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Preparation of CF380mBr of high specific activity
I,,rt'r,,at*o,tat Journal of 4t~phed Rad,at~on and I,otope~ VoL 32. ~p {,)l to t0.-t
0020-708X g[ 02010I-0.~$02.00/0
Pergamon Press Ltd 1981. Pr,ntetI m Great Britain
Preparation of CF3S°mBrof High Specific Activity D. D E J O N G ,
B. W. V A N
HALTEREN
a n d J. T H . V E E N B O E R
Institute for Nuclear Physics Research (IKO), P.O. Box 4395, 1009 AJ Amsterdam, The Netherlands (Receiced 8 June 1980)
Three ways for the preparation of carrier-free CF]°~Br have been tested: the displacement of X in CF3X (X = F, CI, I or H) by S°'Br formed by thermal neutron irradiation of CF3X/CH3Br mixtures, by proton irradiation of CF3X/HzSe mixtures, and by the exchange of Br- coated on inert support material with CF~X'. Neither of these methods fulfils the requirement of a high yield of carrier-free CF]°=Br; however, the irradiation of CFfl/CH3Br mixtures with thermal neutrons gives CF]°'Br with a yield of (11.5 +_ 0.6?/o) and a specific activity of about 32 mCi rag- 1.
Introduction ACHE et al. ~''~ suggested C F ] ° " B r as a source compound for the study of gas-phase recoil chemistry of 8°Br, generated by the isomeric transition. Whereas the ionization potential of CF3Br (12.3 eV) is above that of atomic bromine (ll.84eV), no decharging of ~°Br" ).* formed after isomeric transition can occur when CF3Br is used as s°"Br source compound in the gas mixture. This property of CF3Br is an advantage over the other s°"Br-source gases, such as Br2 (i.p. = 10.55 eV) and CH3Br (i.p. = 10.54eV), where charge-exchange reactions cannot be excluded. The production of C F ] ° ' B r normally involves neutron irradiation of pure gaseous CF3Br, ~'2~ yielding a non-carrier-free product, followed by gas-chromatographic separation. To prevent reactions of recoil 8°Br being influenced by the large amount of inactive CF3Br, we attempted to produce carrier-free CFS°"Br by three different methods: (A) Displacement of X in CF3X (X = F, CI, I and H) by 8°"Br produced by neutron irradiation of gaseous CF3X/CH~Br mixtures in a nuclear reaction. (B) Displacement of X in CF3X (X = CI, I) by B°"Br produced by proton irradiation of Se in gaseous CF3X/H,,Se mixtures. (C) Exchange of s ° " B r - with CF3X (X = CI, I). These three methods are discussed below in the same order.
Materials CH3Br, CF.,, CF3CI and CF3H were obtained from J. T. Baker with stated purities of 99.7, 99.7, 99.0 and 98.09/0 respectively, while CF3Br and CF3I were purchased from P.C.R. lnc. (purity >99.5°~). The gases * Unless Br + is electronically excited. 101
were used without further purification. H.,Se was obtained by the reaction of water with AIzSe 3 (Alfa Products). The gas was dried over P,.O5 before use.
(A) Reaction of s°'
\
Mixtures of CF3X (X = F, CI, I and H) and 20-25,% CH3Br sealed in quartz ampoules (pressure 90 kPa, volume 1 ml) were irradiated for 20 rain at a thermal neutron flux of 1 0 ~ 3 n s - t c m -2 in the IRI nuclear reactor at Delft. The accompanying dose rate was determined, by means of optical absorption measurements with 0.1 mm Ultraphan G (Lonza) foils,(3~ to be about l0 s rad h - '. After irradiation, the contents of a typical ampoule were injected directly onto a chromatographic column by means of an ampoule-cracking device mounted on the injection port of the gas chromatograph. After injection, the splinters of the ampoule were washed with aqueous NazSO3 and chloroform. The two layers were separated and radioassayed with a Ge(Li) detector to determine the s°"Br and S2Br activities ('/-ray energies 617 and 554keV, respectively: 7-ray intensities are taken from Ref. (13). Separation of the different gaseous compounds (CF3Br, CF3X and CH3Br) was performed with a 0.75m Porapak N column, operated temperature programmed (5rain 303K, 1 0 K m i n - t to 393K, flow 60 ml He rain-t). The eluting peaks were trapped in heptane, cooled with dry ice and measured on the Ge(Li) detector. All samples were counted 1 h or more after collection in order to ensure radioactive equilibrium between 8°"Br and 8°Br. Correction was made for the contribution of the 619 keV 7-ray of S-'Br in the determination of 8°"Br activity,
D. de Jong et al.
!q2
TASLE 1, .Absolute >ields of CF~ °~' s'-'Br al~er ~hermal neutron irra&ation" of gaseous CF>k L'( = F. el. I. HI CH~Br Percentage of '~')="~:'Br in CF3Br+ ~')~Br S:Br
Composition CF, 20% CH3Br CF3C1 + 20°, CHaBr CF~I + 20", CH3Br CF3H + 20". CH3Br
6.0 5.9 I 1.5 6.4
-
± 0. I _ 0.1 _- 0.6 ~- 0.2
4.4 4.0 10.8 4.0
: 0.4 _+ 0.2 ± 0,4 +_ 0. I
irradiation
"Neutron flux: 10~3ns-~cm--': 20 min. + Mean value of 2 experiments.
time:
To evaluate the influence of the dose rate and, more particularly', the possible formation of mass a m o u n t s of CF3Br by radio{ysis, all mixtures were irradiated for ~arying times in a e'°Co ;'-source of about 1300kCi with a ;'-dose rate of 4.1 x 1 0 S r a d h -~ These ;'-irradiated samples were analysed in the same way as the neutron-activated mixtures and the level of CF3Br was determined. Restdrs and discu,ssion
The absolute CF~°~(s-')Br yields obtained by this method are summarized in Table 1. Figure 1 illustrates the production of mass a m o u n t s of CF3Br during 7-radiolysis. Some other c o m p o u n d s are also formed in these radiolysis experiments, but they were not investigated further, because only the a m o u n t of CF3Br is important in the determination of the specific activity of the product. Based on these results, estimates of the specific activities of CF3S°~(S2)Br produced in the IRI reactor are given in Table 2. For comparison, the calculated
specific activity of CF~°"s-"Br produced by neutron irradiation of pure CF3Br is also given. Here an absolute CF~°mBr yield of 30",, is used, v, hich ~alue v,e determined for a 20-rain irradiation of pure CF3Br. No literature value is known. The total activity in the final product ,,aries between 10 and 25,uCi of S°mBr and I and 3 #Ci of ~:Br. No real mechanism for the formation of CF~Br by radiolysis can be proposed at this time. since this requires quantitative determination or the other radiolysis products. However. the formation of CF~ radicals in CF_~X. CH3Br mixtures is likel,,. 's~') whereafter abstraction of bromine from CH3Br or from radiolytically formed Br: might occur lthe latter reaction has a relatively high G-value of 3-5 in CF.~(-)). In mixtures with CF3I, iodine is more easily abstracted by C F > whereas the formation of CF3 radicals in CF3H seems not very likely and this results in both cases in a lower CF3Br yield. On the basis of these data it is impossible to make a distinction between CF3~°"Br formed by a direct replacement reaction or by radiolytical processes. The difference between the s°'~Br and S:Br yields is in all likelihood caused because S-'Br is partly formed ,,ia direct In, 7) activation and partly via the isomeric transition of s-''Br. The activity of s>"Br after 20 min irradiation is a factor of 103 larger than the directly produced S-'Br activity, as a consequence of the higher thermal-neutron cross section and the shorter half-life (T t _, s : ' B r = 6.1 min: TI _, S-'Br = 35.4h). After 2 h decay {starting time of the G L C separation} the S-'Br activity formed by the decay of Se~Br is about three times the aZBr activity formed by direct acti,,ation. This decay of 8:~Br proceeds via a highly converted ;'-transition, which gives rise to positively charged bromine ions. The reactions of S-'Br formed by isomeric transition possibly result in a slightly lower
), /
t~ 5x
/
~,o-3 L)
g
2,
CF~ +
o/
20 °/oC H } Br
/ 5x
iO-3
/
/
/
/
% cl + 20%
CH3Br
Z
i
50
~
~oo
tso
5x
b_ t_)
g
5to
Mrad
CFl+
r~
20°/°
u
CH3B r
~oo
5x l0 -3
15o
Mrad
C~H+ 20% CH3Br
/ J c~S L
/ I ~
Z 150
~ Mrad
L~ 50
i~
150
Mrad
FIG. 1. Formation of mass quantities of CF3Br by ;'-radiolysis (dose rate: 4.1 x 105radh-~).
Preparation of CF~°~Br TABLE 2. Specific activity of CF~°"Se'Br produced b v thermal neutron irradiation" Specific acti,,it? SO~Br S:Br {#Ci rag) {uCi mg~
Composition CFa - CH3Br CF3CI -"- CH3Br CF3I - CH3Br CF3H ..a CH~Br CF3Br Ipure)+
5.9 3.8 3.2 >5.0 5.3
x × x × ×
103 I03 10" [0 a 101
5.6 3.9 4.0 5.0 I.I
x x x x x
10: I02 103 103 I0 °
* Irradiation time: 20 min: :'-dose: l0 s rad h "~Calculated specific activity lretention - 30"ol.
CF~-'Br ,yield than the reactions of 8ZBr produced d i r e c t b by (n, 7) activation, as was shown b,, BERG et al. ~ and DaYrEL and ACHff-" for the yields of s2'S°Br-CH3Br produced by the In, ;') reaction or the isomeric transition process in gaseous C H 3 X (X = F, CI, Br). This explains the lower CF]2Br yield, with respect to the CF3SO"Br yield.
(B) Reaction of 76'77~°"~Br in Gas Mixtures of C F 3 X (X = e l , 1) a n d H 2 S e
Experimentul Glass ampoules (pressure 65-90 kPa, volume 10 ml, length l0 cm) containing mixtures of CF3X (X = CI, I) and 20--50°0 H2Se were irradiated for 20 rain with 50-200 nA 20 MeV protons in the external beam of the KVI cyclotron (Groningen). A 0.1 mm nickel foil was used as a current monitor. The gaseous contents of the ampou/es were injected via a loop system into a gas c h r o m a t o g r a p h on a 0.75 m Porapak N column. Collection and radioassay of the products was carried out as described in (A) above. The ;.'-rays of VTBr (239keVI and :~'Br {657keV) (;,'-ray intensities are taken from Re[. 13) were measured. Since the transportation time from the cyclotron to Amsterdam is rather long. no s°"Br (T 1 2 = 4.4h) could be measured in these experiments.
Results and discussion The absolute yields of CF36'7"Br are given in Table 3. It can be seen that these yields are very small. The total activity found in the CF3Br fraction is also not very high {max. 0.1-O.5b~Ci), although the cross sections for the (p, n) reactions on selenium are fairly TABLE 3. Absolute ?ields of CF~6"":Br after 20 MeV proton irradiation* of gaseous CF3X (X = CI. l) HzSe mixtures Composition
Percentage of r~"V"Br Pressure in CF3Br (kPa)
CF3CI + 28°~; H:Se CF3CI + 50°° H2Se CF3I + 2S°~ H_,Se * Irradiation time: 20 min.
90 65 90
103
high. ['e'Se(p. nl-"Br: a = - 6 0 0 r o b at [5MEW s', r'Setp, n ) " B r : ~- = ~ 8 0 0 m b at 12 MeV't'~(] After irradiation, the ampoules were coated on the inside with black and red layers of selenium, presumably due to radiolysis of H.,Se. This radiolysis certainly reduces the total yield of bromine isotopes produced by proton irradiation of H,Se. The cross section for the Ip, n) reaction on S°Se is expected to be lower than that for the reactions on ~'Se and "'Se (a = 80 mb at 6 MeV ~9~, no a-values known at higher energies). No serious differences in absolute CF3Br yields between "e'Br, ' " B r and s°"Br should exist, because below 15 MeV the most important nuclear reaction for the production of these three isotopes is the/P, n) reaction,
(C) Exchange
Reaction
of s°'182)Br-
with
C F 3 X (X = CI, I)
Experimental Bromido-S'Br was prepared with high specific activity as described by ALFASSI et qlJ 1°~ The aqueous S2Br- solution was used to coat kieselguhr 0 8 - 5 2 mesh). Following the exchange procedure of KRUTZIK and ELIAS,I1 t~ 500 mg of 8-'Br coated kieselguhr was brought into contact with gaseous CF3CI, CF3I and CH3Br in a glass ampoule at a pressure of 98 kPa at 353 K. After a reaction time of 2-4 h. the gaseous contents of the ampoule were analysed on a Porapak N column to determine the levels of CFS2Br and CH]2Br. CH3Br was measured as a standard to check the procedure. Results and discussion Table 4 summarizes the results obtained by this method. The CF3S-'Br yields are negligible compared to the CH3Br yield. This p h e n o m e n o n is presumably due to the different electron-density distributions in both molecules: ~j2, the three fluorine atoms in CF3X strongly withdraw electrons from the rest of the molecule, resulting in a CF 3 group with a high electron density and a Br substituent with a low electron density. ~5~ This fact makes the CF3X molecule rather unattractive to nucleophilic attack by 82Br-.
General Conclusions The aim of the study was to develop a method for the preparation of carrier-free C F ] ° " B r . The yields TM}LE 4. S.~nthesis of CFI2Br by exchange of 8:Br- whh CF~.\" iX = CI. I}* Compound
Percentage of S2Br in CF3Br {H3Br)
CF~CI CFfl CH3Br
<0.1 <0. I % 17
*Pressure: 98kPa: reaction time: 2-4h: temperature: 353 K.
I04
D. tie Jon# et al.
resulting from m e t h o d s {B) a n d (CI are m u c h too Io'* to be attractive for a n y labeling p u r p o s e . M e t h o d (A) gives r e a s o n a b l e a m o u n t s of C F S ° ' B r , b u t radiolysis of the gas m i x t u r e in the n u c l e a r r e a c t o r c a u s e s form a t i o n o f m a s s q u a n t i t i e s of C F a B r . H o w e v e r , this m e t h o d gives specific activities that are high c o m pared to t h e direct p r o d u c t i o n by n e u t r o n i r r a d i a t i o n of p u r e CF3Br. In particular, the i r r a d i a t i o n of m i x tures of C F s I a n d C H s B r c o m b i n e s a g o o d yield of C F ] ° " B r with a r a t h e r high specific activity.
Acknowiedqements--We thank the operating team of the |RI nuclear reactor in Delft for the execution of the neutron irradiations and the cyclotron staff of the KVI cyclotron at Groningen for their assistance in performing the proton irradiations. We wish to thank Dr G. A. Brinkman for his suggestions and his helpful discussions. This work is part of the research program of the Institute for Nuclear Physics Research (IKO), made possible by financial support from the Foundation for Fundamental Research on Matter (FOM) and The Netherlands Organization for the Advancement of Pure Research (ZWO).
References l. DANIEL S. H.. ACHE H. J. and STOCKLIN G. J. Phys. Chem. 78, 1043 (1974l.
2. DANIEL S. H. and ACHE H. J. Radiochim. Acta 19, 132 I1973). 3. AIGlY;GER H. and HL'BEN¥ H. Aromkernenergte I0, 45'9 (19651. 4. BERG M. E.. GRALER W. M., HELTON R. W. and RACK E. P, J. Phys. Chem. 79, 1327 (19751. 5. HSIEH T. and HANRAHAN R. J. Radial. Phys. Chem. 12, 153 ,, t978}. 6. SUTCLIEEE H. and NICALPINE I. In Fluorine Chemistr) Review lEd. TARRENT P.) (Dekker. New York). 7. FENG P. Y.. GLASSON W. A,, MA>;ULA L., SCHMUDA K. and SOFFER S. Proc. Jpn. Conf Radioisot. 4, 445 (196lL 8. PAANS A. M. J.. WELLEWEERD J., VAALBURG V~',, REIFEERS S. and WOLDRING N'|. G. Int. J. Appl. Radiat. Isot. 31, 267 II980). 9. DEBUYST R. AND VANDERSTRICHT A. J. lnorq..\'uct. Chem. 30, 691 {19681. I0. ALFASSl Z. B. and FELDMAN L. Int. J. AppI. Radiut. lsot. 27, 125 (1976). ! i. KRUTZIK S. and ELIAS H. Radiochim. Actu 7, 26 (19671. 12. WAZER ','~'.Y and ABSAR I. Electron Densities in Molecules and Molecular Orbitals (Academic Press, New York, 1973). I3. ERDTMAN G. and SOYKA W The Gamma-rays of Radionuclides/Verlag Chemic, I979). 14. VAN DEN BOSH R. L. P. Ph.D. Thesis, University of Techn.. Eindhoven (1979). 15. Cox A. P., DL'XBL'WYG., HARDY J. A. and KAWASFHS~A Y. J. Chem. Soc. Faraday Trans. 2 76, 339 11980).
0020-708X g[ 02010I-0.~$02.00/0
Pergamon Press Ltd 1981. Pr,ntetI m Great Britain
Preparation of CF3S°mBrof High Specific Activity D. D E J O N G ,
B. W. V A N
HALTEREN
a n d J. T H . V E E N B O E R
Institute for Nuclear Physics Research (IKO), P.O. Box 4395, 1009 AJ Amsterdam, The Netherlands (Receiced 8 June 1980)
Three ways for the preparation of carrier-free CF]°~Br have been tested: the displacement of X in CF3X (X = F, CI, I or H) by S°'Br formed by thermal neutron irradiation of CF3X/CH3Br mixtures, by proton irradiation of CF3X/HzSe mixtures, and by the exchange of Br- coated on inert support material with CF~X'. Neither of these methods fulfils the requirement of a high yield of carrier-free CF]°=Br; however, the irradiation of CFfl/CH3Br mixtures with thermal neutrons gives CF]°'Br with a yield of (11.5 +_ 0.6?/o) and a specific activity of about 32 mCi rag- 1.
Introduction ACHE et al. ~''~ suggested C F ] ° " B r as a source compound for the study of gas-phase recoil chemistry of 8°Br, generated by the isomeric transition. Whereas the ionization potential of CF3Br (12.3 eV) is above that of atomic bromine (ll.84eV), no decharging of ~°Br" ).* formed after isomeric transition can occur when CF3Br is used as s°"Br source compound in the gas mixture. This property of CF3Br is an advantage over the other s°"Br-source gases, such as Br2 (i.p. = 10.55 eV) and CH3Br (i.p. = 10.54eV), where charge-exchange reactions cannot be excluded. The production of C F ] ° ' B r normally involves neutron irradiation of pure gaseous CF3Br, ~'2~ yielding a non-carrier-free product, followed by gas-chromatographic separation. To prevent reactions of recoil 8°Br being influenced by the large amount of inactive CF3Br, we attempted to produce carrier-free CFS°"Br by three different methods: (A) Displacement of X in CF3X (X = F, CI, I and H) by 8°"Br produced by neutron irradiation of gaseous CF3X/CH~Br mixtures in a nuclear reaction. (B) Displacement of X in CF3X (X = CI, I) by B°"Br produced by proton irradiation of Se in gaseous CF3X/H,,Se mixtures. (C) Exchange of s ° " B r - with CF3X (X = CI, I). These three methods are discussed below in the same order.
Materials CH3Br, CF.,, CF3CI and CF3H were obtained from J. T. Baker with stated purities of 99.7, 99.7, 99.0 and 98.09/0 respectively, while CF3Br and CF3I were purchased from P.C.R. lnc. (purity >99.5°~). The gases * Unless Br + is electronically excited. 101
were used without further purification. H.,Se was obtained by the reaction of water with AIzSe 3 (Alfa Products). The gas was dried over P,.O5 before use.
(A) Reaction of s°'
\
Mixtures of CF3X (X = F, CI, I and H) and 20-25,% CH3Br sealed in quartz ampoules (pressure 90 kPa, volume 1 ml) were irradiated for 20 rain at a thermal neutron flux of 1 0 ~ 3 n s - t c m -2 in the IRI nuclear reactor at Delft. The accompanying dose rate was determined, by means of optical absorption measurements with 0.1 mm Ultraphan G (Lonza) foils,(3~ to be about l0 s rad h - '. After irradiation, the contents of a typical ampoule were injected directly onto a chromatographic column by means of an ampoule-cracking device mounted on the injection port of the gas chromatograph. After injection, the splinters of the ampoule were washed with aqueous NazSO3 and chloroform. The two layers were separated and radioassayed with a Ge(Li) detector to determine the s°"Br and S2Br activities ('/-ray energies 617 and 554keV, respectively: 7-ray intensities are taken from Ref. (13). Separation of the different gaseous compounds (CF3Br, CF3X and CH3Br) was performed with a 0.75m Porapak N column, operated temperature programmed (5rain 303K, 1 0 K m i n - t to 393K, flow 60 ml He rain-t). The eluting peaks were trapped in heptane, cooled with dry ice and measured on the Ge(Li) detector. All samples were counted 1 h or more after collection in order to ensure radioactive equilibrium between 8°"Br and 8°Br. Correction was made for the contribution of the 619 keV 7-ray of S-'Br in the determination of 8°"Br activity,
D. de Jong et al.
!q2
TASLE 1, .Absolute >ields of CF~ °~' s'-'Br al~er ~hermal neutron irra&ation" of gaseous CF>k L'( = F. el. I. HI CH~Br Percentage of '~')="~:'Br in CF3Br+ ~')~Br S:Br
Composition CF, 20% CH3Br CF3C1 + 20°, CHaBr CF~I + 20", CH3Br CF3H + 20". CH3Br
6.0 5.9 I 1.5 6.4
-
± 0. I _ 0.1 _- 0.6 ~- 0.2
4.4 4.0 10.8 4.0
: 0.4 _+ 0.2 ± 0,4 +_ 0. I
irradiation
"Neutron flux: 10~3ns-~cm--': 20 min. + Mean value of 2 experiments.
time:
To evaluate the influence of the dose rate and, more particularly', the possible formation of mass a m o u n t s of CF3Br by radio{ysis, all mixtures were irradiated for ~arying times in a e'°Co ;'-source of about 1300kCi with a ;'-dose rate of 4.1 x 1 0 S r a d h -~ These ;'-irradiated samples were analysed in the same way as the neutron-activated mixtures and the level of CF3Br was determined. Restdrs and discu,ssion
The absolute CF~°~(s-')Br yields obtained by this method are summarized in Table 1. Figure 1 illustrates the production of mass a m o u n t s of CF3Br during 7-radiolysis. Some other c o m p o u n d s are also formed in these radiolysis experiments, but they were not investigated further, because only the a m o u n t of CF3Br is important in the determination of the specific activity of the product. Based on these results, estimates of the specific activities of CF3S°~(S2)Br produced in the IRI reactor are given in Table 2. For comparison, the calculated
specific activity of CF~°"s-"Br produced by neutron irradiation of pure CF3Br is also given. Here an absolute CF~°mBr yield of 30",, is used, v, hich ~alue v,e determined for a 20-rain irradiation of pure CF3Br. No literature value is known. The total activity in the final product ,,aries between 10 and 25,uCi of S°mBr and I and 3 #Ci of ~:Br. No real mechanism for the formation of CF~Br by radiolysis can be proposed at this time. since this requires quantitative determination or the other radiolysis products. However. the formation of CF~ radicals in CF_~X. CH3Br mixtures is likel,,. 's~') whereafter abstraction of bromine from CH3Br or from radiolytically formed Br: might occur lthe latter reaction has a relatively high G-value of 3-5 in CF.~(-)). In mixtures with CF3I, iodine is more easily abstracted by C F > whereas the formation of CF3 radicals in CF3H seems not very likely and this results in both cases in a lower CF3Br yield. On the basis of these data it is impossible to make a distinction between CF3~°"Br formed by a direct replacement reaction or by radiolytical processes. The difference between the s°'~Br and S:Br yields is in all likelihood caused because S-'Br is partly formed ,,ia direct In, 7) activation and partly via the isomeric transition of s-''Br. The activity of s>"Br after 20 min irradiation is a factor of 103 larger than the directly produced S-'Br activity, as a consequence of the higher thermal-neutron cross section and the shorter half-life (T t _, s : ' B r = 6.1 min: TI _, S-'Br = 35.4h). After 2 h decay {starting time of the G L C separation} the S-'Br activity formed by the decay of Se~Br is about three times the aZBr activity formed by direct acti,,ation. This decay of 8:~Br proceeds via a highly converted ;'-transition, which gives rise to positively charged bromine ions. The reactions of S-'Br formed by isomeric transition possibly result in a slightly lower
), /
t~ 5x
/
~,o-3 L)
g
2,
CF~ +
o/
20 °/oC H } Br
/ 5x
iO-3
/
/
/
/
% cl + 20%
CH3Br
Z
i
50
~
~oo
tso
5x
b_ t_)
g
5to
Mrad
CFl+
r~
20°/°
u
CH3B r
~oo
5x l0 -3
15o
Mrad
C~H+ 20% CH3Br
/ J c~S L
/ I ~
Z 150
~ Mrad
L~ 50
i~
150
Mrad
FIG. 1. Formation of mass quantities of CF3Br by ;'-radiolysis (dose rate: 4.1 x 105radh-~).
Preparation of CF~°~Br TABLE 2. Specific activity of CF~°"Se'Br produced b v thermal neutron irradiation" Specific acti,,it? SO~Br S:Br {#Ci rag) {uCi mg~
Composition CFa - CH3Br CF3CI -"- CH3Br CF3I - CH3Br CF3H ..a CH~Br CF3Br Ipure)+
5.9 3.8 3.2 >5.0 5.3
x × x × ×
103 I03 10" [0 a 101
5.6 3.9 4.0 5.0 I.I
x x x x x
10: I02 103 103 I0 °
* Irradiation time: 20 min: :'-dose: l0 s rad h "~Calculated specific activity lretention - 30"ol.
CF~-'Br ,yield than the reactions of 8ZBr produced d i r e c t b by (n, 7) activation, as was shown b,, BERG et al. ~ and DaYrEL and ACHff-" for the yields of s2'S°Br-CH3Br produced by the In, ;') reaction or the isomeric transition process in gaseous C H 3 X (X = F, CI, Br). This explains the lower CF]2Br yield, with respect to the CF3SO"Br yield.
(B) Reaction of 76'77~°"~Br in Gas Mixtures of C F 3 X (X = e l , 1) a n d H 2 S e
Experimentul Glass ampoules (pressure 65-90 kPa, volume 10 ml, length l0 cm) containing mixtures of CF3X (X = CI, I) and 20--50°0 H2Se were irradiated for 20 rain with 50-200 nA 20 MeV protons in the external beam of the KVI cyclotron (Groningen). A 0.1 mm nickel foil was used as a current monitor. The gaseous contents of the ampou/es were injected via a loop system into a gas c h r o m a t o g r a p h on a 0.75 m Porapak N column. Collection and radioassay of the products was carried out as described in (A) above. The ;.'-rays of VTBr (239keVI and :~'Br {657keV) (;,'-ray intensities are taken from Re[. 13) were measured. Since the transportation time from the cyclotron to Amsterdam is rather long. no s°"Br (T 1 2 = 4.4h) could be measured in these experiments.
Results and discussion The absolute yields of CF36'7"Br are given in Table 3. It can be seen that these yields are very small. The total activity found in the CF3Br fraction is also not very high {max. 0.1-O.5b~Ci), although the cross sections for the (p, n) reactions on selenium are fairly TABLE 3. Absolute ?ields of CF~6"":Br after 20 MeV proton irradiation* of gaseous CF3X (X = CI. l) HzSe mixtures Composition
Percentage of r~"V"Br Pressure in CF3Br (kPa)
CF3CI + 28°~; H:Se CF3CI + 50°° H2Se CF3I + 2S°~ H_,Se * Irradiation time: 20 min.
90 65 90
103
high. ['e'Se(p. nl-"Br: a = - 6 0 0 r o b at [5MEW s', r'Setp, n ) " B r : ~- = ~ 8 0 0 m b at 12 MeV't'~(] After irradiation, the ampoules were coated on the inside with black and red layers of selenium, presumably due to radiolysis of H.,Se. This radiolysis certainly reduces the total yield of bromine isotopes produced by proton irradiation of H,Se. The cross section for the Ip, n) reaction on S°Se is expected to be lower than that for the reactions on ~'Se and "'Se (a = 80 mb at 6 MeV ~9~, no a-values known at higher energies). No serious differences in absolute CF3Br yields between "e'Br, ' " B r and s°"Br should exist, because below 15 MeV the most important nuclear reaction for the production of these three isotopes is the/P, n) reaction,
(C) Exchange
Reaction
of s°'182)Br-
with
C F 3 X (X = CI, I)
Experimental Bromido-S'Br was prepared with high specific activity as described by ALFASSI et qlJ 1°~ The aqueous S2Br- solution was used to coat kieselguhr 0 8 - 5 2 mesh). Following the exchange procedure of KRUTZIK and ELIAS,I1 t~ 500 mg of 8-'Br coated kieselguhr was brought into contact with gaseous CF3CI, CF3I and CH3Br in a glass ampoule at a pressure of 98 kPa at 353 K. After a reaction time of 2-4 h. the gaseous contents of the ampoule were analysed on a Porapak N column to determine the levels of CFS2Br and CH]2Br. CH3Br was measured as a standard to check the procedure. Results and discussion Table 4 summarizes the results obtained by this method. The CF3S-'Br yields are negligible compared to the CH3Br yield. This p h e n o m e n o n is presumably due to the different electron-density distributions in both molecules: ~j2, the three fluorine atoms in CF3X strongly withdraw electrons from the rest of the molecule, resulting in a CF 3 group with a high electron density and a Br substituent with a low electron density. ~5~ This fact makes the CF3X molecule rather unattractive to nucleophilic attack by 82Br-.
General Conclusions The aim of the study was to develop a method for the preparation of carrier-free C F ] ° " B r . The yields TM}LE 4. S.~nthesis of CFI2Br by exchange of 8:Br- whh CF~.\" iX = CI. I}* Compound
Percentage of S2Br in CF3Br {H3Br)
CF~CI CFfl CH3Br
<0.1 <0. I % 17
*Pressure: 98kPa: reaction time: 2-4h: temperature: 353 K.
I04
D. tie Jon# et al.
resulting from m e t h o d s {B) a n d (CI are m u c h too Io'* to be attractive for a n y labeling p u r p o s e . M e t h o d (A) gives r e a s o n a b l e a m o u n t s of C F S ° ' B r , b u t radiolysis of the gas m i x t u r e in the n u c l e a r r e a c t o r c a u s e s form a t i o n o f m a s s q u a n t i t i e s of C F a B r . H o w e v e r , this m e t h o d gives specific activities that are high c o m pared to t h e direct p r o d u c t i o n by n e u t r o n i r r a d i a t i o n of p u r e CF3Br. In particular, the i r r a d i a t i o n of m i x tures of C F s I a n d C H s B r c o m b i n e s a g o o d yield of C F ] ° " B r with a r a t h e r high specific activity.
Acknowiedqements--We thank the operating team of the |RI nuclear reactor in Delft for the execution of the neutron irradiations and the cyclotron staff of the KVI cyclotron at Groningen for their assistance in performing the proton irradiations. We wish to thank Dr G. A. Brinkman for his suggestions and his helpful discussions. This work is part of the research program of the Institute for Nuclear Physics Research (IKO), made possible by financial support from the Foundation for Fundamental Research on Matter (FOM) and The Netherlands Organization for the Advancement of Pure Research (ZWO).
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