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The wall effect of recoil tritium chemistry
J. inorg, nucl. Chem. VoL 43, No. I'. pp. 2585-2587, 1981
f~)22-~902/8I/112585-03502.00/0 Pergamon Press l.td
Printed in Great Britain.
THE WALL EFFECT OF RECOIL TRITIUM CHEMISTRY DAVID J. MALCOLME-LAWES, GRAHAM OLDHAM and YACOUB ZIADEH Nuclear Chemistry Laboratory, Loughborough University of Technology, Leicestershire LEll 3TU, England
(Received 18 February 1981;receivedfor publication 11 May 1981) Abstract--The fate of tritium atoms recoiling into the walls of quartz sample bulbs is examined by analysing the contents of a variety of sample bulbs after neutron irradiation. It is concluded that, contrary to earlier findings, HT is not released into the gas phase as a result of T atoms recoiling directly into the wall, and that the "wall effect" correction applied in some recoil tritium systems may not always be appropriate. situations in which L > 0.1 and in which the fate of the tritium atoms reaching the bulb wall may be important. For example, much information on the energetics of hot atom reactions has been derived from pressure dependence studies [5-7] which have involved experiments at low gas pressures and consequently at large values of r and L. Experiments in which high mole fractions of helium moderator have been used are necessarily carried out under conditions of a large L because of the relatively long range of 192 keV tritons in helium gas. It has been well established that tritons recoiling into the walls of 1720 pyrex sample bulbs tend to remain imbedded within the wall [8]. However 1720 pyrex is not a particularly attractive material for use in conventional recoil tritium experiments because of its poor hand,ling characteristics and because of the presence of boron within its structure, which attenuates the neutron flux and makes the estimation of the induced tritium activity more difficult. Sample bulbs made of quartz have been much more widely used[9-11], although in this case it has been argued that tritium recoiling into the bulb may be released into the gas phase sample as spurious "wallHT". Urch et al. first observed the phenomenon[121 and later estimated the amount of "wall-HT" produced as a function of L i.e. the fraction of tritons which reach the wall, publishing a graph[8] which has been widely used for correcting HT yields for a "wall-HT" contribution. During the course of a moderation study of the recoil tritium/deuterium system we observed that our experimental results did not accord with the "walI-HT" plot and we have therefore re-examined the phenomenon,
Tritium atoms recoiling from the nuclear process 3He(n, p)3H have been extensively used in studies of the reactions of hot tritium atoms with gas phase species such as hydrogen, hydrocarbons and halocarbons[l]. Such systems are generally studied by forming the recoil atoms within a reactant mixture contained in a quartz or 1720 pyrex sample bulb of the order of 10-20cm 3 in volume and of internal diameter of 1-2 cm. It has long been recognised that, when hot atoms are formed in this manner, a fraction of the atoms initially produced recoil directly into the wall of the sample bulb without having the opportunity to react in the gas phase. 2 As the fractional yields of reaction products from such experiments as evaluated as P'
Ai
=A-~T
(1)
where Ai =activity of observed product, and AT = activity of tritium stopped in the gas phase. Several theoretical procedures [2-4] have been devised to enable AT- to be estimated from the total amount of tritium induced in the sample and the recoil loss fraction, L, i.e. that fraction of the induced tritium which is lost directly to the wall. Probably the best known method for estimating L is that due to Estrup and Wolfgang[2]. For a cylindrical sample bulb of length 1 cm and diameter d cm, Estrup concluded that
L=3~[C~c l K(c)+--~-c2+lE(c)]+ d
(c-c 3)
(2)
where K(c) and E(c) are elliptic integrals of the first and second kind, respectively, c = r/d (- 1 and r is the range of a 192 keV triton recoiling within the sample from the 3He(n, p)~I-I process. [A similar equation was deduced ~or the case c > 1].2 It follows from (1) and (2) that the fractional yield of a hot product is given by
Pi
Ai Ao(I-L)
(3)
where Ao is the total tritium activity induced within the sample. In the absence of experimental verification of eqn (2), gas phase recoil tritium experiments are generally performed under conditions chosen so that L is small, (i.e. relatively high gas pressures). However there are several
EXPERIMENTAL Sample bulbs, approximately 60 mm× 18 mm i.d., were fabricated from Vitreosil tubing (Jencons Ltd.) and after heating to red heat under vacuum were filled with reagent gases on a grease-free vacuum line. Details of the procedure and the materials used have been published previously[13], although we note here that the deuterium used for the present work was obtained from the Matheson Company and had a stated purity of > 99.5%. Several different lecture bottles of this deuterium were used during the course of this work and were obtained over a period of two years. Our first series of experiments consisted of measuring the HT yield from samples containing helium-3, deuterium, a noble gas moderator and, in some cases, a scavenger (02 and ICI). Samples were irradiated with thermal neutrons to induce a tritium activity of - 3 × 104 Bq, and then analysed by radiogaschromatography, using a 2 m alumina column operated at liquid nitrogen temperatures. A radiochromatogram showing the separation of HT and DT by our system is depicted in Fig. 1, and it is clear from
2585
D. J. MALCOLME-LAWESet al.
2586
this chromatogram that the quality of the activity measurements in this work is not in doubt. Our gas chromatograph[14] was fitted with a katharometer detector (normally used to check for any mdcroscopic radiation damage of samples) which was of sufficient sensitivity to respond to 10-6 moles of H2 or D2 in the helium carrier gas when the chromatography column was operated at room temperature. However, we observed that the sensitivity of the katharometer to HI2 and D2 decreased by several orders of magnitude when the column was operated at liquid nitrogen temperatures (presumably because the vibrational energy of the diatomics was reduced). For this reason our first experiments were carried out under conditions in which we had no experimental check on the purity of the deuterium gas. The results obtained from our first experiments are shown in Fig. 2, where they are presented as the observed HT yield plotted against the theoretical "wall-I-IT"yield taken from reference. In virtually all cases the HT yield was much higher than could be accounted for by the predicted amount of wall-HT. Furthermore it is clear that the HT yield observed from the unscavenged deuterium samples was almost invariably higher than that observed from scavenged samples. These results suggested that the deuterium sample contains some protium impurity. As similar results were obtained from a second lecture bottle of Dz (also of stated purity > 99.5%), we obtained a catalytic detector[N] consisting of a pair of platinum filaments and operated by mixing the column effluent with oxygen before passing it over one filament, while a helium/oxygen mixture was passed over the second filament. With the filaments connected into a conventional Wheatstone bridge circuit (bridge current 1 A), - 10-6 moles of I,i2 or D2 could be detected even when the chromatography column was operated at liquid nitrogen temperatures. Using the catalytic detector we found that aliquots taken directly from the first lecture bottle of D2 contained - 3.2 atom % of hydrogen, while a second contained about 5%. In view of the fact that the phenomenon of "wall-HT" was postulated as a result of measurements of I-IT yields from deuterated compounds of high stated purities[12], we decided to look for unambiguous evidence that "wall-HT" was actually formed at all. We filled a number of sample bulbs with ~ 5 cm Hg 05, ~ 3 cm Hg 3He and varying amounts of inert gas moderators, He and Ar. After irradiation and analysis we found no measurable HT in any of the samples (detection limit PaT 0.001), even though the recoil loss range examined was L = 0.212-L = 0.848. Repeating the experiment using - 2 cm Hg of ICI in place of the O2 scavenger, small amounts of HT were detected and the yields from both systems are compared with the predicted wall-l-ITin Fig. 3.
DISCUSSION The results shown in Fig. 3 clearly suggest that the conventional correction for wall-HT may not be appropriate for all systems.'No spurious HT of any kind was observed in our oxygen scavenged samples, and the spurious HT detected in the presence of ICl scavenger is consistently much lower in activity than predicted by the "wall-HT" calibration graph[8]. The oxygen scavenged results suggest that tritium recoiling into the walls of a quartz sample bulb does not reappear in the gas phase as HT after reacting with hydrogen atoms within the quartz.
'O8
O 0
'06
0
X /
0 O 0
.04
X
X
0
OxO
J
X
& O X
I##" /
"02 #i AS
'
.c~2
'
.G4
'
predicted "wclll HT " Fig. 2. Experimental yields of HT, PnT, recorded from recoil tritium/D2 system shown as a function of the "wall-HT" yield predicted for each sample. (Broken line shows theoretical function.) Points shown X are for unmoderated and unscavenged D,; O points are He moderated, o Ar moderated and A Kr moderated. Filled symbols represent bulbs containing ICI scavenger.
//
F)NT
)T count
HT
'
.,,i • pl
04
tale
J r
1
/
i J
'02
," i p
÷
," / i.et
36
retention
28 time
20 / rains
Fig. 1. Section of radiochromatogram showing separation of HT and DT achieved using a 2 m activated alumina column operated at liquid nitrogen temperature with helium carrier gas flowingat 50 cm3 rain-'.
'02 predicted
.04 "woll H T "
Fig. 3. Experimental yields of HT, PHT, recorded from samples in which recoil tritium was produced in a hath gas of a moderator (He or Ar) and a scavenger; © = ICl, • = O2. Results shown as a function of the "wall-HT" yield predicted for each sample. (Broken line shows theoretical function.)
The wall effect of recoil tritium chemistry However, this result has been obtained from experiments which are in essence the same as those used earlier to derive the "wall-HT" calibration graph[8]. Apart from pointing out that we were careful to ensure that our sample bulbs were clean and dry (by heating to red heat under vacuum), we can offer no explanation for the contradictory results of the two sets of experiments. The results obtained in the presence of iodine monochloride scavenger indicate that some spurious HT is produced under our experimental conditions when ICI is present. We were unable to detect any hydrogenous impurity in the ICI although the nature of this compound is such that purification and analysis are rather difficult. However, whatever the source of this spurious HT, its activity is significantly lower than the value predicted by the "wall-HT" calibration curve, and we are forced to conclude that there is no longer any unambiguous evidence for either the formation of "wall-HT", or for the validity of the empirical correction of recoil tritium HT yields to allow for spurious HT produced as a result of the recoil loss fraction. Acknowledgements--This work was supported by the Science Research Council. The authors are grateful for the assistance of
2587
staff of the Aldermaston Herald reactor and the University of Manchester Reactor Service who carried out the irradiations. REFERENCES
1. D. S. Urch, Radiochemistry 2, 1 (1975). 2. P. J. Estrup and R. Wolfgang, J. Am. Chem. Soc. 82, 2665 (1%0). 3. J. W. Root, Radiochimica Acta 10, 104 (1968). 4. W. J. Argersinger Jr., J. Phys. Chem. 67, 976 (1%3). 5. A. J. Johnston, D. J. Malcolme-Lawes, D. S. Urch and M. J. Welch, J. Chem. Soc. Chem. Comm. 187 (1966). 6. R. T. K. Baker and R. L. Wolfgang, J. Phys. Chem. 73, 3478 (1%9). 7. C. T. Ting and F. S. Rowland, J. Phys. Chem. 74, 445 (1970). 8. D. J. Malcolme-Lawes, D. S. Urch and M. J. Welch, Radiochimica Acta 6, 185 (1%6). 9. D. S. Urch and M. J. Welch, Trans. Faraday Soc. 64, 547 (1968). 10. D. Seewald and R. Wolfgang, J. Chem. Phys. 47, 143 (1967). 11. P. Volpe, Gazz. Chim. ltal. 104, 237 (1974). 12. D. S. Urch and M. J. Welch, J. Chem. Soc. Chem. Comm. 126 (1%5). 13. D. J. Malcolme-Lawes, G. Oldham and Y. Ziadeh, J. Chem. Soc. Faraday I in press. 14. Y. Ziadeh, Ph.D. Thesis, Loughborough University of Technology (1981).
f~)22-~902/8I/112585-03502.00/0 Pergamon Press l.td
Printed in Great Britain.
THE WALL EFFECT OF RECOIL TRITIUM CHEMISTRY DAVID J. MALCOLME-LAWES, GRAHAM OLDHAM and YACOUB ZIADEH Nuclear Chemistry Laboratory, Loughborough University of Technology, Leicestershire LEll 3TU, England
(Received 18 February 1981;receivedfor publication 11 May 1981) Abstract--The fate of tritium atoms recoiling into the walls of quartz sample bulbs is examined by analysing the contents of a variety of sample bulbs after neutron irradiation. It is concluded that, contrary to earlier findings, HT is not released into the gas phase as a result of T atoms recoiling directly into the wall, and that the "wall effect" correction applied in some recoil tritium systems may not always be appropriate. situations in which L > 0.1 and in which the fate of the tritium atoms reaching the bulb wall may be important. For example, much information on the energetics of hot atom reactions has been derived from pressure dependence studies [5-7] which have involved experiments at low gas pressures and consequently at large values of r and L. Experiments in which high mole fractions of helium moderator have been used are necessarily carried out under conditions of a large L because of the relatively long range of 192 keV tritons in helium gas. It has been well established that tritons recoiling into the walls of 1720 pyrex sample bulbs tend to remain imbedded within the wall [8]. However 1720 pyrex is not a particularly attractive material for use in conventional recoil tritium experiments because of its poor hand,ling characteristics and because of the presence of boron within its structure, which attenuates the neutron flux and makes the estimation of the induced tritium activity more difficult. Sample bulbs made of quartz have been much more widely used[9-11], although in this case it has been argued that tritium recoiling into the bulb may be released into the gas phase sample as spurious "wallHT". Urch et al. first observed the phenomenon[121 and later estimated the amount of "wall-HT" produced as a function of L i.e. the fraction of tritons which reach the wall, publishing a graph[8] which has been widely used for correcting HT yields for a "wall-HT" contribution. During the course of a moderation study of the recoil tritium/deuterium system we observed that our experimental results did not accord with the "walI-HT" plot and we have therefore re-examined the phenomenon,
Tritium atoms recoiling from the nuclear process 3He(n, p)3H have been extensively used in studies of the reactions of hot tritium atoms with gas phase species such as hydrogen, hydrocarbons and halocarbons[l]. Such systems are generally studied by forming the recoil atoms within a reactant mixture contained in a quartz or 1720 pyrex sample bulb of the order of 10-20cm 3 in volume and of internal diameter of 1-2 cm. It has long been recognised that, when hot atoms are formed in this manner, a fraction of the atoms initially produced recoil directly into the wall of the sample bulb without having the opportunity to react in the gas phase. 2 As the fractional yields of reaction products from such experiments as evaluated as P'
Ai
=A-~T
(1)
where Ai =activity of observed product, and AT = activity of tritium stopped in the gas phase. Several theoretical procedures [2-4] have been devised to enable AT- to be estimated from the total amount of tritium induced in the sample and the recoil loss fraction, L, i.e. that fraction of the induced tritium which is lost directly to the wall. Probably the best known method for estimating L is that due to Estrup and Wolfgang[2]. For a cylindrical sample bulb of length 1 cm and diameter d cm, Estrup concluded that
L=3~[C~c l K(c)+--~-c2+lE(c)]+ d
(c-c 3)
(2)
where K(c) and E(c) are elliptic integrals of the first and second kind, respectively, c = r/d (- 1 and r is the range of a 192 keV triton recoiling within the sample from the 3He(n, p)~I-I process. [A similar equation was deduced ~or the case c > 1].2 It follows from (1) and (2) that the fractional yield of a hot product is given by
Pi
Ai Ao(I-L)
(3)
where Ao is the total tritium activity induced within the sample. In the absence of experimental verification of eqn (2), gas phase recoil tritium experiments are generally performed under conditions chosen so that L is small, (i.e. relatively high gas pressures). However there are several
EXPERIMENTAL Sample bulbs, approximately 60 mm× 18 mm i.d., were fabricated from Vitreosil tubing (Jencons Ltd.) and after heating to red heat under vacuum were filled with reagent gases on a grease-free vacuum line. Details of the procedure and the materials used have been published previously[13], although we note here that the deuterium used for the present work was obtained from the Matheson Company and had a stated purity of > 99.5%. Several different lecture bottles of this deuterium were used during the course of this work and were obtained over a period of two years. Our first series of experiments consisted of measuring the HT yield from samples containing helium-3, deuterium, a noble gas moderator and, in some cases, a scavenger (02 and ICI). Samples were irradiated with thermal neutrons to induce a tritium activity of - 3 × 104 Bq, and then analysed by radiogaschromatography, using a 2 m alumina column operated at liquid nitrogen temperatures. A radiochromatogram showing the separation of HT and DT by our system is depicted in Fig. 1, and it is clear from
2585
D. J. MALCOLME-LAWESet al.
2586
this chromatogram that the quality of the activity measurements in this work is not in doubt. Our gas chromatograph[14] was fitted with a katharometer detector (normally used to check for any mdcroscopic radiation damage of samples) which was of sufficient sensitivity to respond to 10-6 moles of H2 or D2 in the helium carrier gas when the chromatography column was operated at room temperature. However, we observed that the sensitivity of the katharometer to HI2 and D2 decreased by several orders of magnitude when the column was operated at liquid nitrogen temperatures (presumably because the vibrational energy of the diatomics was reduced). For this reason our first experiments were carried out under conditions in which we had no experimental check on the purity of the deuterium gas. The results obtained from our first experiments are shown in Fig. 2, where they are presented as the observed HT yield plotted against the theoretical "wall-I-IT"yield taken from reference. In virtually all cases the HT yield was much higher than could be accounted for by the predicted amount of wall-HT. Furthermore it is clear that the HT yield observed from the unscavenged deuterium samples was almost invariably higher than that observed from scavenged samples. These results suggested that the deuterium sample contains some protium impurity. As similar results were obtained from a second lecture bottle of Dz (also of stated purity > 99.5%), we obtained a catalytic detector[N] consisting of a pair of platinum filaments and operated by mixing the column effluent with oxygen before passing it over one filament, while a helium/oxygen mixture was passed over the second filament. With the filaments connected into a conventional Wheatstone bridge circuit (bridge current 1 A), - 10-6 moles of I,i2 or D2 could be detected even when the chromatography column was operated at liquid nitrogen temperatures. Using the catalytic detector we found that aliquots taken directly from the first lecture bottle of D2 contained - 3.2 atom % of hydrogen, while a second contained about 5%. In view of the fact that the phenomenon of "wall-HT" was postulated as a result of measurements of I-IT yields from deuterated compounds of high stated purities[12], we decided to look for unambiguous evidence that "wall-HT" was actually formed at all. We filled a number of sample bulbs with ~ 5 cm Hg 05, ~ 3 cm Hg 3He and varying amounts of inert gas moderators, He and Ar. After irradiation and analysis we found no measurable HT in any of the samples (detection limit PaT 0.001), even though the recoil loss range examined was L = 0.212-L = 0.848. Repeating the experiment using - 2 cm Hg of ICI in place of the O2 scavenger, small amounts of HT were detected and the yields from both systems are compared with the predicted wall-l-ITin Fig. 3.
DISCUSSION The results shown in Fig. 3 clearly suggest that the conventional correction for wall-HT may not be appropriate for all systems.'No spurious HT of any kind was observed in our oxygen scavenged samples, and the spurious HT detected in the presence of ICl scavenger is consistently much lower in activity than predicted by the "wall-HT" calibration graph[8]. The oxygen scavenged results suggest that tritium recoiling into the walls of a quartz sample bulb does not reappear in the gas phase as HT after reacting with hydrogen atoms within the quartz.
'O8
O 0
'06
0
X /
0 O 0
.04
X
X
0
OxO
J
X
& O X
I##" /
"02 #i AS
'
.c~2
'
.G4
'
predicted "wclll HT " Fig. 2. Experimental yields of HT, PnT, recorded from recoil tritium/D2 system shown as a function of the "wall-HT" yield predicted for each sample. (Broken line shows theoretical function.) Points shown X are for unmoderated and unscavenged D,; O points are He moderated, o Ar moderated and A Kr moderated. Filled symbols represent bulbs containing ICI scavenger.
//
F)NT
)T count
HT
'
.,,i • pl
04
tale
J r
1
/
i J
'02
," i p
÷
," / i.et
36
retention
28 time
20 / rains
Fig. 1. Section of radiochromatogram showing separation of HT and DT achieved using a 2 m activated alumina column operated at liquid nitrogen temperature with helium carrier gas flowingat 50 cm3 rain-'.
'02 predicted
.04 "woll H T "
Fig. 3. Experimental yields of HT, PHT, recorded from samples in which recoil tritium was produced in a hath gas of a moderator (He or Ar) and a scavenger; © = ICl, • = O2. Results shown as a function of the "wall-HT" yield predicted for each sample. (Broken line shows theoretical function.)
The wall effect of recoil tritium chemistry However, this result has been obtained from experiments which are in essence the same as those used earlier to derive the "wall-HT" calibration graph[8]. Apart from pointing out that we were careful to ensure that our sample bulbs were clean and dry (by heating to red heat under vacuum), we can offer no explanation for the contradictory results of the two sets of experiments. The results obtained in the presence of iodine monochloride scavenger indicate that some spurious HT is produced under our experimental conditions when ICI is present. We were unable to detect any hydrogenous impurity in the ICI although the nature of this compound is such that purification and analysis are rather difficult. However, whatever the source of this spurious HT, its activity is significantly lower than the value predicted by the "wall-HT" calibration curve, and we are forced to conclude that there is no longer any unambiguous evidence for either the formation of "wall-HT", or for the validity of the empirical correction of recoil tritium HT yields to allow for spurious HT produced as a result of the recoil loss fraction. Acknowledgements--This work was supported by the Science Research Council. The authors are grateful for the assistance of
2587
staff of the Aldermaston Herald reactor and the University of Manchester Reactor Service who carried out the irradiations. REFERENCES
1. D. S. Urch, Radiochemistry 2, 1 (1975). 2. P. J. Estrup and R. Wolfgang, J. Am. Chem. Soc. 82, 2665 (1%0). 3. J. W. Root, Radiochimica Acta 10, 104 (1968). 4. W. J. Argersinger Jr., J. Phys. Chem. 67, 976 (1%3). 5. A. J. Johnston, D. J. Malcolme-Lawes, D. S. Urch and M. J. Welch, J. Chem. Soc. Chem. Comm. 187 (1966). 6. R. T. K. Baker and R. L. Wolfgang, J. Phys. Chem. 73, 3478 (1%9). 7. C. T. Ting and F. S. Rowland, J. Phys. Chem. 74, 445 (1970). 8. D. J. Malcolme-Lawes, D. S. Urch and M. J. Welch, Radiochimica Acta 6, 185 (1%6). 9. D. S. Urch and M. J. Welch, Trans. Faraday Soc. 64, 547 (1968). 10. D. Seewald and R. Wolfgang, J. Chem. Phys. 47, 143 (1967). 11. P. Volpe, Gazz. Chim. ltal. 104, 237 (1974). 12. D. S. Urch and M. J. Welch, J. Chem. Soc. Chem. Comm. 126 (1%5). 13. D. J. Malcolme-Lawes, G. Oldham and Y. Ziadeh, J. Chem. Soc. Faraday I in press. 14. Y. Ziadeh, Ph.D. Thesis, Loughborough University of Technology (1981).