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Secondary nuclear reactions induced in tin by fast protons
amnaN
&NOiLOXd ,LSViI A8 NIL NI SNOIiLL3VBX XVTl3flN AlIvaNoxS
Secondary nuclear reactions induced in tin by fast protons
241
z*Na. A cross-section of 10 milli-barn was assumed for this reaction over the range of proton energies studied. Iodine separation After the irradiation, the sample was dissolved in hot concentrated nitric acid containing potassium persulphate, 30 mg iodine (KIO,) and 50 mg tellurium (nitric acid solution). The solution was diluted to a volume of 80-100 m 1 and the iodate was reduced to the iodide by sodium sulphite. The iodine was successively distilled off through two absorbing vessels, the first of which was filled with a 1 M solution of HNO, and the second with a 1M solution of NaOH. The elementary iodine was then twice extracted with chloroform. For the activity measurements, targets of PdI, were prepared. The chemical yield of the carrier was usually 60-70 per cent. Special tests showed that no loss of radio-iodine occurred as a result of sorption in the tin dioxide formed when metallic tin is dissolved in HNOa. Tellurium separation After distilling off the iodine, the contents of the distillation flask were evaporated and the sediment of tin dioxide was dissolved in concentrated hydrochloric acid. The excess HNO, was removed and tellurium was precipitated by stannous chloride. The precipitate was centrifuged and it was dissolved again in a mixture of hydrochloric and nitric acids. Carriers (selenium, antimony, arsenic, copper etc) were added to 1-2 mg of this solution which was then evaporated to dryness. The selenium, arsenic, tin and antimony were removed by twice evaporating the solution with 3 ml of HBr, traces of HNO, being removed at the same time. The residue was dissolved in a 3 M solution of HCl and the tellurium was precipitated from the boiling solution by sulphur dioxide. The evaporation of the solution with HBr and the precipitation of tellurium by sulphur dioxide was repeated twice. In order to remove noble metal impurities, the tellurium was distilled in a stream of hydrogen at a temperature of 800-900°C. The condensate was washed off with nitric acid and evaporated with HCl and HBr. Metallic tellurium was then precipitated by sulphur dioxide and deposited on a target. The chemical yield was 20-40 per cent. Radioactioity measurements The measurements of the activity of the samples were made using an end window counter having a mica window of thickness -3 mg/cm2. The following iodine isotopes were observed : T(/?, K, T = 13 day); ltiI(/3, K, T = 4.5 day); rBI(K, T = 13 hr); i21T(p, K, T = 1.8 hr); 1201(p,T = 30 min). We found K capture branching ratios for 1261,1241,and 1211equal to 50, ~60 and -60 per cent respectively. An account of this work will be published later. For tellurium, a single half-life of about 6 days was observed. This activity was ascribed to ii8Te (K, T = 6 day) and it can be detected through the decay of its daughter nucleus ‘lsSb (/?, T = 3.5 min, EB = 3.1 MeV). Owing to the fact that the half-life of llgTe (K, T = 4.5 days) is close to that of llsTe and because of the low X-ray sensitivity of the counter, it was not possible to determine the llgTe contribution from the decay curves. The 2.5 hour activity of l17Te was not studied. 16
\
242
M.‘IA.
KUZNETSOVA, V. N. MEKHED~V and V. A.
KHALKIN
Experimental results The formation cross-sections of those reaction products identified as having nuclear charges 2 or 3 units greater than that of the target nuclei are tabulated in Table 1 for a number of proton energies. The Table also gives the mean arithmetic errors derived from the results of three experiments. The determination of the yields of the individual products by analysing the decay curves was rather difficult because of the low activity of the samples and the large number of isotopes which were present. We believe that the lzlI formation cross-section is completely reliable but the total cross-section may be somewhat over-estimated because the internal conversion electrons from lBI were also counted .* It would seem that the absolute error of our total cross-section determination cannot be very large because our results are in agreement with those of KURCHATOVet al.,@) which were obtained by a different method. Moreover, our formation cross-sections are close to the formation crosssections of the germanium isotopes and of 211Atproduced in the proton bombardment of copper and leadts) (see Table 2). It follows from our results that the secondary reaction cross-sections for the capture of lithium by tin at 340 MeV incident proton energy are 50 times less than the values obtained by MARQUEZand PERLMAN.(~) Furthermore, although we observed the same iodine isotopes we found relative yields which were very different from those reported by these authors. It has been suggested that our results underestimate the secondary reaction crosssections because of the loss of iodine caused by target heating in the synchrocyclotron vacuum chamber. This suggestion was shown to be incorrect by making some control experiments. Samples of tin were sealed up inside a thin glass ampoule and irradiated. From these experiments we obtained iodine formation cross-sections which were identical with those presented in Table 1. The contribution to the radio-iodine production of likely sample contaminants was also investigated and shown to be negligible. ‘For example, by separating barium (Z = 56), it was shown that only l/25 part of the observed iodine activity could be accounted for by spallation products. The change in the iodine formation cross-section with increasing proton energy is shown by the bottom curve of the Figure 1. The increase in the rate of rise of the cross-section which occurs for proton energies above 400 MeV is caused mainly by an increase in the yield of the lighter iodine isotopes. This can be explained by supposing that these isotopes are produced in fast proton capture reactions in which the subsequent emission of two r-mesons and several neutrons occurs. Unfortunately, the experimental data is insufficient to allow an estimate of the contribution of this reaction to be made, nor can the energy region over which it takes place be specified. From the secondary a-particle capture cross-sections only the ll*Te formation cross-section was determined (see Table 1). This cross-section rises to a maximum for protons in the energy interval 340-500 MeV and then falls again (see Fig. 1). A very similar behaviour was observed in the formation of light actinium isotopes from bismuth(l) and in the formation of 6sGa from copper.(5) This rather odd energy dependence can be explained most simply by supposing that an additional reaction takes place in which IlaTe is formed by the capture of a fast proton followed by the * The internal conversion electrons were taken into account when computing the Is11 formation crosssection. The X-ray efficiency of the end window counter was found to be 2 per cent by using a standard sample of’s61 (K, 2’= 60 day). Other measurements with a cylindrical aluminium walled counter confirmed the accuracy of the procedure used in analysing the decay curves. leaI was not detected in these experiments but the yields of the other isotopes were in agreement with the end window counter results.
/
/
6 day 30 min 1.8 hr 13hr 4,5 day 13 day
Half-life
Capture of lithium nuclei
Capture of cc-particles
Type of reaction
’
!
3.6 0.02 0.02 0.11
* 1.0 I- 0.01 ! 0035 I; 0.08
170MeV I
, j
044 + 0.084
16.5 & 1.5 0.03 f 0.01 0.067 I 0.003 0.3 * 0.07 0.024 0.02
340 MeV
/
(
I
I
ssGe i- 88Ge + 88Ge
cu Sn Pb =“At
laOIi 1211 __f1231 + 1241 + IZil
GBGa $- s7Ga i- @Ga *leTe 210At + 9”At
cu
Sn Bi
Reaction products
_
’
BY COPPER,TlN, BISMUTH
Target element
I
480 MeV
LEAD
.~ 0.3 (340) 0.9 (480) 0.04 - 0.08
26 (340) 8 (480) 80 (480)
/
1
0.51
(0.11
I
5.0+ 0.05 0.17 0.18
340 MeV
’
/ /
0.6 0.9 0.16 -- 0.32
90 40 80
-
(3) This work (1)
(3) This work (1)
Reference
1-24
0.45 0.38 0.41
11
480 MeV
Reference(B’
cross-section (1O-3o cm%)
/ Extrapolated
I
1 ,
LITHIUMNUCLEI
0.2 0.007 0.2 0.008 0.01 0.5
t: 1.3 .! f f f f i
CL-PARTICLESAND
10 0.27 0.24 0.97 0.06 0.06 1.6
Experimental cross-section (10-Y cm2)
AND
i
1
660 MeV
TELLURIUMANDIODINEISOTOPFS
14.5 f 7.7 0.10 f 0.01 0.15 f 0.03 0.56 :k 0.16 0.035 0.048 i 0.006 0’9 f 0.2
This work
TABLE~.--SECONDARYREACTIONCROSS-SECTIONSFORTHECAPTUREOP
Total iodine cross-section
lisTe 1ZOI lZlI 123I WI l%I
Isotope
/ / ’
l.-CROSS-SECIIONSFORTKEFORMATIONOF
INTHEPROT~NBOMBARDMENT~F.~NAT~AR~~~~PROTONENEROIES(~BARN)
TABLE
244
M. IA. KUZNETSOVA, V. N. MEKHEDOV and V. A. KHALKIN
emission of a r-meson and some neutrons. It is difficult to explain the form of the curve in terms of a change in the spectrum and emission probability of the a-particles because such an explanation will not at the same time explain why the curve falls again in the high energy region. A
:!O-
15-
E
10-
Y
A
b’
Actinium
5-
Iodine
// 0_ 0
200
I
I
500
600
t$5
MA’
FIG. I.-Yield
of iodine and tellurium
from tin for protons
of different
energies.
The contribution of (CC,xn) reactions to the rr8Te formation cross-section in the energy interval 180-660 MeV can be estimated by assuming that (1) these reactions account for most of the l1sTe which is formed at energies 180 and 660 MeV; (2) over this energy interval the cross-section for llsTe production by (cc, xn) reactions changes linearly with proton energy (broken line in Fig. 1). By interpolating the energy dependence of the total cross-section for (tl, xn) reactions with tin in this way, one obtains an energy dependence which closely resembles that obtained for the formation cross-section of actinium in the proton bombardment of bismuth (full line in Fig 1). This similarity between the cross-section curves lends support to our interpretation of the experimental results. The interpolated value of the cross-section for ll*Te formation by (IX,xn) reactions at 480 MeV proton energy is 8 pbarns. DISCUSSION
Before we can compare our results with those of other authorso3) it is necessary first to estimate the total formation cross-section of tellurium. For this purpose, it is sufficient to multiply the llsTe formation cross-section by 5.* This procedure leads * In the work on the formation of actinium from bismuth, (1)it was found that the actinium was produced mainly by the (q 2n) and (a, 3n) reactions. Assuming that ‘rsTe is produced in the same way, and allowing (20 per cent), we obtain a factor of 5. for the abundance of the isotopes ?Sn and”%
Secondarynuclear reactions induced in tin by fast protons
245
to a total tellurium formation cross-section of 40 pbarns at E, = 480MeV, a value which is close to the formation cross-sections of gallium isotopes from topper(3) and of the isotopes 210At and “llAt from bismuth”) over a comparable proton energy interval (Table 2). It is interesting to compare the available secondary reaction cross-section data which have been brought together in Table 2. The experimental cross-sections given in this table were obtained at different proton energies; the proton energies in MeV being given by the values in brackets. In addition to this difference, there are a number of cases where some of the reaction products were not observed. Therefore, in order to compare the results of these experiments it is necessary not only to extrapolate them to a single incident proton energy but also to introduce a correction for the unobserved reaction products. In the last column but one of Table 2 these crosssections have been extrapolated to give cross-sections at E, = 480 MeV. In making this extrapolation, it was assumed that the yields for secondary reactions with copper depended on the proton energy in the same way as for secondary reactions with tin and bismuth. The correction factor which allows for the increase of the cross-section between 340 and 480 MeV was 1.0 for the gallium isotopes and 2.0 for germanium. A correction factor ~3 was applied to allow for the unobserved products resulting from x-particle capture by copper (the isotopes ‘j6Ga, 67Ga and @Ga are produced mainly by (a, 2n) and (R, 3~2)reactions with the 30 per cent abundant isotope ‘j5Cu). In the case of tin, as mentioned above, this correction factor was equal to 5. The total formation cross-section of actinium from lead was obtained by multiplying the 211At formation cross-section by 4. However, it would appear that the extrapolated actinium cross-section has been under-estimated because of the incomplete chemical separation et u/.(~) of zllAt in the work of KURCHATOV It will be seen from Table 2 that the a-particle capture cross-sections change very little for target nuclei having very different atomic numbers. The cross-sections for , the capture of lithium nuclei are roughly two orders of magnitude less than the M-capture cross-sections but they also are essentially independent of the atomic number of the target nucleus. This fact is rather unexpected. It is well known that the yields of the light nuclei sLi and ‘Be decrease rapidly as the atomic number of the target nucleus increases. (2,7) One would expect the secondary reaction cross-section for the capture of lithium nuclei to fall by a factor of 10 between copper and lead. Although the observed a-particle capture cross-sections can be explained satisfactorily by assuming that the x-particles are emitted as a result of the evaporation process, some other assumption is required to explain the reactions with lithium nuclei. MARQUEZand PERLMAN(~) analysed their results on the yield of iodine from tin by supposing that all the lithium nuclei were emitted with an energy of 80 MeV. Even so, their calculations led to 8Li and ‘Be formation cross-sections which were 500 times greater than those observed experimentally. Calculations similar to those of MARQUEZand PERLMANusing the iodine yields obtained from our experiments at E, = 340 MeV, gave a formation cross-section for lithium nuclei of about 0.5 mb. This is about 10 times greater than the formation cross-sections of 8Li from xenon(7) or of ‘Be from silveF at about the same incident proton energy. However, this cannot be regarded as a serious discrepancy because of the qualitative nature of the calculations and the fact that other lithium isotopes besides 8Li can take part in secondary reactions. The formation cross-section of
246
M. IA. KUZNET~~VA,V. N. MEKHE~~Vand V. A. KHALKIN
lithium nuclei having energies exceeding the Coulomb barrier can be estimated from the results of photographic emulsion studies of stars containing tracks of fragments. Experiments of this kind in which fragments with Z > 4 were observed have been made by LOZHKIN and PERFILOV@) using the synchrocyclotron. An analysis of these experiments leads to a lithium nucleus formation cross-section of 0.1 milli-barn, a value which agrees reasonably well with our calculated value. A calculation in which it was assumed that all the lithium nuclei were emitted with an energy of 40 MeV gave a lithium formation cross-section which was 15 times greater than that in which 80 MeV was assumed for the energy of these nuclei. An attempt was made to estimate the energy spectrumf(E) of the lithium nuclei from the yield of the iodine isotopes. This estimate was made using the following formula. *
This analysis showed that the calculated lithium formation cross-section depends sensitively upon the rate at which the spectrum is assumed to fall off with energy. Thus, for a spectrum extending to 80 MeV and falling off according to l/P, one finds aLi = 1 milli-barn. For a spectrum which falls off according to l/P, oLi N 20 millibarn. This value is considerably greater than the experimentally observed 8Li and ‘Be formation cross-sections. In making these estimates, the principle errors arise from the fact that the excitation function for reactions of the type (Li, xn) are not known. It was necessary to deduce an approximate excitation function by analogy with the excitation function for (a, xn) reactions. Because of the qualitative nature of these calculations it is not possible to come to a definite conclusion about the energy spectra of the nuclei causing the reactions. However, these estimates show that the observed secondary lithium capture crosssections can only be explained by supposing that the lithium nuclei are emitted with energies considerably exceeding those which could be acquired as a result of fission or evaporation of the target nucleus. It might be supposed that the fast fragments emitted in secondary reactions are due to collisions between the incident protons and sub-structural groups of nucleons in the nucleus formed as a result of density fluctuations in the nuclear matter. Such a possibility has been discussed in the literature several times before(8*9) but no definite theoretical conclusions have been reached. We believe that, as with other problems of this kind, further work on secondary reactions will help to elucidate the nature of the reaction mechanism. a&hors thank B. V. KTJRCHATOV, V. G. S~LOVEVand I. Iu. LEVENBERG for their assistancein the work and also V. P. DZHELEPOV, M. G. MFSHCHERIAKOV and G. A. LEKSIN for valuable discussion.
AcknowleGqements-The
REFERENCES 1. KIJRCHATOV B. V., MEKHEDOV V. N., KUZNETCWVA M. IA., KIJRCHAT~VA L. I. and B~RI~OVAN. I.,
Summary Report of the Joint Institute of Nuclear Studies, No. 633 (1951). 2. MARQUEZL. and PERLMANI., Phys. Rev. 81,953 (1951). 3. BATZEL R. E., MILLER D. R. and SEABORGL. T., Phys. Rev. 84, 671 (1951). * In this formula, B is the yield of the capture products, n is the number of tin m&i per ems; -dE/dx is the ionization energy loss suffered by the lithium nuclei; and I$, is the lower energy limit in the excitation function u(E) for reactions in which lithium nuclei are captured by tin.
Secondary nuclear reactions induced in tin by fast protons
241
4. T~RNEVICHA. and SUGARMANN., Phys. Rev. 94,128 (1954). 5. VIN~CRAD~VA. P., ALIMARINI. P., BARANOVV. I., LAVRUKHINAA. N., BARANOVAT. V. and PAVLO~KAIAF. I., Conference of the U.S.S.R. Academj of Sciences on the Peaceful Uses of Atomic Energy, Chemical Sciences Division. Consultants Bureau, New York (1955). 6. KURCHATOV B. V., MEKHED~V V. N., KURCHATOVAL. I., KUZNETX~JA M. IA. and KUZNET~OVA L. V., Report of the Joint Institute of Nuclear Studies No. 258 (1953). 7. WRIGHTS. C., Phys. Rev. 79, 838 (1950). 8. L~ZHKIN 0. V. and PERFILOV N. A., Zh. eksp. teor.jiz. 31,913 (1956). 9. BLOKHINTSEV D. I., Achievements of Physical Sciences 61, 137 (1957).
&NOiLOXd ,LSViI A8 NIL NI SNOIiLL3VBX XVTl3flN AlIvaNoxS
Secondary nuclear reactions induced in tin by fast protons
241
z*Na. A cross-section of 10 milli-barn was assumed for this reaction over the range of proton energies studied. Iodine separation After the irradiation, the sample was dissolved in hot concentrated nitric acid containing potassium persulphate, 30 mg iodine (KIO,) and 50 mg tellurium (nitric acid solution). The solution was diluted to a volume of 80-100 m 1 and the iodate was reduced to the iodide by sodium sulphite. The iodine was successively distilled off through two absorbing vessels, the first of which was filled with a 1 M solution of HNO, and the second with a 1M solution of NaOH. The elementary iodine was then twice extracted with chloroform. For the activity measurements, targets of PdI, were prepared. The chemical yield of the carrier was usually 60-70 per cent. Special tests showed that no loss of radio-iodine occurred as a result of sorption in the tin dioxide formed when metallic tin is dissolved in HNOa. Tellurium separation After distilling off the iodine, the contents of the distillation flask were evaporated and the sediment of tin dioxide was dissolved in concentrated hydrochloric acid. The excess HNO, was removed and tellurium was precipitated by stannous chloride. The precipitate was centrifuged and it was dissolved again in a mixture of hydrochloric and nitric acids. Carriers (selenium, antimony, arsenic, copper etc) were added to 1-2 mg of this solution which was then evaporated to dryness. The selenium, arsenic, tin and antimony were removed by twice evaporating the solution with 3 ml of HBr, traces of HNO, being removed at the same time. The residue was dissolved in a 3 M solution of HCl and the tellurium was precipitated from the boiling solution by sulphur dioxide. The evaporation of the solution with HBr and the precipitation of tellurium by sulphur dioxide was repeated twice. In order to remove noble metal impurities, the tellurium was distilled in a stream of hydrogen at a temperature of 800-900°C. The condensate was washed off with nitric acid and evaporated with HCl and HBr. Metallic tellurium was then precipitated by sulphur dioxide and deposited on a target. The chemical yield was 20-40 per cent. Radioactioity measurements The measurements of the activity of the samples were made using an end window counter having a mica window of thickness -3 mg/cm2. The following iodine isotopes were observed : T(/?, K, T = 13 day); ltiI(/3, K, T = 4.5 day); rBI(K, T = 13 hr); i21T(p, K, T = 1.8 hr); 1201(p,T = 30 min). We found K capture branching ratios for 1261,1241,and 1211equal to 50, ~60 and -60 per cent respectively. An account of this work will be published later. For tellurium, a single half-life of about 6 days was observed. This activity was ascribed to ii8Te (K, T = 6 day) and it can be detected through the decay of its daughter nucleus ‘lsSb (/?, T = 3.5 min, EB = 3.1 MeV). Owing to the fact that the half-life of llgTe (K, T = 4.5 days) is close to that of llsTe and because of the low X-ray sensitivity of the counter, it was not possible to determine the llgTe contribution from the decay curves. The 2.5 hour activity of l17Te was not studied. 16
\
242
M.‘IA.
KUZNETSOVA, V. N. MEKHED~V and V. A.
KHALKIN
Experimental results The formation cross-sections of those reaction products identified as having nuclear charges 2 or 3 units greater than that of the target nuclei are tabulated in Table 1 for a number of proton energies. The Table also gives the mean arithmetic errors derived from the results of three experiments. The determination of the yields of the individual products by analysing the decay curves was rather difficult because of the low activity of the samples and the large number of isotopes which were present. We believe that the lzlI formation cross-section is completely reliable but the total cross-section may be somewhat over-estimated because the internal conversion electrons from lBI were also counted .* It would seem that the absolute error of our total cross-section determination cannot be very large because our results are in agreement with those of KURCHATOVet al.,@) which were obtained by a different method. Moreover, our formation cross-sections are close to the formation crosssections of the germanium isotopes and of 211Atproduced in the proton bombardment of copper and leadts) (see Table 2). It follows from our results that the secondary reaction cross-sections for the capture of lithium by tin at 340 MeV incident proton energy are 50 times less than the values obtained by MARQUEZand PERLMAN.(~) Furthermore, although we observed the same iodine isotopes we found relative yields which were very different from those reported by these authors. It has been suggested that our results underestimate the secondary reaction crosssections because of the loss of iodine caused by target heating in the synchrocyclotron vacuum chamber. This suggestion was shown to be incorrect by making some control experiments. Samples of tin were sealed up inside a thin glass ampoule and irradiated. From these experiments we obtained iodine formation cross-sections which were identical with those presented in Table 1. The contribution to the radio-iodine production of likely sample contaminants was also investigated and shown to be negligible. ‘For example, by separating barium (Z = 56), it was shown that only l/25 part of the observed iodine activity could be accounted for by spallation products. The change in the iodine formation cross-section with increasing proton energy is shown by the bottom curve of the Figure 1. The increase in the rate of rise of the cross-section which occurs for proton energies above 400 MeV is caused mainly by an increase in the yield of the lighter iodine isotopes. This can be explained by supposing that these isotopes are produced in fast proton capture reactions in which the subsequent emission of two r-mesons and several neutrons occurs. Unfortunately, the experimental data is insufficient to allow an estimate of the contribution of this reaction to be made, nor can the energy region over which it takes place be specified. From the secondary a-particle capture cross-sections only the ll*Te formation cross-section was determined (see Table 1). This cross-section rises to a maximum for protons in the energy interval 340-500 MeV and then falls again (see Fig. 1). A very similar behaviour was observed in the formation of light actinium isotopes from bismuth(l) and in the formation of 6sGa from copper.(5) This rather odd energy dependence can be explained most simply by supposing that an additional reaction takes place in which IlaTe is formed by the capture of a fast proton followed by the * The internal conversion electrons were taken into account when computing the Is11 formation crosssection. The X-ray efficiency of the end window counter was found to be 2 per cent by using a standard sample of’s61 (K, 2’= 60 day). Other measurements with a cylindrical aluminium walled counter confirmed the accuracy of the procedure used in analysing the decay curves. leaI was not detected in these experiments but the yields of the other isotopes were in agreement with the end window counter results.
/
/
6 day 30 min 1.8 hr 13hr 4,5 day 13 day
Half-life
Capture of lithium nuclei
Capture of cc-particles
Type of reaction
’
!
3.6 0.02 0.02 0.11
* 1.0 I- 0.01 ! 0035 I; 0.08
170MeV I
, j
044 + 0.084
16.5 & 1.5 0.03 f 0.01 0.067 I 0.003 0.3 * 0.07 0.024 0.02
340 MeV
/
(
I
I
ssGe i- 88Ge + 88Ge
cu Sn Pb =“At
laOIi 1211 __f1231 + 1241 + IZil
GBGa $- s7Ga i- @Ga *leTe 210At + 9”At
cu
Sn Bi
Reaction products
_
’
BY COPPER,TlN, BISMUTH
Target element
I
480 MeV
LEAD
.~ 0.3 (340) 0.9 (480) 0.04 - 0.08
26 (340) 8 (480) 80 (480)
/
1
0.51
(0.11
I
5.0+ 0.05 0.17 0.18
340 MeV
’
/ /
0.6 0.9 0.16 -- 0.32
90 40 80
-
(3) This work (1)
(3) This work (1)
Reference
1-24
0.45 0.38 0.41
11
480 MeV
Reference(B’
cross-section (1O-3o cm%)
/ Extrapolated
I
1 ,
LITHIUMNUCLEI
0.2 0.007 0.2 0.008 0.01 0.5
t: 1.3 .! f f f f i
CL-PARTICLESAND
10 0.27 0.24 0.97 0.06 0.06 1.6
Experimental cross-section (10-Y cm2)
AND
i
1
660 MeV
TELLURIUMANDIODINEISOTOPFS
14.5 f 7.7 0.10 f 0.01 0.15 f 0.03 0.56 :k 0.16 0.035 0.048 i 0.006 0’9 f 0.2
This work
TABLE~.--SECONDARYREACTIONCROSS-SECTIONSFORTHECAPTUREOP
Total iodine cross-section
lisTe 1ZOI lZlI 123I WI l%I
Isotope
/ / ’
l.-CROSS-SECIIONSFORTKEFORMATIONOF
INTHEPROT~NBOMBARDMENT~F.~NAT~AR~~~~PROTONENEROIES(~BARN)
TABLE
244
M. IA. KUZNETSOVA, V. N. MEKHEDOV and V. A. KHALKIN
emission of a r-meson and some neutrons. It is difficult to explain the form of the curve in terms of a change in the spectrum and emission probability of the a-particles because such an explanation will not at the same time explain why the curve falls again in the high energy region. A
:!O-
15-
E
10-
Y
A
b’
Actinium
5-
Iodine
// 0_ 0
200
I
I
500
600
t$5
MA’
FIG. I.-Yield
of iodine and tellurium
from tin for protons
of different
energies.
The contribution of (CC,xn) reactions to the rr8Te formation cross-section in the energy interval 180-660 MeV can be estimated by assuming that (1) these reactions account for most of the l1sTe which is formed at energies 180 and 660 MeV; (2) over this energy interval the cross-section for llsTe production by (cc, xn) reactions changes linearly with proton energy (broken line in Fig. 1). By interpolating the energy dependence of the total cross-section for (tl, xn) reactions with tin in this way, one obtains an energy dependence which closely resembles that obtained for the formation cross-section of actinium in the proton bombardment of bismuth (full line in Fig 1). This similarity between the cross-section curves lends support to our interpretation of the experimental results. The interpolated value of the cross-section for ll*Te formation by (IX,xn) reactions at 480 MeV proton energy is 8 pbarns. DISCUSSION
Before we can compare our results with those of other authorso3) it is necessary first to estimate the total formation cross-section of tellurium. For this purpose, it is sufficient to multiply the llsTe formation cross-section by 5.* This procedure leads * In the work on the formation of actinium from bismuth, (1)it was found that the actinium was produced mainly by the (q 2n) and (a, 3n) reactions. Assuming that ‘rsTe is produced in the same way, and allowing (20 per cent), we obtain a factor of 5. for the abundance of the isotopes ?Sn and”%
Secondarynuclear reactions induced in tin by fast protons
245
to a total tellurium formation cross-section of 40 pbarns at E, = 480MeV, a value which is close to the formation cross-sections of gallium isotopes from topper(3) and of the isotopes 210At and “llAt from bismuth”) over a comparable proton energy interval (Table 2). It is interesting to compare the available secondary reaction cross-section data which have been brought together in Table 2. The experimental cross-sections given in this table were obtained at different proton energies; the proton energies in MeV being given by the values in brackets. In addition to this difference, there are a number of cases where some of the reaction products were not observed. Therefore, in order to compare the results of these experiments it is necessary not only to extrapolate them to a single incident proton energy but also to introduce a correction for the unobserved reaction products. In the last column but one of Table 2 these crosssections have been extrapolated to give cross-sections at E, = 480 MeV. In making this extrapolation, it was assumed that the yields for secondary reactions with copper depended on the proton energy in the same way as for secondary reactions with tin and bismuth. The correction factor which allows for the increase of the cross-section between 340 and 480 MeV was 1.0 for the gallium isotopes and 2.0 for germanium. A correction factor ~3 was applied to allow for the unobserved products resulting from x-particle capture by copper (the isotopes ‘j6Ga, 67Ga and @Ga are produced mainly by (a, 2n) and (R, 3~2)reactions with the 30 per cent abundant isotope ‘j5Cu). In the case of tin, as mentioned above, this correction factor was equal to 5. The total formation cross-section of actinium from lead was obtained by multiplying the 211At formation cross-section by 4. However, it would appear that the extrapolated actinium cross-section has been under-estimated because of the incomplete chemical separation et u/.(~) of zllAt in the work of KURCHATOV It will be seen from Table 2 that the a-particle capture cross-sections change very little for target nuclei having very different atomic numbers. The cross-sections for , the capture of lithium nuclei are roughly two orders of magnitude less than the M-capture cross-sections but they also are essentially independent of the atomic number of the target nucleus. This fact is rather unexpected. It is well known that the yields of the light nuclei sLi and ‘Be decrease rapidly as the atomic number of the target nucleus increases. (2,7) One would expect the secondary reaction cross-section for the capture of lithium nuclei to fall by a factor of 10 between copper and lead. Although the observed a-particle capture cross-sections can be explained satisfactorily by assuming that the x-particles are emitted as a result of the evaporation process, some other assumption is required to explain the reactions with lithium nuclei. MARQUEZand PERLMAN(~) analysed their results on the yield of iodine from tin by supposing that all the lithium nuclei were emitted with an energy of 80 MeV. Even so, their calculations led to 8Li and ‘Be formation cross-sections which were 500 times greater than those observed experimentally. Calculations similar to those of MARQUEZand PERLMANusing the iodine yields obtained from our experiments at E, = 340 MeV, gave a formation cross-section for lithium nuclei of about 0.5 mb. This is about 10 times greater than the formation cross-sections of 8Li from xenon(7) or of ‘Be from silveF at about the same incident proton energy. However, this cannot be regarded as a serious discrepancy because of the qualitative nature of the calculations and the fact that other lithium isotopes besides 8Li can take part in secondary reactions. The formation cross-section of
246
M. IA. KUZNET~~VA,V. N. MEKHE~~Vand V. A. KHALKIN
lithium nuclei having energies exceeding the Coulomb barrier can be estimated from the results of photographic emulsion studies of stars containing tracks of fragments. Experiments of this kind in which fragments with Z > 4 were observed have been made by LOZHKIN and PERFILOV@) using the synchrocyclotron. An analysis of these experiments leads to a lithium nucleus formation cross-section of 0.1 milli-barn, a value which agrees reasonably well with our calculated value. A calculation in which it was assumed that all the lithium nuclei were emitted with an energy of 40 MeV gave a lithium formation cross-section which was 15 times greater than that in which 80 MeV was assumed for the energy of these nuclei. An attempt was made to estimate the energy spectrumf(E) of the lithium nuclei from the yield of the iodine isotopes. This estimate was made using the following formula. *
This analysis showed that the calculated lithium formation cross-section depends sensitively upon the rate at which the spectrum is assumed to fall off with energy. Thus, for a spectrum extending to 80 MeV and falling off according to l/P, one finds aLi = 1 milli-barn. For a spectrum which falls off according to l/P, oLi N 20 millibarn. This value is considerably greater than the experimentally observed 8Li and ‘Be formation cross-sections. In making these estimates, the principle errors arise from the fact that the excitation function for reactions of the type (Li, xn) are not known. It was necessary to deduce an approximate excitation function by analogy with the excitation function for (a, xn) reactions. Because of the qualitative nature of these calculations it is not possible to come to a definite conclusion about the energy spectra of the nuclei causing the reactions. However, these estimates show that the observed secondary lithium capture crosssections can only be explained by supposing that the lithium nuclei are emitted with energies considerably exceeding those which could be acquired as a result of fission or evaporation of the target nucleus. It might be supposed that the fast fragments emitted in secondary reactions are due to collisions between the incident protons and sub-structural groups of nucleons in the nucleus formed as a result of density fluctuations in the nuclear matter. Such a possibility has been discussed in the literature several times before(8*9) but no definite theoretical conclusions have been reached. We believe that, as with other problems of this kind, further work on secondary reactions will help to elucidate the nature of the reaction mechanism. a&hors thank B. V. KTJRCHATOV, V. G. S~LOVEVand I. Iu. LEVENBERG for their assistancein the work and also V. P. DZHELEPOV, M. G. MFSHCHERIAKOV and G. A. LEKSIN for valuable discussion.
AcknowleGqements-The
REFERENCES 1. KIJRCHATOV B. V., MEKHEDOV V. N., KUZNETCWVA M. IA., KIJRCHAT~VA L. I. and B~RI~OVAN. I.,
Summary Report of the Joint Institute of Nuclear Studies, No. 633 (1951). 2. MARQUEZL. and PERLMANI., Phys. Rev. 81,953 (1951). 3. BATZEL R. E., MILLER D. R. and SEABORGL. T., Phys. Rev. 84, 671 (1951). * In this formula, B is the yield of the capture products, n is the number of tin m&i per ems; -dE/dx is the ionization energy loss suffered by the lithium nuclei; and I$, is the lower energy limit in the excitation function u(E) for reactions in which lithium nuclei are captured by tin.
Secondary nuclear reactions induced in tin by fast protons
241
4. T~RNEVICHA. and SUGARMANN., Phys. Rev. 94,128 (1954). 5. VIN~CRAD~VA. P., ALIMARINI. P., BARANOVV. I., LAVRUKHINAA. N., BARANOVAT. V. and PAVLO~KAIAF. I., Conference of the U.S.S.R. Academj of Sciences on the Peaceful Uses of Atomic Energy, Chemical Sciences Division. Consultants Bureau, New York (1955). 6. KURCHATOV B. V., MEKHED~V V. N., KURCHATOVAL. I., KUZNETX~JA M. IA. and KUZNET~OVA L. V., Report of the Joint Institute of Nuclear Studies No. 258 (1953). 7. WRIGHTS. C., Phys. Rev. 79, 838 (1950). 8. L~ZHKIN 0. V. and PERFILOV N. A., Zh. eksp. teor.jiz. 31,913 (1956). 9. BLOKHINTSEV D. I., Achievements of Physical Sciences 61, 137 (1957).