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Chemical state of nitrogen-13 formed by the (n, 2n) reaction in solid nitrogen compounds
J. lnorsani¢ and Nuclear Chemistry. 1935. VoL 1. Pp. 296-300. Purllamon Pres8 Ltd.. London
CHEMICAL STATE OF NITROGEN-13 FORMED BY THE (n, 2n) REACTION IN SOLID NITROGEN COMPOUNDS By R. D. SMITH* a n d A . H. W . ATEN, JUN. lnstituut veer Kernphysisch Onderzoek, Ooster Ringdijk 18, Amsterdam-O.
(Receiced 22 April 1955; in final form 21 June 1955) Abstract--The distribution of N xs over different chemical states after irradiation with fast neutrons is investigated for NaNOs, NaNO3, NH,NO3, KNOs, and LiNes. INTRODUCTION WHEN nitrogen a t o m s a r e e x p o s e d to a flux o f fast neutrons, N 14 is partly, b u t o n l y for a very small fraction, c o n v e r t e d to N la as a result o f a n n,2n-reaction. T h e half-life o f the p r o d u c t is 10"1 min, a n d the r a d i a t i o n e m i t t e d consists o f p o s i t r o n s . T h e nuclear process causes a c h a n g e in the c h e m i c a l state o f the n i t r o g e n a t o m s involved, similar to the S z i l a r d - C h a l m e r s effect in the case o f the n,y-process. A s t u d y o f the c h e m i c a l f o r m o f N la in the i r r a d i a t e d m a t e r i a l s h o u l d p r o v i d e s o m e i n f o r m a t i o n c o n c e r n i n g the c h e m i c a l reactions which t a k e p l a c e between highly energetic n i t r o g e n ions a n d the target material. W e have investigated the c h e m i c a l state o f N la after fast n e u t r o n i r r a d i a t i o n o f several salts c o n t a i n i n g nitrogen. EXPERIMENTAL The salts were of analytical reagent purity, except for the lithium nitrate. This was made from lithium carbonate, the product being melted in vacuo, and dried over phosphorus pentoxide. The nitrate content was 98 per cent of the theoretical, and the discrepancy was presumably due to water. The salts were powdered and (except for the lithium nitrate) r,ir-dried at 110°. They were exposed at the ordinary ambient temperature of the cyclotron to the neutrons emitted from a beryllium target bombarded with deuterons of energy 26 MeV. The N 1. in the irradiated material and in the analytical products was determined either by means of a thin window counter using thin layers of solid or by means of a liquid counter, which registered the annihilation radiation from solutions. In the former case corrections were applied for selfabsorption. '1~ Activities induced in oxygen and sodium in tl~e salts differed sufficiently in half-life not to interfere with the N ~s determination, but a strong CP 8 activity induced on bombardment of potassium nitrate had to be removed by precipitating with silver nitrate. Experiments with the ammonium halides failed because of the difficulty of removing the activities induced in the halogens. The proportion of N ta activity in the ammonium ion was found by precipitating a sample of ammonium hydrogen tartrate. Nitrate was isolated as nitron nitrate, after removal of nitrite with hydrazine sulphate, and nitrite as silver nitrite precipitated slowly from the hot dilute solution. Carriers and hold-back carriers were employed in the usual way. The proportion of N ~s in gaseous f o r m s was measured by comparing the activities of two solutions, one of which had been made up in a stoppered tube without loss of gas, and the other of which had been boiled or bubbled through with nitrogen until there was no further loss in activity. In the sodium and ammonium salts, the fraction of the total N 1. activity in any particular chemical form could be reproduced to within a few percent of the total activity. In the case of lithium nitrate, however, the reproducibility was much poorer. All experiments, except a few concerning ammonium nitrate, were performed at least in duplicate. The results are quoted as the range of values actually * Present address: I.C.I., Nobel Division, Stevenston, Ayrshire. ~x~A. H. W. AT~N,JU'N.N,,cleonicJ 6, No. 1, 68 (1950). 296
Chemical state of nitrogen-13 formed by the (n, 2n) reaction in solid nitrogen compounds
297
found; where only one figure is quoted, only one determination was made. Experimental errors cause the totals to differ from 100 per cent since the analysis was not done by difference. RESULTS The values found in the main experiments are given in the table. TABLE--PAR'r OF TOTAL N13-AC'rlvtrv FOUND IN I~IWEREN-r FRACTIONS OF V A R I O U S N I T R O G E N C O M P O U N D S
NO3-
Compound
NOz-
!i
NH4 +
i NaNO3 NaNO2 NH4NO3 LiNOa
0.44 0.02 0.44 0"51
± 0'02 ::_ 0.01 ~ 0.01 ± 0'04
0.46 0.45 0.01 0.47
×~ 0.01 - 0.01 ± 0.01 :- 0.08
Recrystallized material 0.52 ~ 0.02
I 0.03 - 0.00 I
0.43 - 0.01
Gases
0.07 0.58 0.57 0.07
~-~0.02 z 0.02 ~ 0.02 .- 0.00
Attempts, which were only partially successful, were made to extend this series in different directions. When KNOa was irradiated, the total activity of the N TMcould not be determined, owing to the large quantity of CP 8 which was formed. We did, however, determine the ratio [N~aO3-]/[N13Oz-], for which we found 0'41 ~ 0-04. We repeated the experiment with a sample recrystallized from water through which hydrogen was bubbled, and found 0"39 ~- 0.05 for this ratio. With a sample similarly treated with oxygen we found 0-29 ~ 0.02. It would have been very interesting to obtain similar data on the distribution of N 13 in ammonium halides. With these compounds we were, however, unable to measure the total nitrogen activity owing to the presence of strong radio-halogen periods, although it was possible to detect N13H~' among the products. Special attention was paid to the case of the ammonium nitrate. It was ascertained that the fraction of the activity found as gases was essentially unchanged if the ammonium nitrate had been recrystallized in the presence of hydrogen (0.50) or in the presence of oxygen (0.52 : 0.02) (these samples had been dried at 120 ~ after recrystallization). We also made sure that the fraction of the activity retained in the ammonium nitrate on recrystallization was :not notably affected if the irradiated crystals were dissolved in 0.1 N sulphuric acid (0.44 ! 0-01), dilute sodium hydroxide (0.42), or a solution of potassium permanganate (0.43). An attempt was made to determine the constituents of the active gas fraction. Oxidizable forms were sought by adding a little acidified permanganate solution before boiling to remove dissolved gases. The permanganate had no effect on the retention of activit), by the solution, showing that there could be no appreciable amount of NO or NO2 originally anaong the active dissolved gases: free NO~ is in any case unlikely at very low concentrations. Similarly, the loss in activity was no less when nitrogen was passed through a cold mixed solution of the irradiated ammonium nitrate and concentrated ferrous sulphate than when it was passed through the simple solution of the nitrate in water. Molecular nitrogen and nitrous oxide are the only two remaining likely constituents of the active gas. Nitrous oxide was sought by dissolving the irradiated amlnonium nitrate in water saturated with nitrous oxide as carrier, and then drawing nitrogen through until all gaseous activity had been swept out of the solution and through a liquid air-trap, where the nitrous oxide condensed. The contents of this trap were later allowed to evaporate into one of the stoppered tubes generally used for liquid counting. 8 per cent of the total N ~3activity was isolated in this way. Although separation of nitrous oxide was probably not complete, this figure does suggest that a considerable part of the active products consists of nitrous oxide inside ammonium nitrate crystals. DISCUSSION W i t h m o s t c o m p o u n d s t h e r a d i o c h e m i c a l s e p a r a t i o n s e e m s t o give s a t i s f a c t o r y results. I n t h i s c o n n e c t i o n it s h o u l d b e r e a l i z e d t h a t in t h e c a s e o f s o d i u m n i t r a t e t h e r e c r y s t a l l i z e d f r a c t i o n w o u l d b e e x p e c t e d t o give t h e N O a - - a c t i v i t y , a n d in t h e case o f the ammonium nitrate the sum of the NOa--activity and the NH~+-activity.
298
R . D . S t a r k a n d A. H. W. ATtar, JUN.
The fact that in all irradiated compounds the sum of the activities of nitrate, nitrite, and the gaseous compounds amounts to about 100 per cent indicates that nitrate and nitrite are the only non-volatile reaction products which are important. With lithium nitrate the reproducibility is not nearly so good as it is in the other cases. A possible explanation might be found in the extremely hygroscopic nature of this substance, but the fact that it was handled in a dry box detracts from the weight of this consideration. In view of the fact that the chemical state of radio-sulphur formed by fast neutron irradiatien of potassium chloride is strongly dependent ~2) on the presence of oxygen in the crystal lattice, it seemed worth while to test for a similar influence in our experiments. Our results with ammonium nitrate indicated that with N xs such an influence could not be clearly recognized. However, the ratio [NxaO3-]/[NlaOs-], which was about unity in NaNO s and iNO 3, proved to be appreciably lower in KNOs, so we had to reconsider the possioility of such an oxidation in the crystal. Although for the latter salt we were unable to exclude wholly such an ~nterference (though we are not certain that the difference actually exceeds the possible error), we consider that the [NlSOs-]/[NlSOs-] ratio is definitely lower in this compound than it is in the two other alkali nitrates investigated. An even more serious question was whether the valences found for N xa are uniquely determined by the processes which take place in the crystal, or whether the final distribution is influenced by processes taking place during solution, as seems to be the case with neutron-irradiated KMnO4. cs~ For this reason the experiments were undertaken in which irradiated ammonium nitrate was dissolved in thepresence of H +-, OH--, or MnO4--ions. The fact that these additions did not influence the activity distribution makes it almost certain that the final form in which the N~3-atoms will be found is already determined entirely in the crystal. This does not, of course, mean that radioactive nitrate and nitrite ions are necessarily present in the solid as such. It is not impossible that they are formed by hydration of radioactive nitrogen atoms or oxides carrying the appropriate charge at the moment these go into solution. Neutrons must have an energy around '10 MeV to form N ~a from N x* by the n,2n-process. Under these circumstances they have also an excess energy, which is on the average of the order of 1 MeV. Most of this excess energy will be dissipated as kinetic energy of the two neutrons formed, but an appreciable fraction will turn up as recoil energy of the NXS-atoms. This energy not only enables the NXa-atoms to break their chemical bonds, but also to strip offsome of their electrons and to adopt a positive charge. Similar circumstances occur in n,p- and n,~-processes, t*~ For this reason it is to be expected that if different nitrogen compounds are irradiated with fast neutrons the primary condition of N ~a will be a highly oxidized one, and presumably one which depends very little on the nature of the irradiated substance. This state of affairs does not, of course, determine by itself the results of the chemical analysis, as the highly charged positive N ~a will be able to react chemically with the components of the crystal lattice after it has lost most of its excess energy. The activity measurement indicates therefore the results of the chemical processes which have taken place between N xaatoms or ions in essentially similar conditions and the ions which make up the different solids. ~s) W. S. KOSKI, J. Amer. Chem. Soc. 70, 4251 (1948).
W. F. Liner, J. Amer. Chem. See. 62, 1930 (1940). t*~ A. H. W. ATEN, /UN., Recueil Tray. CAlm. Pays-Bus 61,467 (1942); Phys. Rev. 71,641 (1947). csJ
Chemical state of nltrogem-13 formed by the (n, 2n) reaction in solid nitrogen compounds 299 It is to be expected that the composition and the structure of the target material will exert a strong influence on the activity distribution. A very striking example is presented by the difference between NaNO s and NaNO~. The lack of radioactive NOa--ions in the latter solid may perhaps be taken as an argument for the assumption that the complex radioactive anions, or at least the nitrate ions, are already formed in the crystal lattice before the material is dissolved, but it certainly does not definitely disprove the opposite. In any case this difference shows clearly the influence of the target material on the chemical form of the reaction products, in agreement with the observations made with n,p- and n,~-processes. ~4) As KNO3 gives results which are quite different from those found with NaNO3 and LiNO3, it is clear that even the cation can make a difference. It is worth pointing out that, in all the materials studied, an important fraction of the N 13 is found in an oxidized state, as is to be expected if the N 13 passes through a stage in which it is positively charged. It should be kept in mind, however, that in the ordinary Szilard-Chalmers process the n,y-reaction sometimes leads to a similar situation, as in the case of the iodates. ~5~ We want to make it quite clear that, although the N ~3is found in an oxidized state to an important degree, the three alkali nitrates and sodium nitrite all show a partial loss of oxygen during the nuclear reaction, relative to the combination state of nitrogen in the substrate. The only compound which does not suffer a reduction during activation is ammonium nitrate. This substance constitutes a more complicated case, for the medium can exert both an oxidizing and a reducing influence on the N la. It should be kept in mind that in this case secondary reactions may be possible with radioactive ions in an excited state. If an intermediate formation of active NO 2 -ions has occurred, these have disappeared by the reaction NO 2 q - N H a + - ~ 2H20 q - N z before the energy liberated by the nuclear process has been dissipated. Similarly, excited radioactive ions of NH4+ and of NO3-- may have been lost by the reaction NO 3- + NH4 * --~ 2H20 q-N20. The uncertainty of the determination of N20 in the reaction products makes it impossible to reach quantitative conclusions concerning these possibilities. It must be stated clearly, however, that the radioactive gases N 2 and N~O may very well have been formed "directly," that is, from a mixture of highly energetic atoms (one of which was radioactive) without passing through an intermediate stage in which they had the composition of stable ions or compounds. The abundance of NO a- relative to NH~+ in the active products from ammonium nitrate is noteworthy. It suggests, without proving it, that NO a- is indeed a more favoured first product of the reaction of energetic N 13 atoms with the substrate, for if the relative lack of N~ZHa+ is to be ascribed solely to the secondary reaction postulated above, it is difficult tO explain why the N~303- has not disappeared to a corresponding degree. In any case, by making the extreme assumption that all the active gases are the reaction products of active ammonium ions, one is led to the conclusion that the original concentration of the latter cannot have appreciably exceeded the combined concentrations of active nitrate and nitrite ions. Discussion of a more quantitative nature, e.g., of the difference between the [NlaO3-]/[N~302-] ratios in NaNO a and LiNOa on the one hand and KNO a on the other, is at present hardly possible. InterpretatioTi of the observations on ammonium nitrate would have been greatly helped by a knowledge of the processes in ammonium ts~ R. E. CLEARY,W. S . HAM1LL, and R. R. WILLIAMS,J. Amer. Chem. Soc. 74, 4675 (1952).
300
R . D . SMrm and A. H. ATEN, JUN.
salts with stable anions, but experiments with this aim were unsuccessful for the reasons given above. ACKNOWLEDGEMENTS The authors wish to thank Miss A. C. KLOPPER foLcarrying out some of the analyses. We also wish to express our thanks to the personnel of the Philips' cyclotron for the irradiations. This investigation forms part of the Research Programme of the Foundation for Fundamental Research of Matter (F.O.M.). It was carried out with financial aid from the Netherlands Organization for Pure Research (Z.W.O.). Grateful acknowledgement is also made to Imperial Chemical Industries Limited (Nobel Division) for allowing one of the authors (R. D. S.) to take part in this work.
CHEMICAL STATE OF NITROGEN-13 FORMED BY THE (n, 2n) REACTION IN SOLID NITROGEN COMPOUNDS By R. D. SMITH* a n d A . H. W . ATEN, JUN. lnstituut veer Kernphysisch Onderzoek, Ooster Ringdijk 18, Amsterdam-O.
(Receiced 22 April 1955; in final form 21 June 1955) Abstract--The distribution of N xs over different chemical states after irradiation with fast neutrons is investigated for NaNOs, NaNO3, NH,NO3, KNOs, and LiNes. INTRODUCTION WHEN nitrogen a t o m s a r e e x p o s e d to a flux o f fast neutrons, N 14 is partly, b u t o n l y for a very small fraction, c o n v e r t e d to N la as a result o f a n n,2n-reaction. T h e half-life o f the p r o d u c t is 10"1 min, a n d the r a d i a t i o n e m i t t e d consists o f p o s i t r o n s . T h e nuclear process causes a c h a n g e in the c h e m i c a l state o f the n i t r o g e n a t o m s involved, similar to the S z i l a r d - C h a l m e r s effect in the case o f the n,y-process. A s t u d y o f the c h e m i c a l f o r m o f N la in the i r r a d i a t e d m a t e r i a l s h o u l d p r o v i d e s o m e i n f o r m a t i o n c o n c e r n i n g the c h e m i c a l reactions which t a k e p l a c e between highly energetic n i t r o g e n ions a n d the target material. W e have investigated the c h e m i c a l state o f N la after fast n e u t r o n i r r a d i a t i o n o f several salts c o n t a i n i n g nitrogen. EXPERIMENTAL The salts were of analytical reagent purity, except for the lithium nitrate. This was made from lithium carbonate, the product being melted in vacuo, and dried over phosphorus pentoxide. The nitrate content was 98 per cent of the theoretical, and the discrepancy was presumably due to water. The salts were powdered and (except for the lithium nitrate) r,ir-dried at 110°. They were exposed at the ordinary ambient temperature of the cyclotron to the neutrons emitted from a beryllium target bombarded with deuterons of energy 26 MeV. The N 1. in the irradiated material and in the analytical products was determined either by means of a thin window counter using thin layers of solid or by means of a liquid counter, which registered the annihilation radiation from solutions. In the former case corrections were applied for selfabsorption. '1~ Activities induced in oxygen and sodium in tl~e salts differed sufficiently in half-life not to interfere with the N ~s determination, but a strong CP 8 activity induced on bombardment of potassium nitrate had to be removed by precipitating with silver nitrate. Experiments with the ammonium halides failed because of the difficulty of removing the activities induced in the halogens. The proportion of N ta activity in the ammonium ion was found by precipitating a sample of ammonium hydrogen tartrate. Nitrate was isolated as nitron nitrate, after removal of nitrite with hydrazine sulphate, and nitrite as silver nitrite precipitated slowly from the hot dilute solution. Carriers and hold-back carriers were employed in the usual way. The proportion of N ~s in gaseous f o r m s was measured by comparing the activities of two solutions, one of which had been made up in a stoppered tube without loss of gas, and the other of which had been boiled or bubbled through with nitrogen until there was no further loss in activity. In the sodium and ammonium salts, the fraction of the total N 1. activity in any particular chemical form could be reproduced to within a few percent of the total activity. In the case of lithium nitrate, however, the reproducibility was much poorer. All experiments, except a few concerning ammonium nitrate, were performed at least in duplicate. The results are quoted as the range of values actually * Present address: I.C.I., Nobel Division, Stevenston, Ayrshire. ~x~A. H. W. AT~N,JU'N.N,,cleonicJ 6, No. 1, 68 (1950). 296
Chemical state of nitrogen-13 formed by the (n, 2n) reaction in solid nitrogen compounds
297
found; where only one figure is quoted, only one determination was made. Experimental errors cause the totals to differ from 100 per cent since the analysis was not done by difference. RESULTS The values found in the main experiments are given in the table. TABLE--PAR'r OF TOTAL N13-AC'rlvtrv FOUND IN I~IWEREN-r FRACTIONS OF V A R I O U S N I T R O G E N C O M P O U N D S
NO3-
Compound
NOz-
!i
NH4 +
i NaNO3 NaNO2 NH4NO3 LiNOa
0.44 0.02 0.44 0"51
± 0'02 ::_ 0.01 ~ 0.01 ± 0'04
0.46 0.45 0.01 0.47
×~ 0.01 - 0.01 ± 0.01 :- 0.08
Recrystallized material 0.52 ~ 0.02
I 0.03 - 0.00 I
0.43 - 0.01
Gases
0.07 0.58 0.57 0.07
~-~0.02 z 0.02 ~ 0.02 .- 0.00
Attempts, which were only partially successful, were made to extend this series in different directions. When KNOa was irradiated, the total activity of the N TMcould not be determined, owing to the large quantity of CP 8 which was formed. We did, however, determine the ratio [N~aO3-]/[N13Oz-], for which we found 0'41 ~ 0-04. We repeated the experiment with a sample recrystallized from water through which hydrogen was bubbled, and found 0"39 ~- 0.05 for this ratio. With a sample similarly treated with oxygen we found 0-29 ~ 0.02. It would have been very interesting to obtain similar data on the distribution of N 13 in ammonium halides. With these compounds we were, however, unable to measure the total nitrogen activity owing to the presence of strong radio-halogen periods, although it was possible to detect N13H~' among the products. Special attention was paid to the case of the ammonium nitrate. It was ascertained that the fraction of the activity found as gases was essentially unchanged if the ammonium nitrate had been recrystallized in the presence of hydrogen (0.50) or in the presence of oxygen (0.52 : 0.02) (these samples had been dried at 120 ~ after recrystallization). We also made sure that the fraction of the activity retained in the ammonium nitrate on recrystallization was :not notably affected if the irradiated crystals were dissolved in 0.1 N sulphuric acid (0.44 ! 0-01), dilute sodium hydroxide (0.42), or a solution of potassium permanganate (0.43). An attempt was made to determine the constituents of the active gas fraction. Oxidizable forms were sought by adding a little acidified permanganate solution before boiling to remove dissolved gases. The permanganate had no effect on the retention of activit), by the solution, showing that there could be no appreciable amount of NO or NO2 originally anaong the active dissolved gases: free NO~ is in any case unlikely at very low concentrations. Similarly, the loss in activity was no less when nitrogen was passed through a cold mixed solution of the irradiated ammonium nitrate and concentrated ferrous sulphate than when it was passed through the simple solution of the nitrate in water. Molecular nitrogen and nitrous oxide are the only two remaining likely constituents of the active gas. Nitrous oxide was sought by dissolving the irradiated amlnonium nitrate in water saturated with nitrous oxide as carrier, and then drawing nitrogen through until all gaseous activity had been swept out of the solution and through a liquid air-trap, where the nitrous oxide condensed. The contents of this trap were later allowed to evaporate into one of the stoppered tubes generally used for liquid counting. 8 per cent of the total N ~3activity was isolated in this way. Although separation of nitrous oxide was probably not complete, this figure does suggest that a considerable part of the active products consists of nitrous oxide inside ammonium nitrate crystals. DISCUSSION W i t h m o s t c o m p o u n d s t h e r a d i o c h e m i c a l s e p a r a t i o n s e e m s t o give s a t i s f a c t o r y results. I n t h i s c o n n e c t i o n it s h o u l d b e r e a l i z e d t h a t in t h e c a s e o f s o d i u m n i t r a t e t h e r e c r y s t a l l i z e d f r a c t i o n w o u l d b e e x p e c t e d t o give t h e N O a - - a c t i v i t y , a n d in t h e case o f the ammonium nitrate the sum of the NOa--activity and the NH~+-activity.
298
R . D . S t a r k a n d A. H. W. ATtar, JUN.
The fact that in all irradiated compounds the sum of the activities of nitrate, nitrite, and the gaseous compounds amounts to about 100 per cent indicates that nitrate and nitrite are the only non-volatile reaction products which are important. With lithium nitrate the reproducibility is not nearly so good as it is in the other cases. A possible explanation might be found in the extremely hygroscopic nature of this substance, but the fact that it was handled in a dry box detracts from the weight of this consideration. In view of the fact that the chemical state of radio-sulphur formed by fast neutron irradiatien of potassium chloride is strongly dependent ~2) on the presence of oxygen in the crystal lattice, it seemed worth while to test for a similar influence in our experiments. Our results with ammonium nitrate indicated that with N xs such an influence could not be clearly recognized. However, the ratio [NxaO3-]/[NlaOs-], which was about unity in NaNO s and iNO 3, proved to be appreciably lower in KNOs, so we had to reconsider the possioility of such an oxidation in the crystal. Although for the latter salt we were unable to exclude wholly such an ~nterference (though we are not certain that the difference actually exceeds the possible error), we consider that the [NlSOs-]/[NlSOs-] ratio is definitely lower in this compound than it is in the two other alkali nitrates investigated. An even more serious question was whether the valences found for N xa are uniquely determined by the processes which take place in the crystal, or whether the final distribution is influenced by processes taking place during solution, as seems to be the case with neutron-irradiated KMnO4. cs~ For this reason the experiments were undertaken in which irradiated ammonium nitrate was dissolved in thepresence of H +-, OH--, or MnO4--ions. The fact that these additions did not influence the activity distribution makes it almost certain that the final form in which the N~3-atoms will be found is already determined entirely in the crystal. This does not, of course, mean that radioactive nitrate and nitrite ions are necessarily present in the solid as such. It is not impossible that they are formed by hydration of radioactive nitrogen atoms or oxides carrying the appropriate charge at the moment these go into solution. Neutrons must have an energy around '10 MeV to form N ~a from N x* by the n,2n-process. Under these circumstances they have also an excess energy, which is on the average of the order of 1 MeV. Most of this excess energy will be dissipated as kinetic energy of the two neutrons formed, but an appreciable fraction will turn up as recoil energy of the NXS-atoms. This energy not only enables the NXa-atoms to break their chemical bonds, but also to strip offsome of their electrons and to adopt a positive charge. Similar circumstances occur in n,p- and n,~-processes, t*~ For this reason it is to be expected that if different nitrogen compounds are irradiated with fast neutrons the primary condition of N ~a will be a highly oxidized one, and presumably one which depends very little on the nature of the irradiated substance. This state of affairs does not, of course, determine by itself the results of the chemical analysis, as the highly charged positive N ~a will be able to react chemically with the components of the crystal lattice after it has lost most of its excess energy. The activity measurement indicates therefore the results of the chemical processes which have taken place between N xaatoms or ions in essentially similar conditions and the ions which make up the different solids. ~s) W. S. KOSKI, J. Amer. Chem. Soc. 70, 4251 (1948).
W. F. Liner, J. Amer. Chem. See. 62, 1930 (1940). t*~ A. H. W. ATEN, /UN., Recueil Tray. CAlm. Pays-Bus 61,467 (1942); Phys. Rev. 71,641 (1947). csJ
Chemical state of nltrogem-13 formed by the (n, 2n) reaction in solid nitrogen compounds 299 It is to be expected that the composition and the structure of the target material will exert a strong influence on the activity distribution. A very striking example is presented by the difference between NaNO s and NaNO~. The lack of radioactive NOa--ions in the latter solid may perhaps be taken as an argument for the assumption that the complex radioactive anions, or at least the nitrate ions, are already formed in the crystal lattice before the material is dissolved, but it certainly does not definitely disprove the opposite. In any case this difference shows clearly the influence of the target material on the chemical form of the reaction products, in agreement with the observations made with n,p- and n,~-processes. ~4) As KNO3 gives results which are quite different from those found with NaNO3 and LiNO3, it is clear that even the cation can make a difference. It is worth pointing out that, in all the materials studied, an important fraction of the N 13 is found in an oxidized state, as is to be expected if the N 13 passes through a stage in which it is positively charged. It should be kept in mind, however, that in the ordinary Szilard-Chalmers process the n,y-reaction sometimes leads to a similar situation, as in the case of the iodates. ~5~ We want to make it quite clear that, although the N ~3is found in an oxidized state to an important degree, the three alkali nitrates and sodium nitrite all show a partial loss of oxygen during the nuclear reaction, relative to the combination state of nitrogen in the substrate. The only compound which does not suffer a reduction during activation is ammonium nitrate. This substance constitutes a more complicated case, for the medium can exert both an oxidizing and a reducing influence on the N la. It should be kept in mind that in this case secondary reactions may be possible with radioactive ions in an excited state. If an intermediate formation of active NO 2 -ions has occurred, these have disappeared by the reaction NO 2 q - N H a + - ~ 2H20 q - N z before the energy liberated by the nuclear process has been dissipated. Similarly, excited radioactive ions of NH4+ and of NO3-- may have been lost by the reaction NO 3- + NH4 * --~ 2H20 q-N20. The uncertainty of the determination of N20 in the reaction products makes it impossible to reach quantitative conclusions concerning these possibilities. It must be stated clearly, however, that the radioactive gases N 2 and N~O may very well have been formed "directly," that is, from a mixture of highly energetic atoms (one of which was radioactive) without passing through an intermediate stage in which they had the composition of stable ions or compounds. The abundance of NO a- relative to NH~+ in the active products from ammonium nitrate is noteworthy. It suggests, without proving it, that NO a- is indeed a more favoured first product of the reaction of energetic N 13 atoms with the substrate, for if the relative lack of N~ZHa+ is to be ascribed solely to the secondary reaction postulated above, it is difficult tO explain why the N~303- has not disappeared to a corresponding degree. In any case, by making the extreme assumption that all the active gases are the reaction products of active ammonium ions, one is led to the conclusion that the original concentration of the latter cannot have appreciably exceeded the combined concentrations of active nitrate and nitrite ions. Discussion of a more quantitative nature, e.g., of the difference between the [NlaO3-]/[N~302-] ratios in NaNO a and LiNOa on the one hand and KNO a on the other, is at present hardly possible. InterpretatioTi of the observations on ammonium nitrate would have been greatly helped by a knowledge of the processes in ammonium ts~ R. E. CLEARY,W. S . HAM1LL, and R. R. WILLIAMS,J. Amer. Chem. Soc. 74, 4675 (1952).
300
R . D . SMrm and A. H. ATEN, JUN.
salts with stable anions, but experiments with this aim were unsuccessful for the reasons given above. ACKNOWLEDGEMENTS The authors wish to thank Miss A. C. KLOPPER foLcarrying out some of the analyses. We also wish to express our thanks to the personnel of the Philips' cyclotron for the irradiations. This investigation forms part of the Research Programme of the Foundation for Fundamental Research of Matter (F.O.M.). It was carried out with financial aid from the Netherlands Organization for Pure Research (Z.W.O.). Grateful acknowledgement is also made to Imperial Chemical Industries Limited (Nobel Division) for allowing one of the authors (R. D. S.) to take part in this work.