- Email: [email protected]
The fission neutron averaged cross-section of the 72Ge(n,p)72Ga reaction
Appl. Radial.
Pergamon
0%9-8043(93)EOOlO-1
hf. Vol. 45, No. 5, pp. 573-576, 1994 Copyright C 1994 Elsevier Science Ltd Printed in Great Brilain. All rights reserved 0969-8043194 $7.00 + 0.00
The Fission Neutron Averaged Cross-section of the 72Ge(n,p)72Ga Reaction J. C. FURNARI’,
I. M. COHEN’*
and
A. J. KESTELMAN’
‘Gerencia de Area Radioisotopes y Radiaciones, Comision National de Energia Atomica, Av. de1 Libertador 8250, 1429 Buenos Aires, Argentina and *Centro Atomic0 Bariloche. Comision National de Energia
Atbmica,
8400 S. C. de Bariloche.
Rio Negro,
Argentina
(Received July 1992; in revised form 22 March 1993: receioed,for publication 3 November 1993)
Determination of the 7*Ge(n,p)72Ga reaction cross-section, averaged over a fission spectrum, has been carried out at the Comision National de Energia Atomica, using the facilities of Centro Atomic0 Ezeiza and Centro Atomic0 Bariloche. The overall average value is (0.0959 + 0.0049) mb. The experimental method is described and the implications of the results are discussed.
Introduction One of the problems that affect the quality of reactor neutron activation analysis (RNAA) is the possibility of reporting erroneous data, because of an inefficient control of the threshold reactions induced on the matrix by the fast component of the neutron spectrum. Interferences from elements other than those to be determined can be produced by (n,p) and (n,a) reactions. Even different isotopes of the investigated elements can lead to wrong results, through (n,n’) and (n,2n) reactions, when single comparators are used. In both cases the errors can reach significant figures, as it was demonstrated e.g. by Cohen (1991) and De Corte et al. (1987). Thus, the additional determiof the neutron fast flux and subnation sequent evaluation of the interferences caused by threshold reactions is often a mandatory requirement to correctly accomplish an analysis based on reactor activation. The authors have investigated the feasibility of using germanium as comparator and flux monitor in RNAA. The results, which will be published elsewhere, proved that germanium could be an excellent comparator and an acceptable monitor for both the thermal and epithermal components of the neutron spectrum. Another advantage analyzed in the course of the evaluation, and so far not explored, was the possibility of utilizing one of the threshold reactions induced in germanium, namely the “Ge(n,p)‘*Ga reaction [threshold: 3.255 MeV; effective energy, as defined by Calamand (1974): 8.5 MeV], to monitor the fast flux. For such purpose, a good knowledge of the cross-section value was an obvious prerequisite. *Author for correspondence.
The published data found by the authors concerning the ‘*Ge(n,p)‘*Ga reaction cross-section averaged over a fission neutron spectrum were scarce and very discrepant. The only experimental references were an upper limit [O.Ol mb; Rochlin (1959)] and two effective values: (0.044 f 0.07) mb (De Neve et al., 1966) and 0.0218 mb (Rau, 1967), whereas three estimated values were: 0.089 mb (Pearlstein, 1973); 0.18 mb (Calamand, 1974); and 0.0789 mb (Horibe, 1983). It was apparent that neither the dispersion nor the errors of the individual data were compatible with the proposed objective. In consequence, the authors carried out two independent determinations using the facilities of Centro Atomic0 Ezeiza and Centro Atomic0 Bariloche, both belonging to Comision National de Energia Atomica. The present work describes the experimental methods, and discusses the results and their implications.
Experimental Slices of hyperpure grade germanium (quality required for detector manufacture) were treated with a mixture: 5: 1: 1 HNO,(c)-HF(ctH202, in order to remove superficial impurities, and then washed with water, dried and weighed with 0.1 pg precision. They were irradiated together with suitable fast neutron flux monitors, in a cadmium cover, at several core positions of the RA-3 reactor (Centro Atomic0 Ezeiza) and one of the positions next to the core of the RA-6 reactor (Centro Atomic0 Bariloche). The fast flux ranged from 2 x 10” n. cm-*. s-1 to 7 x lOr2 n. cm-*. s-’ , in the RA-3 reactor and was about 8 x 10” n. cm-* s-I in the RA-6 reactor. 573
574
J. C. FURNARIet al.
The monitors used for the irradiations at the RA-3 and RA-6 reactors were respectively iron and titanium, the standard reactions being: S4Fe(n,p)54Mn; 46Ti(n,p)46Sc; 47Ti(n,p)47Sc and 48Ti(n,p)48Sc. In separate experiments, irradiations of sets of monitors having varying threshold energies for reactions induced by fast neutrons were performed at the selected positions of the RA-3 reactor, in order to compare the results of the different flux measurements. They were: manganese, through the “Mn(n, 2n)“Mn reaction, and iron and titanium (reactions already quoted). Similar experiments had been carried out at the RA-6 reactor in previous work (Salas Bacci, 1991), using the standard reactions: 24Mg(n,p)24Na, 27Al(n,cr)24Na, 54Fe(n,p)54Mn, and 58Ni(n,p)S8Co. The nuclear constants for the reactions involved in this work, as quoted by Baard et al. (1989) and Tub (1990) are indicated in Table 1. As a result of all these measurements with well characterized threshold monitors, the authors concluded that the reactor spectrum did not deviate significantly from a fission spectrum in both the Ezeiza and Bariloche reactors. Irradiation times at the RA-3 reactor were 3-4 h and 5-6 h, measurements were made sequentially after 6-24 h decay. The experiments at the RA-6 reactor were performed in conjunction with other cross-section determinations for threshold reactions on germanium, involving measurement of shorterlived nuclides (Cohen et al., 1992); thus, the samples were irradiated for 3-4min and measured, after the decay of 75mGeand ““Ge (about 5-6 min), for 15 min and then for 30 min. In some of the experiments, 6 h measurements were made with the samples which had decayed for several hours. High resolution gamma spectrometry systems, composed of HP Ge detectors, multichannel analyzers and peripheral components, were used for the measurements. The samples were measured at
geometries sufficiently distant from the detectors, to avoid pile-up or coincidence effects. In order to enhance the detection of the 72Ga gamma rays with respect to those emitted by “Ge and “Ge, most of them having low energies, absorbers were interposed between samples and detectors: lead and leadcadmiumcopper absorbers, for the measurements performed respectively at Centro Atomic0 Ezeiza and Centro Atomic0 Bariloche. Since the energy of the most intense transition of 72Ga (834.1 keV) was practically coincident with on iron (834.9 keV), that of the 54Mn induced measurements of the monitors irradiated at the RA3 reactor in similar conditions to the samples, allowed the direct standardization of that peak. For the samples measured after irradiation in the RA-6 reactor, the 834.1 and 2201.7 keV 72Ga peaks were used for calculations. In this case, relative efficiency curves were determined by measurements of a 60Co-“omAg-‘82Ta mixed source, with and without absorbers, which were then normalized to absolute values using curves previously adjusted through measurements of well calibrated reference sources. The relative curves were also checked by measuring the numerous peaks from the “Ge produced in the samples. While the germanium slices irradiated at the RA-3 reactor weighed 2&30 mg, the weights of the germanium samples used for short irradiations at the RA-6 reactor were considerably higher, thicknesses being about 0.15 cm. Self-attenuation factors were calculated and each measurement was consequently corrected, the corrections being respectively the order of 2.5% and 1.5% for the peaks at 834.1 and 2201.7 keV. The relevant data of germanium and gallium isotopes necessary for the final calculation of the crosssection studied, included in Table 2, were extracted from the tables of Tub (1990) and, for the gamma
Table I. Nuclear data of the flux monitors Reaction Vi(n,p)YSc
EL, (MeV) 5.6
M
a,(mb)
tI(%)
Half-life
E. (keV)
i (%)
47.88 (0.03)
I I.8 (0.4)
8.0 (0.1)
83.810 d (0.010)
889.3
99.9840 (0.00l0) 99.9870 (0.00l0)
1120.5 P’Ti(n,p)‘7Sc
3.7
47.88 (0.03)
18.0 (0.6)
7.3 (0.1)
3.345 d (0.003)
159.4
68.3 (0.4)
“Ti(n,p)“Sc
8.3
47.88 (0.03)
0.307 (0.011)
73.8 (0.1)
43.7 h (0.1)
983.5
100.0 (0.3) 97.5 (0.5) 100.0 (0.5)
1037.5 1312.1 “Mn(n,2n)YMn “Fe(n,p)“Mn
13 4.1
54.93805 (0.00007)
0.258 (0.013)
55.847 (0.003)
81.7 (2.2)
IO0
312.14d (0.05)
834.8
99.97s (0.001)
5.8 (0.1)
312.34d (0.05)
834.8
99.97s 10.001 b
Parentheses indicate uncertainty of the magnitude*. E,.,, median of energy response range; M, relative atomic mass of element; 0, isotopic abundance. averaged cross-section for a fission spectrum; E:, gamma energy of product; i mtensity.
bt,
Table 2. Nuclear constants for the calculation reaction cross-section
of the “Ge(n,p)“Ga
Germanium
Atomic
72.61 (0.02)
‘*CC
Isotopic
“Ga
Half-life:
weight:
abundance
(%):
27.66 (0.03) 14.10 h (0.02) 834.088 95.63 *
Gamma energy (keV): Intensity (%):
Parentheses indicate uncertainty of the magnitude. *Systematic uncertainty
575
fission averaged cross-section
“Ge(n,p)‘*Ga
2201.73 25.9 (0.5) Browne
and
energies and intensities, from the tables of Browne and Firestone (1986). Although no impurities were detected in the irradiated germanium, the possible presence of gallium was specially checked, since the “Ga(n,y)‘*Ga reaction could have led to a systematic excess error in the determination of the cross-section. The procedure adopted was the determination of the activities of two gallium isotopes, namely ‘*Ga and 73Ga after irradiation of germanium in a cadmium cove; and in the undisturbed reactor spectrum. While the 73Ge(n,p)73Ga reaction induced by the fast component of the neutron spectrum was the only method of ‘)Ga production, 72Ga could be produced by thermal and epithermal neutrons on gallium and by fast neutrons on germanium as well, so that any eventual difference for the 72Ga/73Ga activity ratios would have provided evidence of gallium contamination. A simple method for radiochemical separation of gallium was developed, the steps being: dissolution of the sample in a mixture 5 : 1: 1 HNO, (c)-HF(ctH,O, (30%); heating till dryness and redissolution in a solution 7 N NH,SCN in 0.5 N HCl (4 mL); extraction with ethyl ether (two aliquots, 4 mL and 1.5 mL); evaporation of the organic extracts; measurement. Chemical yield was 97.6% and both gallium isotopes could be easily measured. The results of the search for gallium impurities in the germanium used are summarized in Table 3. The average value for the quotient: (72Ga/73Ga, undisturbed spectrum)/( 72Ga/73Ga, epicadmium spectrum) was 0.998 f 0.006, thus proving the absence
of gallium contamination. (Incidentally, the fact that coincident values for the cross-section were obtained after irradiation at different positions and reactors, with variable epithermal to fast flux ratios, gave an indirect evidence about the purity of the germanium samples).
Results and Discussion The average of five determinations at Centro Atomic0 Ezeiza and four determinations at Centro Atomic0 Bariloche for the fission neutron averaged cross-section of the “Ge(n,p)‘*Ga reaction was (0.0959 f 0.0049) mb. The expression for the uncertainty includes the statistical dispersion of the average and the systematic errors of the nuclear constants involved in the calculations (the error of the weighing was negligible). As mentioned in the introduction, the possibility of using of the 72Ge(n,p)72Ga reaction for evaluation of the fast component of the reactor spectrum depends on the reliability of the cross-section value. In this respect, the present datum is the result of two entirely independent sets of determinations at different reactors and facilities and in some aspects different methodology; the difference between the averages for both sets was < 3%. As the precision is also good, the conclusion is that this reaction can be effectively used as monitor of the fast neutron flux. It is evident that a variety of reactions, e.g. those used in the present work for standardization, are more suitable for fast flux determination. Nevertheless, the positive advantage of the use of germanium as flux monitor is the feasibility of simultaneously performing the determination of not only the fast component but also of the thermal and epithermal components of the neutron spectrum. However, it should be mentioned that the detection of 72Ga becomes difficult when germanium is irradiated in the undisturbed reactor spectrum. The radiochemical method described in the present paper can also be applied in routine work for gallium separation and determination of the fast flux. Acknowledgement-The authors wish Arribere for her valuable collaboration.
to
thank
M.
A.
References Table
3. Control of gallium impurities in germanium (“Ga/72Ga activity ratios for different gamma emissions)
E.., “Ga/E.. 297.3 325.7 297.3 325.7 297.3 325.7 297.3 325.7
, ‘*Ga
k&/630.0 keV keVi630.0 keV keV/834. I keV k&‘/834.1 keV keV/894.3 keV keV/894.3 keV k&/1050.9 keV keV/l050.0 keV
‘, Undisturbed E, Epicadmium
spectrum. spectrum.
(A “Ga/A
“Ga)“/(A
“Ga/A
I .007l 0.9872 I .0030 I .0242 I .0042 0.9839 0.9973 0.9770 averase: 0.998 _ + 0.006
72Ga)E
Baard J. H., Zijp W. L. and Nolthenius H. J. (1989) Nuclear Data Guide for Reactor Neurron Metrology. Kluwer Academic Publishers, The Netherlands. Browne E. and Firestone R. B. (1986) Table of Radioactive Isotopes (Ed. Shirley V. S.). John Wiley and Sons, New York. Calamand A. (1974) Cross-sections for fission neutron spectrum induced reactions. In Handbook on Nuclear Activation Cross-Sections, p, 273. IAEA Technical Report Series 156. Cohen I. M. (1991) Analytical importance of some secondary and threshold reactions induced in nuclear reactors. J. Radioanal. Nucl. Chem. Articles 148, 155.
516
J. C. FURNARIet al.
Cohen 1. M., Kestelman A. J. and Furnari J. C. (1992) Determination of 70Ge(n,p)7”Ga and “Ge(n,p)74Ga reaction cross-sections for a fission neutron spectrum. Presented at the Third International Conference on Nuclear and Radiochemistry. Vienna, 7-1 I September. To be published in J. Radioanal. Nucl. Chem. Articles, De Corte F., Simonits A., De Wispelaere A. and Hoste J. (1987) Accuracy and applicability of the k,-standardization method. J. Radioanal. Nucl. Chem. Articles 113, 145. De Neve R., De Soete D. and Hoste J. (1966) The cross72Ge(n,p)7’Ga 7’-Ge(n.a)6“mZn reactor and sections, Radiochim. Acta 5, 188. Horibe 0. (1983) A new empirical rule for the estimation of fission neutron spectrum averaged cross-sections of
the (n,p) and (n,a) reactions. .4nn. NW/. Energy 10, 359. Pearlstein S. (1973) Neutron-induced reactions in medium mass nuclei. J. Nucl. Energ? 27, 8 I. Rau G. (1967) Bestimmung verschiedener iiber ein Spaltneutronenspektrum gemittelter n.p und n,% Wirkungsquerschnitte. Nukleonik 9, 226. Rochlin R. S. (1959) Fission-neutron cross-sections for threshold reactions. Nucleonics 17, 54. Salas Bacci A. (1991) Determination Experimental de la Section Eficaz Promedio de la Reaction “Cr(n.p)“V, para un Espectro de Neutrones de Fision. Thesis, Universidad Mayor de San And& La Paz. Bolivia. Tuli J. K. (1990) Nuclear IVaNe/ Cards. National Nuclear Data Center. Brookhaven National Laboratory.
Pergamon
0%9-8043(93)EOOlO-1
hf. Vol. 45, No. 5, pp. 573-576, 1994 Copyright C 1994 Elsevier Science Ltd Printed in Great Brilain. All rights reserved 0969-8043194 $7.00 + 0.00
The Fission Neutron Averaged Cross-section of the 72Ge(n,p)72Ga Reaction J. C. FURNARI’,
I. M. COHEN’*
and
A. J. KESTELMAN’
‘Gerencia de Area Radioisotopes y Radiaciones, Comision National de Energia Atomica, Av. de1 Libertador 8250, 1429 Buenos Aires, Argentina and *Centro Atomic0 Bariloche. Comision National de Energia
Atbmica,
8400 S. C. de Bariloche.
Rio Negro,
Argentina
(Received July 1992; in revised form 22 March 1993: receioed,for publication 3 November 1993)
Determination of the 7*Ge(n,p)72Ga reaction cross-section, averaged over a fission spectrum, has been carried out at the Comision National de Energia Atomica, using the facilities of Centro Atomic0 Ezeiza and Centro Atomic0 Bariloche. The overall average value is (0.0959 + 0.0049) mb. The experimental method is described and the implications of the results are discussed.
Introduction One of the problems that affect the quality of reactor neutron activation analysis (RNAA) is the possibility of reporting erroneous data, because of an inefficient control of the threshold reactions induced on the matrix by the fast component of the neutron spectrum. Interferences from elements other than those to be determined can be produced by (n,p) and (n,a) reactions. Even different isotopes of the investigated elements can lead to wrong results, through (n,n’) and (n,2n) reactions, when single comparators are used. In both cases the errors can reach significant figures, as it was demonstrated e.g. by Cohen (1991) and De Corte et al. (1987). Thus, the additional determiof the neutron fast flux and subnation sequent evaluation of the interferences caused by threshold reactions is often a mandatory requirement to correctly accomplish an analysis based on reactor activation. The authors have investigated the feasibility of using germanium as comparator and flux monitor in RNAA. The results, which will be published elsewhere, proved that germanium could be an excellent comparator and an acceptable monitor for both the thermal and epithermal components of the neutron spectrum. Another advantage analyzed in the course of the evaluation, and so far not explored, was the possibility of utilizing one of the threshold reactions induced in germanium, namely the “Ge(n,p)‘*Ga reaction [threshold: 3.255 MeV; effective energy, as defined by Calamand (1974): 8.5 MeV], to monitor the fast flux. For such purpose, a good knowledge of the cross-section value was an obvious prerequisite. *Author for correspondence.
The published data found by the authors concerning the ‘*Ge(n,p)‘*Ga reaction cross-section averaged over a fission neutron spectrum were scarce and very discrepant. The only experimental references were an upper limit [O.Ol mb; Rochlin (1959)] and two effective values: (0.044 f 0.07) mb (De Neve et al., 1966) and 0.0218 mb (Rau, 1967), whereas three estimated values were: 0.089 mb (Pearlstein, 1973); 0.18 mb (Calamand, 1974); and 0.0789 mb (Horibe, 1983). It was apparent that neither the dispersion nor the errors of the individual data were compatible with the proposed objective. In consequence, the authors carried out two independent determinations using the facilities of Centro Atomic0 Ezeiza and Centro Atomic0 Bariloche, both belonging to Comision National de Energia Atomica. The present work describes the experimental methods, and discusses the results and their implications.
Experimental Slices of hyperpure grade germanium (quality required for detector manufacture) were treated with a mixture: 5: 1: 1 HNO,(c)-HF(ctH202, in order to remove superficial impurities, and then washed with water, dried and weighed with 0.1 pg precision. They were irradiated together with suitable fast neutron flux monitors, in a cadmium cover, at several core positions of the RA-3 reactor (Centro Atomic0 Ezeiza) and one of the positions next to the core of the RA-6 reactor (Centro Atomic0 Bariloche). The fast flux ranged from 2 x 10” n. cm-*. s-1 to 7 x lOr2 n. cm-*. s-’ , in the RA-3 reactor and was about 8 x 10” n. cm-* s-I in the RA-6 reactor. 573
574
J. C. FURNARIet al.
The monitors used for the irradiations at the RA-3 and RA-6 reactors were respectively iron and titanium, the standard reactions being: S4Fe(n,p)54Mn; 46Ti(n,p)46Sc; 47Ti(n,p)47Sc and 48Ti(n,p)48Sc. In separate experiments, irradiations of sets of monitors having varying threshold energies for reactions induced by fast neutrons were performed at the selected positions of the RA-3 reactor, in order to compare the results of the different flux measurements. They were: manganese, through the “Mn(n, 2n)“Mn reaction, and iron and titanium (reactions already quoted). Similar experiments had been carried out at the RA-6 reactor in previous work (Salas Bacci, 1991), using the standard reactions: 24Mg(n,p)24Na, 27Al(n,cr)24Na, 54Fe(n,p)54Mn, and 58Ni(n,p)S8Co. The nuclear constants for the reactions involved in this work, as quoted by Baard et al. (1989) and Tub (1990) are indicated in Table 1. As a result of all these measurements with well characterized threshold monitors, the authors concluded that the reactor spectrum did not deviate significantly from a fission spectrum in both the Ezeiza and Bariloche reactors. Irradiation times at the RA-3 reactor were 3-4 h and 5-6 h, measurements were made sequentially after 6-24 h decay. The experiments at the RA-6 reactor were performed in conjunction with other cross-section determinations for threshold reactions on germanium, involving measurement of shorterlived nuclides (Cohen et al., 1992); thus, the samples were irradiated for 3-4min and measured, after the decay of 75mGeand ““Ge (about 5-6 min), for 15 min and then for 30 min. In some of the experiments, 6 h measurements were made with the samples which had decayed for several hours. High resolution gamma spectrometry systems, composed of HP Ge detectors, multichannel analyzers and peripheral components, were used for the measurements. The samples were measured at
geometries sufficiently distant from the detectors, to avoid pile-up or coincidence effects. In order to enhance the detection of the 72Ga gamma rays with respect to those emitted by “Ge and “Ge, most of them having low energies, absorbers were interposed between samples and detectors: lead and leadcadmiumcopper absorbers, for the measurements performed respectively at Centro Atomic0 Ezeiza and Centro Atomic0 Bariloche. Since the energy of the most intense transition of 72Ga (834.1 keV) was practically coincident with on iron (834.9 keV), that of the 54Mn induced measurements of the monitors irradiated at the RA3 reactor in similar conditions to the samples, allowed the direct standardization of that peak. For the samples measured after irradiation in the RA-6 reactor, the 834.1 and 2201.7 keV 72Ga peaks were used for calculations. In this case, relative efficiency curves were determined by measurements of a 60Co-“omAg-‘82Ta mixed source, with and without absorbers, which were then normalized to absolute values using curves previously adjusted through measurements of well calibrated reference sources. The relative curves were also checked by measuring the numerous peaks from the “Ge produced in the samples. While the germanium slices irradiated at the RA-3 reactor weighed 2&30 mg, the weights of the germanium samples used for short irradiations at the RA-6 reactor were considerably higher, thicknesses being about 0.15 cm. Self-attenuation factors were calculated and each measurement was consequently corrected, the corrections being respectively the order of 2.5% and 1.5% for the peaks at 834.1 and 2201.7 keV. The relevant data of germanium and gallium isotopes necessary for the final calculation of the crosssection studied, included in Table 2, were extracted from the tables of Tub (1990) and, for the gamma
Table I. Nuclear data of the flux monitors Reaction Vi(n,p)YSc
EL, (MeV) 5.6
M
a,(mb)
tI(%)
Half-life
E. (keV)
i (%)
47.88 (0.03)
I I.8 (0.4)
8.0 (0.1)
83.810 d (0.010)
889.3
99.9840 (0.00l0) 99.9870 (0.00l0)
1120.5 P’Ti(n,p)‘7Sc
3.7
47.88 (0.03)
18.0 (0.6)
7.3 (0.1)
3.345 d (0.003)
159.4
68.3 (0.4)
“Ti(n,p)“Sc
8.3
47.88 (0.03)
0.307 (0.011)
73.8 (0.1)
43.7 h (0.1)
983.5
100.0 (0.3) 97.5 (0.5) 100.0 (0.5)
1037.5 1312.1 “Mn(n,2n)YMn “Fe(n,p)“Mn
13 4.1
54.93805 (0.00007)
0.258 (0.013)
55.847 (0.003)
81.7 (2.2)
IO0
312.14d (0.05)
834.8
99.97s (0.001)
5.8 (0.1)
312.34d (0.05)
834.8
99.97s 10.001 b
Parentheses indicate uncertainty of the magnitude*. E,.,, median of energy response range; M, relative atomic mass of element; 0, isotopic abundance. averaged cross-section for a fission spectrum; E:, gamma energy of product; i mtensity.
bt,
Table 2. Nuclear constants for the calculation reaction cross-section
of the “Ge(n,p)“Ga
Germanium
Atomic
72.61 (0.02)
‘*CC
Isotopic
“Ga
Half-life:
weight:
abundance
(%):
27.66 (0.03) 14.10 h (0.02) 834.088 95.63 *
Gamma energy (keV): Intensity (%):
Parentheses indicate uncertainty of the magnitude. *Systematic uncertainty
575
fission averaged cross-section
“Ge(n,p)‘*Ga
2201.73 25.9 (0.5) Browne
and
energies and intensities, from the tables of Browne and Firestone (1986). Although no impurities were detected in the irradiated germanium, the possible presence of gallium was specially checked, since the “Ga(n,y)‘*Ga reaction could have led to a systematic excess error in the determination of the cross-section. The procedure adopted was the determination of the activities of two gallium isotopes, namely ‘*Ga and 73Ga after irradiation of germanium in a cadmium cove; and in the undisturbed reactor spectrum. While the 73Ge(n,p)73Ga reaction induced by the fast component of the neutron spectrum was the only method of ‘)Ga production, 72Ga could be produced by thermal and epithermal neutrons on gallium and by fast neutrons on germanium as well, so that any eventual difference for the 72Ga/73Ga activity ratios would have provided evidence of gallium contamination. A simple method for radiochemical separation of gallium was developed, the steps being: dissolution of the sample in a mixture 5 : 1: 1 HNO, (c)-HF(ctH,O, (30%); heating till dryness and redissolution in a solution 7 N NH,SCN in 0.5 N HCl (4 mL); extraction with ethyl ether (two aliquots, 4 mL and 1.5 mL); evaporation of the organic extracts; measurement. Chemical yield was 97.6% and both gallium isotopes could be easily measured. The results of the search for gallium impurities in the germanium used are summarized in Table 3. The average value for the quotient: (72Ga/73Ga, undisturbed spectrum)/( 72Ga/73Ga, epicadmium spectrum) was 0.998 f 0.006, thus proving the absence
of gallium contamination. (Incidentally, the fact that coincident values for the cross-section were obtained after irradiation at different positions and reactors, with variable epithermal to fast flux ratios, gave an indirect evidence about the purity of the germanium samples).
Results and Discussion The average of five determinations at Centro Atomic0 Ezeiza and four determinations at Centro Atomic0 Bariloche for the fission neutron averaged cross-section of the “Ge(n,p)‘*Ga reaction was (0.0959 f 0.0049) mb. The expression for the uncertainty includes the statistical dispersion of the average and the systematic errors of the nuclear constants involved in the calculations (the error of the weighing was negligible). As mentioned in the introduction, the possibility of using of the 72Ge(n,p)72Ga reaction for evaluation of the fast component of the reactor spectrum depends on the reliability of the cross-section value. In this respect, the present datum is the result of two entirely independent sets of determinations at different reactors and facilities and in some aspects different methodology; the difference between the averages for both sets was < 3%. As the precision is also good, the conclusion is that this reaction can be effectively used as monitor of the fast neutron flux. It is evident that a variety of reactions, e.g. those used in the present work for standardization, are more suitable for fast flux determination. Nevertheless, the positive advantage of the use of germanium as flux monitor is the feasibility of simultaneously performing the determination of not only the fast component but also of the thermal and epithermal components of the neutron spectrum. However, it should be mentioned that the detection of 72Ga becomes difficult when germanium is irradiated in the undisturbed reactor spectrum. The radiochemical method described in the present paper can also be applied in routine work for gallium separation and determination of the fast flux. Acknowledgement-The authors wish Arribere for her valuable collaboration.
to
thank
M.
A.
References Table
3. Control of gallium impurities in germanium (“Ga/72Ga activity ratios for different gamma emissions)
E.., “Ga/E.. 297.3 325.7 297.3 325.7 297.3 325.7 297.3 325.7
, ‘*Ga
k&/630.0 keV keVi630.0 keV keV/834. I keV k&‘/834.1 keV keV/894.3 keV keV/894.3 keV k&/1050.9 keV keV/l050.0 keV
‘, Undisturbed E, Epicadmium
spectrum. spectrum.
(A “Ga/A
“Ga)“/(A
“Ga/A
I .007l 0.9872 I .0030 I .0242 I .0042 0.9839 0.9973 0.9770 averase: 0.998 _ + 0.006
72Ga)E
Baard J. H., Zijp W. L. and Nolthenius H. J. (1989) Nuclear Data Guide for Reactor Neurron Metrology. Kluwer Academic Publishers, The Netherlands. Browne E. and Firestone R. B. (1986) Table of Radioactive Isotopes (Ed. Shirley V. S.). John Wiley and Sons, New York. Calamand A. (1974) Cross-sections for fission neutron spectrum induced reactions. In Handbook on Nuclear Activation Cross-Sections, p, 273. IAEA Technical Report Series 156. Cohen I. M. (1991) Analytical importance of some secondary and threshold reactions induced in nuclear reactors. J. Radioanal. Nucl. Chem. Articles 148, 155.
516
J. C. FURNARIet al.
Cohen 1. M., Kestelman A. J. and Furnari J. C. (1992) Determination of 70Ge(n,p)7”Ga and “Ge(n,p)74Ga reaction cross-sections for a fission neutron spectrum. Presented at the Third International Conference on Nuclear and Radiochemistry. Vienna, 7-1 I September. To be published in J. Radioanal. Nucl. Chem. Articles, De Corte F., Simonits A., De Wispelaere A. and Hoste J. (1987) Accuracy and applicability of the k,-standardization method. J. Radioanal. Nucl. Chem. Articles 113, 145. De Neve R., De Soete D. and Hoste J. (1966) The cross72Ge(n,p)7’Ga 7’-Ge(n.a)6“mZn reactor and sections, Radiochim. Acta 5, 188. Horibe 0. (1983) A new empirical rule for the estimation of fission neutron spectrum averaged cross-sections of
the (n,p) and (n,a) reactions. .4nn. NW/. Energy 10, 359. Pearlstein S. (1973) Neutron-induced reactions in medium mass nuclei. J. Nucl. Energ? 27, 8 I. Rau G. (1967) Bestimmung verschiedener iiber ein Spaltneutronenspektrum gemittelter n.p und n,% Wirkungsquerschnitte. Nukleonik 9, 226. Rochlin R. S. (1959) Fission-neutron cross-sections for threshold reactions. Nucleonics 17, 54. Salas Bacci A. (1991) Determination Experimental de la Section Eficaz Promedio de la Reaction “Cr(n.p)“V, para un Espectro de Neutrones de Fision. Thesis, Universidad Mayor de San And& La Paz. Bolivia. Tuli J. K. (1990) Nuclear IVaNe/ Cards. National Nuclear Data Center. Brookhaven National Laboratory.