr G. Hirtz for his assistance related to the irradiation scheduling, are acknowledged with appreciation. This research was supported in part by the Office of Health and Environmental Research, U.S. Department of Energy, under contract DE-AC05-960R22464, with Lockheed Martin Energy Research Corporation.
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