Neutron activation cross-sections for Cr isotopes at 14.6 MeV neutron energy

Ann. nucl. Energy, Vol. 12, No. 11, pp. 577-581, 1985

0306-4549/85$3.00+0.00 Copyright© 1985PergamonPressLtd

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N E U T R O N ACTIVATION CROSS-SECTIONS FOR Cr ISOTOPES AT 14.6 MeV N E U T R O N E N E R G Y I. RIBANSK~"

Institute of Physics, EPRC, Slovak Academy of Sciences, 84228 Bratislava, Czechoslovakia Ts. PANTELEEVand L. STOEVA Institute of Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, Sofia 1181, Bulgaria (Received 28 November 1984; accepted in revised form 29 April 1985)

Al~tract--Neutron activation cross-sections for Cr isotopes were measured using Gc(Li) y-Sl~Ctroscopyof the reaction products. The linear least-squares method was used to resolve the interfering reactions. The results are compared with other data measured at 14.6-14.8 MeV.

1. INTRODUCTION Activation techniques have been used to measure the cross-sections for neutron-induced reactions of Cr isotopes with emphasis on the (n,p) and (n, np)* processes. Though many such data have been measured in the past, it is generally difficult to determine which value is to be preferred in varying applications or comparisons with theoretical calculations. The point is that the measured data often exhibit mutual disagreement substantially exceeding the quoted uncertainties. This is probably the reason why the new data are still requested in W R E N D A 83/84 (1983). MoreoTer, Cr is a potential component of fusion reactor construction material and, obviously, precise and reliable activation data are needed for that reason alone. Recently, the Coordinate Research Programme was launched by the IAEA (Vienna) to encourage this kind of measurements. We believe that one of the efficient ways to improve the existing situation in activation data for Cr is their careful remeasurement using modern experimental procedures, dataprocessing techniques and error-handling methods. The activation cross-sections measured in this work will be compared with all other results contained in CINDA. (Index to the literature and computer files on Microscopic Neutron Data. IAEA, Vienna.) However, only those data measured between 14.6 and 14.8 MeV incident neutron energy will be considered in order to minimize the influence of the excitation function's behaviour around 14 MeV. These data will be

referenced by the year in which they were included in CINDA. The original references are omitted as many of them were not available to the present authors. They can, however, be found in CINDA.

2. EXPERIMENTAL PROCEDURE

The details of the experimental technique and dataacquisition system have been described previously (Gmuca and Ribansk~,, 1983a; Ribansk~' and Gmuca, 1983). Here we report only details pertinent to the present experiment. The samples used were prepared by pressing C r 2 0 3 powder into cylindrical Plexiglas containers (dia = 16 mm, wall thickness = 1 mm, stopper = 3 ram). The enriched samples were supplied by TECHNABEXPORT (Moscow). All samples were of spectral purity and at least two targets were prepared from each isotopic mixture. The isotopic abundances are listed in Table 1. The sample thickness varied from 200 to 400 mg cm-2. The required neutrons were produced via the T(d,n)4He reaction using a small electrostatic accelerator (120 kV) and a thick Ti-T target on a 1 mm Cu backing cooled by a 2 mm layer of water. The frontwall thickness of the target housing (AI) was 2 mm. All samples (including 16 mm dia Fe and AI foils) were irradiated at 0 ° to the D-beam in a fixed geometry. The distance between the T i - T layer and the front surface of the samples was 9 mm; the D-beam spot dia was 8 mm. Taking into account the large-angle geometry employed and the neutron-producing target pro* The (n, np) symbol stands for the sum of the (n, n'p), (n, pn) perties, it is believed that the mean neutron energy was 14.6 MeV (Lewis, 1984). The time variation of the and (n, d) contributions. 577 ANE ±2:I±-A

578

I. RIBANSK~"et al. Table2. The principalsourcesof uncertainties

Table 1. Isotopicabundances(%)of Cr samples Isotope Sample

50

52

53

54

natCr S°Cr ~2Cr 53Cr

4.35 90.5 0.01 0.03 0.13

83.79 8.5 99.8 2.19 4.06

9.50 0.8 0.19 97.7 2.01

2.36 0.2 0.01 0.08 93.8

54Cr

neutron flux was monitored by two neutron detectors and was taken into account in the cross-section calculations. They were positioned at 90° to the Dbeam and at distances of 15 cm (proton recoil detector) and 3 m (Li glass scintillation detector). Both detectors were calibrated several times before each run using the 56Fe(n, p) reaction, assuming cq,.p) = 109.5 + 1.1 mbarn (Ryves et al., 1978a). To check the experimental procedure the AI samples were also irradiated and the cross-section for the 27Al(n,p) reaction was determined. Its value--relative to a~n.p~(56Fe)--turned out to be 74.6+ 2.4 mbarn. This is not in as good agreement with the 68 + 2 mbarn reported by Ryves et al. (1978b) as one would expect. This may be due to lowenergy neutrons from the considerably large target assembly. On the other hand, our value is in good agreement with the ENDF/B-V (1979) evaluation (73.2 mbarn). When possible, the irradiation of the Cr samples was repeated in order to lower the statistical uncertainties. The activities of the reaction products were measured with a Ge(Li) detector (2.5 keV at 1332 keV) and at least two y-ray spectra were taken after each irradiation and recorded on a disk for off-line processing. The fullenergy peak areas were evaluated using the code GWENN (Gmuca and Ribansk~, 1983b). They were corrected for self-absorption and coincidence summing effects using the code KORSUM (Debertin and Schttzig, 1979). To determine the contributions of the (n, p) and (n, np) processes on neighbouring isotopes leading to the same reaction product, measurements were made on samples with several different isotopic abundances. The corresponding cross-sections were evaluated using a linear least-squares method. UNCERTAINTIES Great care was taken in correctly treating both the correlated (systematic) and uncorrelated (statistical) errors connected with the present experiment. The covariance matrix method (Mannhart, 1981; Smith, 1981) was employed for estimation of our total data

Source Countingstatistics Samplemass Isotopicabundances 7-Rayintensities Detector efficiency(FEP) Reactionproducthalf-life Irradiationand countinggeometry (position) Coincidencesummingcorrections 7-Rayself-absorption Monitorcalibration[includingthe 56Fe(n,p) referencereaction]

Resulting uncertainty (%) 0.5-6 0.1 0.2 0.0-7.8 1.5 0.3-3.0 0.2 0.7 0.5 2.8

uncertainties. The sources of uncertainties which were taken into account are listed in Table 2. No scattering corrections were considered. All uncertainties quoted in this paper represent one standard deviation. 4. RESULTSAND DISCUSSION The results of our measurements are presented in Table 3, Those data which are intercorrelated are grouped and the corresponding covariance matrix is given in column 5. The number of independent irradiations is shown in column 4. The decay data of the reaction products are presented in Table 4. The coincidence summing corrections (factors by which the measured full-energy peak areas were multiplied) are given in the last column. Note that for the 54Cr(n, p) reaction these corrections amount to 20~o. The numbers in parentheses (columns 2 and 4) represent the uncertainties of the half-lives and y-transition intensities in the same format as used by Lederer and Shirley (1978). Our results are compared with the previous measurements performed at 14.6-14.8 MeV in Tables 5 and 6. In general, detailed comparison with other data is difficult because many important experimental details are often not presented by the authors, different reference reactions were frequently used and a different experimental technique (e.g. //-counting) was sometimes employed. Therefore, only very general observations based on Tables 5 and 6 will be presented. The data are ordered according to the year in which they were included in CINDA. Table 5

Our cross-section for the S°Cr(n, 2n) reaction is in very good agreement with the majority of other results. This statement also includes those data which do not

579

Neutron activation cross-sections for Cr isotopes Table 3. Neutron activationcross-sectionsfor Cr isotopesat 14.6MeV Reaction S°Cr(n,2n)'~gcr S2Cr(n,2n)~Cr 52Cr(n,p)S2v

53Cr(n,np)52V

53Cr(n' p)53V S4Cr(n,np)S3V s'*Cr(n,p)5'*V 5'*Cr(n,~t)st Ti

o (mbarn)

Uncertainty No. of Correlation (mbarn) irradiations matrix

21.2 375 73.4 7.6 37.0 1.6 15.3 11.0

1.2 23 3.2 0.6 1.6 0.2 0.7 0.5

overlap with our value within the quoted uncertainties by less than + 2 mbarn. The reason is that the measured excitation function (Chatterjee et al., 1969; Borman, 1965) exhibits a gradient of ~ 2 mbarn/0.1 MeV between 14 and 15 MeV. The remaining three values are compatible with our results only at the 3tr confidence level. For the 52Cr(n,2n) reaction the gradient of the excitation function is ,-~ 18 mbarn/0.1 MeV. Bearing this in mind one can see that our result is in very good agreement with about 50~o of the other data. The result of Araminovicz and Dresler (1972) is clearly too low. The value reported by Maslov et al. (1972) is very probably too high and this is also probably true for the Sailer et al. (1977) value. The last observation is based on the non-activation measurement of the ~atCr(n, 2n) cross-section (406 + 32 mbarn) measured by Frehaut et al. (1980) at 14.76 MeV. The contributions of the (n, 2n) reactions of the S4Cr and 53Cr isotopes to the Frehaut et al. (1980) value can be estimated from the semiempirical calculations of Pearlstein (1973). They represent 23 and 87 mbarn, respectively. The contribution of the 5°Cr(n, 2n) reaction is only ~ 1 m b a r n and we are left with a value of 353 + 28 m b a r n for the cross-section of the s 2Cr(n ' 2n) reaction. This value is in agreement with our result and supports our abovementioned assertion.

2 1 33 27 27 7

100 -14 100 100 -25 100 --

Table 6

Our cross-section for the 52Cr(n,p) reaction is in agreement with only 4 out of the 10 other measurements presented in Table 6. The disturbing fact is that the rest of the data values are substantially larger than our result. The contribution of the 53Cr(n, np) reaction (not taken into account) can not explain the observed discrepancy. For natural samples, assuming trCn,np)(53Cr) ~ 7 mbarn, this contribution represents 1 ~ because the product r/(53Cr)a¢n,np~(53Cr)--r/is the isotopic a b u n d a n c e - - i s ,-~ 1/100 r/(52Cr)tren,p)(52Cr). For samples enriched in 52Cr ' the situation is even more favourable. The incident neutron energy spread is also irrelevant because the excitation function of the 52Cr(n, p) reaction is practically constant around 14 MeV (Clator, 1969; Kern et al., 1959). It appears that our cross-section is probably too low, though no obvious reasons can be identified. The measured proton spectra of the 52Cr(n, xp) reaction (Grimes et al., 1979) can also be used to deduce the (n, p) activation cross-section. Assuming that the second particle (in this case the neutron) is definitely emitted from the 52V nucleus once its excitation energy reaches the neutron binding energy, the (n, p) activation cross-section can be approximated by the integral of the experimental proton spectrum from the pn-threshold to the

Table 4. Decaydata of reaction products Reaction product

t~

49Cr

42.09(15)m

51Cr 52V 53V 54V

27.703(4)d 3.760(8)m 1.61(5kn 49.8(1.0)s*

SlTi

5.76(3)rn

aReus et al. (1979).

E~ (keV) 90.6 152.9 320.1 1434.1 1006.0 834.8 986.0 319.7

Ir (%)

Coincidence summing correction

54.2(9) 30.9(5) 9.85(9) 100.0 90.0(2.0) 100.0 81.8(6.4) 93.4(9)

1.113 1.108) 1.0 1.007 1.001 1.215 1.224} 1.001

Reference ENDF/B-V (1979) IAEA(1983) Lederer and Shirley(1978) Lederer and Shirley(1978) Ledererand Shirley(1978) Lederer and Shirley(1978)

580

I. RIBANSKY et al. Table 5. Comparison of our (n, 2n) cross-sections with other measurements at 14.6-14.8 MeV 5°Cr(n, 2n)'tgCr (7 (mbarn)

E. (MeV)

21.2 5:1.2 21.9_+0.5 24 + 5 26.9 ± 2.0 26 4- 3 31.4 ± 2.5 21 ±2 24 ± 2 32.0 5- 3.2 27.0± 6.7 18.8 4-1.9

14.6 14.8 14.7 14.6 14.7 14.6 14.7 14.7 14.8 14.8 14.8

52Cr(n, 2n) 5t Cr

Reference This work Sailer et al. (1977) Valkonen (1976) Sigg (1976) Qaim (1972) Araminowicz and Dresler (1972) Chatterjce et al. (1969), EF Borman (1965), EF Mukhcrjos et al. (1961) Khurana and Hans (1961) Chittenden (1961)

(7 (mbarn)

E, (MeV)

Reference

375 4- 23 377 +_45 439 5:12 304 5:20 143 5:17 543 5:50 370 + 26

14.6 14.8 14.8 14.7 14.6 14.6 14.7

This work Molla et al. (1983) Sailer et al. (1977) Qaim (1972) Araminowicz and Dresler (1972) Maslov et al. (1972) Borman (1968), EF

EF = deduced from the measured excitation function.

maximum proton energy. From the data of Grimes et al. (1979) we obtained a value of ~ 8 0 + 1 5 mbarn, which is in agreement with our measurement. For the other two (n,p) reactions the situation is much better. Our ~r(n.p)(saCt) is in very good agreement with other results; only the Qaim and Molla (1977) value is noticeably higher. No comment is necessary on the S4Cr(n, p) reaction as all the results are in perfect mutual agreement. The (n, np) data are characterized by large discrepancies between our measurements and those of

Qaim and MoUa (1977), which are substantially higher. Whilst for the 53Cr(n, np) reaction their value is very possibly in error (the other two results are practically identical with our result), new measurements are needed for the 54Cr(n, np) reaction in order to resolve the observed discrepancy. The present value for the 54Cr(n, u) cross-section is in very good agreement with the measurements of Valkonen (1976) and Husian and Kuroda (1967). Our result differs slightly from that of Sailer et al. (1977) and significantly from that of Qaim (1974).

Table 6. Comparison of our (n, p), (n, np) and (n,,,) cross-sections with other measurements at 14.6-14.8 MeV 52Cr(n, p)52V (7 (mbarn) 73.4+3.2 80 4-6 94 5:10 96 4- 3 73 5:3 1104-24 1155:15 83 4- 6 105 4-10 83 4-9 113 5:12

Reference This work Qaim and Molla (1977) Valkonen (1976) Dresler et al. (1972) Prasad and Sarkar (1969) Clator (1969), EF Husain and Kuroda (1967) Mitra and Ghose (1965) Mukherjee et al. (1961) Chittenden (1961) Kern et al. (1959), EF

53Cr(n, p)S3V (7 (mbarn) 37.04-1.6 48 + 7 40 + 7 36 4- 6 44 5:5 37.35:3.7

53Cr(n, np)52V (7

(mbarn)

Reference

7.6 _ 0.6 125=3 7.3 4-0.4 7.1 4-1.5

This work Qaim and MoNa (1977) Webber and Duggan (1968) Husain and Kuroda (1967)

Reference This work Qaim and MoUa (1977) Valkonen (1976) Prasad and Sarkar (1969) Husain and Kurnda (1967) Chittenden (1961)

S('Cr(n, p)S4v (7 (mbarn)

Reference

15.34-0.7 18 4- 3 15 5:4 13.5 4-1.5

This work Qaim and MoUa (1977) Valkonen (1976) Husain and Kuroda (1967)

54Cr(n, np)53V (7 (mbarn)

Reference

1.65:0.2 2.05:0.8

This work Qaim and Molla (1977)

EF = deduced from the measured excitation function (at E. = 14.7 MeV).

S'Cr(n, u)51V (7 (mbarn)

Reference

11.04- 0.5 13.45:1.2 7 5:4 15.0 4-1.6 12.5 5:1,3

This work Sailer et al. (1977) Valkonen (1976) Qaim (1974) Husain and Kuroda (1967)

Neutron activation cross-sections for Cr isotopes Acknowledgement--This work was supported by the IAEA (Vienna) under Research Agreement No. 3436/CF.

REFERENCES

Araminowicz J. and Dresler J. (1972) CINDA 76/77. Borman M. (1965) CINDA 76/77. Borman M. (1968) CINDA 76/77. Chatterjee A., Nath A. and Ghose M. (1969) CINDA 76/77. Chittenden D. M. and Gardner D. G. (1961) CINDA 76/77. Clator I. G. (1969) CINDA 76/77. Debertin K. and SchStzig U. (1979) Nucl. Instrum. Meth. 158, 471. Dresler J., Araminowicz J. and Garuska U. (1972) CINDA 76/77. ENDF/B-V (1979) Dosimetry file. Frehaut J., Bertin A., Bois R. and Jary J. (1980) CINDA 83. Gmuca ~. and Ribansk~' I. (1983a) Acta phys. slov. 33, 9. Gmuca ~. and Ribansk~, I. (1983b) Jadern/~ Energ. 29, 56. Grimes S. M., Haight R. C., Alvar K. R., Barshall H. H. and Borchers R. R. (1979) Phys. Rev. C19, 2127. Husian L. and Kuroda P. K. (1967) CINDA 76/77. 1AEA (1983) Nuclear Data Standards for Nuclear Measurements. IAEA Tech. Rep. Ser. No. 227, Vienna. Kern B. D., Thompson W. E. and Ferguson J. M. (1959) CINDA 76/77. Khurana C. S. and Hans H. S. (1961) CINDA 76/77. Lederer C. M. and Shirley V. S. (1978) Tables of Isotopes, 7th edn. Wiley, New York.

581

Lewis V. E. (1984) Metrologia 20, 49. Mannhart W. (1981) Report PTB-FMRB-84. Maslov G. N., Nasyrov F. and Pashkin N. F. (1972) CINDA 76/77. Mitra B. and Ghose A. M. (1965) CINDA 76/77. Molla N. I., Mizanul I. M., Mizanur R. M. and Khatun S. (1983) CINDA 83. Mukherjee S. K., Ganguly A. K. and Majumder N. K. (1961) CINDA 76/77. Pearlstein S. (1973) J. nucl. Energy 27, 81. Prasad R. and Sarkar D. C. (1969) CINDA 76/77. Qaim S. M. (1972) CINDA 76/77. Qaim S. M. (1974) CINDA 76/77. Qaim S. M. and Molla N. I. (1977) CINDA 76/77. Reus U., Westermeier W. and Warnecke I. (1979) GSI Report 79-2. Ribansk~ I. and Gmuca ~. (1983) J. Phys. G9, 1537. Ryves T. B., Kolkowski P. and Zieba K. J. (1978a) Metrologia 14, 127. Ryves T. B., Kolkowski P. and Zieba K. J. (1978b) J. Phys. G4, 1783, Sailer K., Daroczy S., Raics P. and Nagy S. (1977) Prec. 4th All Union Conf. on Neutron Physics, Kiev, April, Vol. 1, p. 246 (in Russian). Sigg R. A. (1976) CINDA 76/77. Smith D. L. (1981) Report ANL/NDM-62. Valkonen M. (1976) CINDA 76/77. Webber L. D. and Duggan J. L. (1968) CINDA 76/77. WRENDA 83/84 (1983) Report IAEA INCD(SEC)88AJRSF.