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Some activation measurements with 3H(d, n)4He neutrons
Nuclear Instruments and Methods 184 (1981) 439 -444 North-Holland Publishing Company
439
SOME ACTIVATION MEASUREMENTS WITH all(d, n)4He NEUTRONS R.A. JARJIS * Department of Physics, The University of Aston, Birmingham B4 7ET, England Reccived 29 September 1980
Accuracy of the concentric ring geometry for fast neutron monitoring is verified by activating concentric rings and discs, which were made of copper, and measuring the neutron fluxes using the 6aCu(n, 2n)62Cu reaction. Consistent results were obtained using 3tl(d, n)4He neutrons from Van de Graaff and Dynamitron accelerators. In addition, 12.9-15.9 MeV neutrons were used to measure the excitation function for the 121Sb(n, 2n)12°gSb reaction and the ratios of 63Cu(n, 2n)62Cu and 27Al(n, p)27Mg reaction cross-sections, which were used for neutron energy determination. The investigation includes also a novel gamma ray counting metbod for the determination of the antimony cross-section, and recommendations for establishing the concentric ring technique as a standard.
1. Introduction Activation measurements with neutrons from the all(d, n)4He reaction have had much appeal for the measurement o f fast neutron cross-sections. This was invoked by the relative simplicity of the activation method and the favourable features of the all(d, n)4He neutron source, i.e., production of copious energetic neutrons at low deuteron b o m b a r d m e n t energies. However, accurate measurements can be hindered by some errors in neutron source stability, source/sample geometry, neutron flux monitoring, neutron energy evaluation, and induced activity determination. Developments in the fundamental studies of nuclear reaction mechanisms and the new surge of activity in the design o f fusion nuclear power reactors have stimulated further interests in this venerable field. This has motivated several investigations aiming primarily towards improving the accuracy of the activation measurements; see for example ref. 1 - 4 . It is the purpose of this paper to complement those efforts, and to follow up the initial work on the concentric ring technique for fast neutron monitoring [2]. Hence, we report on the accuracy o f this technique and extend the measurements o f neutron energies using the method of cross-sections ratio. Furthermore, we present new cross-section data for the
* Present address: Department of Physics, The University of Nottingham, Nottingham NG7 2RD, England. 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © North-Holland
12aSb(n, 2n)12°gSb reaction in the neutron energy region of 1 2 . 9 - 1 5 . 9 MeV.
2. Neutron source Accelerated ions of 0.3 MeV d~ and 1.4 MeV d2 were produced by the 0,5 MV Van de Graaff as Aston University and the 3.0 MV Dynamitron at Birmingham Radiation Centre. The ion beams impinged on 1.08 mg/cm 2 tritiated titanium targets producing neutrons by the 3H(d, n)4He nuclear reaction. The neutron producing targets were placed inside special water cooled target assemblies, through which a uniform neutron attenuation is exerted [5]. Irradiations of samples were conducted under low neutron scattering conditions, and samples were subjected to neutrons o f different energies by varying the angles made by their positions with respect to the incident deuteron beam direction. In order to determine the characteristics of the neutron beams, we assumed a uniform tritium distribution prof'fle in the tritiated targets, which have a tritium loading factor of 1.2. Such targets have a deuteron energy equivalent of about 0.37 MeV; which implies that at higher deuteron bombardment energies the neutron energy spread is reduced and the mean neutron energy range is enhanced. However, at such bombardment energies the targets exhibit reduction in the neutron yields, as shown in fig. 1. This, on the other hand, can be compensated for by using the molecular beam.
440
R.A. Jarjis / Activation measurements I
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The procedure which was implemented for the determination of neutron yields and energies is based on dividing the deuteron energy range, in the target, into small energy steps, and taking into account neutron anisotropy. For this purpose, the deuteron energy loss data were taken from the estimates of Benveniste and Zenger [6] and Saker and Wood [7]; whilst the cross-section data for the all(d, n)4He reaction were taken from the calculations of Liskein and Paulsen [8]. Mean neutron energies, fig. 2, were determined from the computed energy spectra, e.g. fig. 3, in such a manner that the effect of tritium target finite thickness was taken into account. It is important to point out that if we consider the neutron energy spread due to tritium target thickness only; then, the neutron source can be found to be most monoenergetic at 98 °. At this laboratory angle, the mean neutron energy and energy spread are most stable against changes in the energy of incident deuterons. From this point of view, one can utilize the activations at 98 ° for inter-laboratory comparisons. Hence, a fixed irradiation position at 98 ° was incorporated in our sample holder.
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Jarjis and Hunt [1]. This has led the authors to propose a geometry of concentric monitoring ring and circular sample disc. The technique was extended to the measurement of neutron energies in the 1 3 . 5 14.9 MeV region by activating concentric rings, or a disc and a ring, of two different materials whose ratio of activation cross-sections is a steep function of neutron energy [2]. For a given sensitivity, dimensions of the monitoring ring are determined by the monitor reaction cross-section, neutron flux and sample dimensions. It follows that for a typical monitor with a high activation cross-section, the amount of the monitor material which is required to obtain good gamma counting statistics is small. This means that thin rings can be used, such that the additional neutron energy spreads which are experienced by the rings are kept very small. The following experiment was performed on the Van de Graaff and Dynamitron accelerators in order to verify the accuracy of the concentric ring technique. Copper samples, each consisting of a disc and two concentric rings, were irradiated with 3H(d, n)4He neutrons for periods of h at a distance of 10 cm from the source. Components of the concentric samples were then separated and their induced annihilation gamma activities, which were produced by the 63Cu(n,2n)62Cu reaction, were determined using a NaI(T1) gamma ray spectrometer. Fig. 4 shows the ratios of neutron fluxes (~b) which were determined
by the induced activities of the inner (rl) and outer (r2) rings relative to that of the disc (d). It is interesting to point out that accuracy of the flux measurements, 1%, is very good inspite of the steep variation of the 6SCu(n,2n)62Cu reaction cross-section with neutron energy [5]. The method of neutron energy determination using the ratio of 63Cu(n, 2n)62Cu (T1/2 = 9.8 min) and 27Al(n, p)27Mg (T1/2 = 9.5 rain) reaction crosssections has been reported for 13.4-14.9 MeV neutrons [2]. In the present work, the molecular deuteron beam was used to extend these measurements to the 12.9-15.9 MeV neutron energy region. Samples consisting of aluminium discs and copper concentric rings were irradiated by neutrons, and the ratios of cross-sections were determined by measuring the decay rates of 62Cu and 27Mg through the integration under the 0.511 MeV and 0.842 MeV gamma ray peaks, respectively. A fixed gamma source/detector geometry was maintained throughout the measurements, and the irradiated copper samples, ~+ emitters, were enclosed in a special aluminum can in order to stop the positrons. The results which were produced on the Van de Graaff and Dynamitron accelerators are in good agreement, fig. 5, and in general the curve can be used to measure neutron energies with an accuracy of +1.2%. Whilst this article was being prepared for publication, a paper suggesting a transfer standard for
R.A. Jarjis / Activation measurements
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3H(d, n)4He neutron fluence and energy was published [9]. The standard consists of niobium and zirconium rods which were irradiated by 13.5-14.7 MeV neutrons. The neutron fluence was measured by the 9aNb(n, 2n)92mNb reaction whilst a combination of this and the 9°Zr(n, 2n)89g+mZr reaction was used for the energy measurements. The reported method for the energy measurements is based on the ratio of activities which is similar to our present work and to the original development investigations which were carried out at this laboratory [1,2,10,11]. In comparison with our technique, Lewis and Zieba [9] have failed to recommend an improved geometry for accurate inter-laboratory measurements. In addition, the half-lives of their product nuclei, 10.15 d for 92Nb and 78.4 h for 89Zr, differ by a factor of three;
therefore, introducing some undesirable errors due to the instabilities in neutron flux during prolonged irradiations.
4. The 12lSb(n, 2n) 120gSb cross-section The (n, 2n) reaction on 121Sb leads to the formation of 12°Sb nucleus in its ground or metastable state, with half-lives of 15.18 min and 5.8 d, respectively. Previous measurements at this laboratory have shown that the excitation function for the 121Sb(n,2n)12°gSb reaction is nearly fiat in the 13.4-14.9 MeV region [2]. This and the fact that the reaction has a high cross-section and reasonable halflife have led Jarjis and Hunt to recommend using the
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Fig. 5. The ratio of 63Cu(n, 2n)62Cu and 27Al(n, p)27Mg reaction cross-sections as a function of neutron energy. The solid line represents the data which were determined from the previously measured excitation functions [5].
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reaction for fast neutron monitoring [2]. Accordingly, it was successfully used as a standard for carrying out cross-section measurements [12,13]. In the present investigation, the neutron energy region was enhanced in order to understand the actual trend of the excitation function. The 121Sb(n, 2n) 12OgSb reaction cross-section was determined relative to the 6aCu(n,2n)62Cu crosssection which we have reported elsewhere [5]. Each of the irradiated samples consisted of an antimony disc and concentric copper and alumiuium rings serving as monitors for fast neutron flux and energy. The concentric samples were designed in such a manner that the induced activity on their components were of similar order of magnitude, and produce good counting statistics. The counting procedure consisted of, first, counting the copper and aluminium rings simultaneously, followed by the antimony disc. Furthermore, we devised a special cylindrical alumin u m enclosure for stopping the positrons which were emitted by the copper rings. The arrangement utilizes the radioactive aluminium ring as a position absorber in the horizontal direction, and therefore, it determined the dimensions of the aluminium ring. This novel method ensured a simultaneous gamma ray counting of the copper and aluminium rings, and an approximately uniform 4rr stoppage of the 62Cu positrons. The present 121Sb(n, 2n) 120gSb cross-section data, which were obtained on the Dynamitron accelerator,
complement those previously obtained on the Van de Graaff accelerator [2], as shown in fig. 6. The excitation function exhibits a broad maximum around 13.8-14.4 MeV with gradual reduction in the crossTable i Cross-section data for the 121Sb(n, 2n)12°gSb reaction En (MeV)
Cross-section (mbarn) Present work
12.83 + 0.10 13.08 4- 0.12 13.26_+0.13 13.50_+ 0.15 13.604-0.11 13.83 -+0.06 14.01 -+0.06
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43 45 56 77 57 50 56
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1000.0 4- 50 975.0 4- 45
14.49 _+0.14 14.61_+0.16 14.71 4- 0.18 14.80 4- 0.18 14.86 _+0.24 14.90 4- 0.26 14.91 4- 0.29 15.32 4- 0.23 15.85 4- 0.25 15.95 + 0.25
982.1 _+ 93 924.8 4- 46 908.0 4- 78 940.1 4- 93 920.2 4- 103 931.1 4- 87 905.9 4- 108 875.0 4- 44 780.5 4- 46 760.7 4- 42
other measurements a)
ref
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15 14
a) Neutron energy in parenthesis.
444
R.A. Jarfis / Activation measurements
section above 14.4 MeV. This points to the existance of a competing process for neutron interaction with 1z i Sb nucleus. This is in slight disagreement with the early results of Rayburn [14], table 1, which show a similar behaviour at higher neutron energy. However, there is a good general agreement between our results and those which were reported by the other researches.
5. Conclusions The purpose o f our experiments was to investigate the accuracy of the concentric ring technique for fast neutron monitoring. This was implemented by performing the tests at two different accelerator facilities and using different deuteron beam compositions for the production of neutrons. The results have shown that the special monitor geometry and the method of cross-section ratio can be adopted for the simultaneous accurate measurements of neutron flux and energy, with relative ease. Therefore, the technique can be recommended as a standard for inter-laboratory tests. Furthermore, its principles can be exploited to cover broader energy regions and halflives by selecting the appropriate reactions. The lZlSb(n, 2n)lZ°gSb reaction, for example, can be used jointly with other suitable reactions for the measurement of neutron energies in the regions 13.8 MeV > E n > 14.4 MeV. This means that a set of standard concentric detectors can be established and tested at different laboratories as described in this paper.
I am grateful to Professor S.E. Hunt for his stimulating advice and interest in this work.
References [1] R.A. Jarjis and S.E. Hunt, J. Radioanal. Chem. 16 (1973) 611. [2] R.A. Jarjis and S.E. Hunt, Int. J. Appl. Radiat. Isotopes 26 (1975) 57. [3] K. Kudo, Nucl. Instr. and Meth. 141 (1977) 325. [4] R. V//nsk// and Rieppo, Nuel. Instr. and Meth. 171 (1980) 281. [5] R.A. Jarjis, J. Phys. G: Nucl. Phys. 4 (1978) 445. [6] J. Benveniste and J. Zenger, UCRL-4266 (1954). [7] E.W. Saker and J.D.L.H. Wood, S.E.R.L. Teeh. Report No. 61 (1958). [8] H. Liskein and A. Paulsen, Nuclear Data Tables 11 (1973) 569. [9] V.E. Lewis and K.J. Zieba, Nucl. Instr. and Meth. 174 (1980) 141. [ 10] A.J. Cox and R.A. Jarjis, Int. 1. Appl. Radiat. Isotopes 23 (1972) 301. [11] R.A, Jarjis, Ph.D. Thesis, The University of Aston in Birmingham (1976). [12] S.C. Misra and U.C. Gupta, J. Phys. G.: Nuel. Phys. 5 (1979) 855. [13] R.A. Jarjis, to be published. [14] L.A. Rayburn, Phys. Rev. 130 (1963) 731. [15] Y. Kanda, J. Phys. Soc. Japan 24 (1968) 17. [16] W. Lu, N. Ranakumar and R.W. Fink, Phys. Rev. 1C (1970) 350. [17] E.B. Paul and R.L. Clarke, Can. J. Phys. 31 (1953) 267. [18] C.S. Khurana and H.S. Hans, Nu¢l. Phys. 28 (1961) 560.
439
SOME ACTIVATION MEASUREMENTS WITH all(d, n)4He NEUTRONS R.A. JARJIS * Department of Physics, The University of Aston, Birmingham B4 7ET, England Reccived 29 September 1980
Accuracy of the concentric ring geometry for fast neutron monitoring is verified by activating concentric rings and discs, which were made of copper, and measuring the neutron fluxes using the 6aCu(n, 2n)62Cu reaction. Consistent results were obtained using 3tl(d, n)4He neutrons from Van de Graaff and Dynamitron accelerators. In addition, 12.9-15.9 MeV neutrons were used to measure the excitation function for the 121Sb(n, 2n)12°gSb reaction and the ratios of 63Cu(n, 2n)62Cu and 27Al(n, p)27Mg reaction cross-sections, which were used for neutron energy determination. The investigation includes also a novel gamma ray counting metbod for the determination of the antimony cross-section, and recommendations for establishing the concentric ring technique as a standard.
1. Introduction Activation measurements with neutrons from the all(d, n)4He reaction have had much appeal for the measurement o f fast neutron cross-sections. This was invoked by the relative simplicity of the activation method and the favourable features of the all(d, n)4He neutron source, i.e., production of copious energetic neutrons at low deuteron b o m b a r d m e n t energies. However, accurate measurements can be hindered by some errors in neutron source stability, source/sample geometry, neutron flux monitoring, neutron energy evaluation, and induced activity determination. Developments in the fundamental studies of nuclear reaction mechanisms and the new surge of activity in the design o f fusion nuclear power reactors have stimulated further interests in this venerable field. This has motivated several investigations aiming primarily towards improving the accuracy of the activation measurements; see for example ref. 1 - 4 . It is the purpose of this paper to complement those efforts, and to follow up the initial work on the concentric ring technique for fast neutron monitoring [2]. Hence, we report on the accuracy o f this technique and extend the measurements o f neutron energies using the method of cross-sections ratio. Furthermore, we present new cross-section data for the
* Present address: Department of Physics, The University of Nottingham, Nottingham NG7 2RD, England. 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © North-Holland
12aSb(n, 2n)12°gSb reaction in the neutron energy region of 1 2 . 9 - 1 5 . 9 MeV.
2. Neutron source Accelerated ions of 0.3 MeV d~ and 1.4 MeV d2 were produced by the 0,5 MV Van de Graaff as Aston University and the 3.0 MV Dynamitron at Birmingham Radiation Centre. The ion beams impinged on 1.08 mg/cm 2 tritiated titanium targets producing neutrons by the 3H(d, n)4He nuclear reaction. The neutron producing targets were placed inside special water cooled target assemblies, through which a uniform neutron attenuation is exerted [5]. Irradiations of samples were conducted under low neutron scattering conditions, and samples were subjected to neutrons o f different energies by varying the angles made by their positions with respect to the incident deuteron beam direction. In order to determine the characteristics of the neutron beams, we assumed a uniform tritium distribution prof'fle in the tritiated targets, which have a tritium loading factor of 1.2. Such targets have a deuteron energy equivalent of about 0.37 MeV; which implies that at higher deuteron bombardment energies the neutron energy spread is reduced and the mean neutron energy range is enhanced. However, at such bombardment energies the targets exhibit reduction in the neutron yields, as shown in fig. 1. This, on the other hand, can be compensated for by using the molecular beam.
440
R.A. Jarjis / Activation measurements I
I
I
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The procedure which was implemented for the determination of neutron yields and energies is based on dividing the deuteron energy range, in the target, into small energy steps, and taking into account neutron anisotropy. For this purpose, the deuteron energy loss data were taken from the estimates of Benveniste and Zenger [6] and Saker and Wood [7]; whilst the cross-section data for the all(d, n)4He reaction were taken from the calculations of Liskein and Paulsen [8]. Mean neutron energies, fig. 2, were determined from the computed energy spectra, e.g. fig. 3, in such a manner that the effect of tritium target finite thickness was taken into account. It is important to point out that if we consider the neutron energy spread due to tritium target thickness only; then, the neutron source can be found to be most monoenergetic at 98 °. At this laboratory angle, the mean neutron energy and energy spread are most stable against changes in the energy of incident deuterons. From this point of view, one can utilize the activations at 98 ° for inter-laboratory comparisons. Hence, a fixed irradiation position at 98 ° was incorporated in our sample holder.
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Fig. 2. Mean neutron energy curves for a 1.08 mg/cm ~ tritiated titanium target with a tritium loading factor of 1.2.
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Fig. 3. Calculated neutron spectra from the 3H(d, n)4He reaction by 0.7 MeV deuterons impinging on a thick tritiated titanium target. The neutron yield at 98° has been reduced by a factor of two.
Jarjis and Hunt [1]. This has led the authors to propose a geometry of concentric monitoring ring and circular sample disc. The technique was extended to the measurement of neutron energies in the 1 3 . 5 14.9 MeV region by activating concentric rings, or a disc and a ring, of two different materials whose ratio of activation cross-sections is a steep function of neutron energy [2]. For a given sensitivity, dimensions of the monitoring ring are determined by the monitor reaction cross-section, neutron flux and sample dimensions. It follows that for a typical monitor with a high activation cross-section, the amount of the monitor material which is required to obtain good gamma counting statistics is small. This means that thin rings can be used, such that the additional neutron energy spreads which are experienced by the rings are kept very small. The following experiment was performed on the Van de Graaff and Dynamitron accelerators in order to verify the accuracy of the concentric ring technique. Copper samples, each consisting of a disc and two concentric rings, were irradiated with 3H(d, n)4He neutrons for periods of h at a distance of 10 cm from the source. Components of the concentric samples were then separated and their induced annihilation gamma activities, which were produced by the 63Cu(n,2n)62Cu reaction, were determined using a NaI(T1) gamma ray spectrometer. Fig. 4 shows the ratios of neutron fluxes (~b) which were determined
by the induced activities of the inner (rl) and outer (r2) rings relative to that of the disc (d). It is interesting to point out that accuracy of the flux measurements, 1%, is very good inspite of the steep variation of the 6SCu(n,2n)62Cu reaction cross-section with neutron energy [5]. The method of neutron energy determination using the ratio of 63Cu(n, 2n)62Cu (T1/2 = 9.8 min) and 27Al(n, p)27Mg (T1/2 = 9.5 rain) reaction crosssections has been reported for 13.4-14.9 MeV neutrons [2]. In the present work, the molecular deuteron beam was used to extend these measurements to the 12.9-15.9 MeV neutron energy region. Samples consisting of aluminium discs and copper concentric rings were irradiated by neutrons, and the ratios of cross-sections were determined by measuring the decay rates of 62Cu and 27Mg through the integration under the 0.511 MeV and 0.842 MeV gamma ray peaks, respectively. A fixed gamma source/detector geometry was maintained throughout the measurements, and the irradiated copper samples, ~+ emitters, were enclosed in a special aluminum can in order to stop the positrons. The results which were produced on the Van de Graaff and Dynamitron accelerators are in good agreement, fig. 5, and in general the curve can be used to measure neutron energies with an accuracy of +1.2%. Whilst this article was being prepared for publication, a paper suggesting a transfer standard for
R.A. Jarjis / Activation measurements
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Fig. 4. The ratios of neutron fluxes which were measured by using the 63Cu(n, 2n)62Cu reaction on concentric copper ring/disc samples. 4'r and ~d represent the fluxes which were measured by the rings and disc, respectively: whilst ~ r l and ~r2 are the angles which were subtended by the inner and outer rings on the neutron source. The corresponding angle for the disc was 10.2 ° .
3H(d, n)4He neutron fluence and energy was published [9]. The standard consists of niobium and zirconium rods which were irradiated by 13.5-14.7 MeV neutrons. The neutron fluence was measured by the 9aNb(n, 2n)92mNb reaction whilst a combination of this and the 9°Zr(n, 2n)89g+mZr reaction was used for the energy measurements. The reported method for the energy measurements is based on the ratio of activities which is similar to our present work and to the original development investigations which were carried out at this laboratory [1,2,10,11]. In comparison with our technique, Lewis and Zieba [9] have failed to recommend an improved geometry for accurate inter-laboratory measurements. In addition, the half-lives of their product nuclei, 10.15 d for 92Nb and 78.4 h for 89Zr, differ by a factor of three;
therefore, introducing some undesirable errors due to the instabilities in neutron flux during prolonged irradiations.
4. The 12lSb(n, 2n) 120gSb cross-section The (n, 2n) reaction on 121Sb leads to the formation of 12°Sb nucleus in its ground or metastable state, with half-lives of 15.18 min and 5.8 d, respectively. Previous measurements at this laboratory have shown that the excitation function for the 121Sb(n,2n)12°gSb reaction is nearly fiat in the 13.4-14.9 MeV region [2]. This and the fact that the reaction has a high cross-section and reasonable halflife have led Jarjis and Hunt to recommend using the
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Fig. 5. The ratio of 63Cu(n, 2n)62Cu and 27Al(n, p)27Mg reaction cross-sections as a function of neutron energy. The solid line represents the data which were determined from the previously measured excitation functions [5].
443
R.A. Jar]is/Activation measurements i
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MeV
Fig. 6. Excitation function for the 121Sb(n, 2n)120gSb reaction.
reaction for fast neutron monitoring [2]. Accordingly, it was successfully used as a standard for carrying out cross-section measurements [12,13]. In the present investigation, the neutron energy region was enhanced in order to understand the actual trend of the excitation function. The 121Sb(n, 2n) 12OgSb reaction cross-section was determined relative to the 6aCu(n,2n)62Cu crosssection which we have reported elsewhere [5]. Each of the irradiated samples consisted of an antimony disc and concentric copper and alumiuium rings serving as monitors for fast neutron flux and energy. The concentric samples were designed in such a manner that the induced activity on their components were of similar order of magnitude, and produce good counting statistics. The counting procedure consisted of, first, counting the copper and aluminium rings simultaneously, followed by the antimony disc. Furthermore, we devised a special cylindrical alumin u m enclosure for stopping the positrons which were emitted by the copper rings. The arrangement utilizes the radioactive aluminium ring as a position absorber in the horizontal direction, and therefore, it determined the dimensions of the aluminium ring. This novel method ensured a simultaneous gamma ray counting of the copper and aluminium rings, and an approximately uniform 4rr stoppage of the 62Cu positrons. The present 121Sb(n, 2n) 120gSb cross-section data, which were obtained on the Dynamitron accelerator,
complement those previously obtained on the Van de Graaff accelerator [2], as shown in fig. 6. The excitation function exhibits a broad maximum around 13.8-14.4 MeV with gradual reduction in the crossTable i Cross-section data for the 121Sb(n, 2n)12°gSb reaction En (MeV)
Cross-section (mbarn) Present work
12.83 + 0.10 13.08 4- 0.12 13.26_+0.13 13.50_+ 0.15 13.604-0.11 13.83 -+0.06 14.01 -+0.06
773.9 4820.0 4870.0__ + 922.5 4950.2__ 4980.0 4965.3 -+
43 45 56 77 57 50 56
14.23 _+0.05 14.37 4- 0.10
1000.0 4- 50 975.0 4- 45
14.49 _+0.14 14.61_+0.16 14.71 4- 0.18 14.80 4- 0.18 14.86 _+0.24 14.90 4- 0.26 14.91 4- 0.29 15.32 4- 0.23 15.85 4- 0.25 15.95 + 0.25
982.1 _+ 93 924.8 4- 46 908.0 4- 78 940.1 4- 93 920.2 4- 103 931.1 4- 87 905.9 4- 108 875.0 4- 44 780.5 4- 46 760.7 4- 42
other measurements a)
ref
893_+ 884_+ 929 _+ 878 _+ 951_+ 979 _+ 1062 _+
80(13.00__+0.42) 75(13.20_+0.08) 83 (13.47 _+0.42) 78 (13.60 _+0.15) 81(13.7 _+0.30) 88 (14.05 _+0.62) 82 (14.05 _+0.31)
14 15 14 15 15 14 15
1056_+ 95 (14.4 +0.30) 1188_+ 60(14.4 _+0.30) 750 __+187 (14.5)
14 16 17
998 4- 83 (14.72 _+0.31) 1180_+177(14.8 +0.5)
15 18
1120__+ 95(15.16_+0.15) 1053 _+ 94 (15.24 4- 0.66)
15 14
a) Neutron energy in parenthesis.
444
R.A. Jarfis / Activation measurements
section above 14.4 MeV. This points to the existance of a competing process for neutron interaction with 1z i Sb nucleus. This is in slight disagreement with the early results of Rayburn [14], table 1, which show a similar behaviour at higher neutron energy. However, there is a good general agreement between our results and those which were reported by the other researches.
5. Conclusions The purpose o f our experiments was to investigate the accuracy of the concentric ring technique for fast neutron monitoring. This was implemented by performing the tests at two different accelerator facilities and using different deuteron beam compositions for the production of neutrons. The results have shown that the special monitor geometry and the method of cross-section ratio can be adopted for the simultaneous accurate measurements of neutron flux and energy, with relative ease. Therefore, the technique can be recommended as a standard for inter-laboratory tests. Furthermore, its principles can be exploited to cover broader energy regions and halflives by selecting the appropriate reactions. The lZlSb(n, 2n)lZ°gSb reaction, for example, can be used jointly with other suitable reactions for the measurement of neutron energies in the regions 13.8 MeV > E n > 14.4 MeV. This means that a set of standard concentric detectors can be established and tested at different laboratories as described in this paper.
I am grateful to Professor S.E. Hunt for his stimulating advice and interest in this work.
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