Activation and mass spectrometric study of3He particle emission in the interactions of fast neutrons with medium mass nuclei

LIM

(1979) 63-72 ; © North-Holland Publishing Co ., Amsterdam Not to be reproduced by phOtoprint or microfilm without written permission from the publisher Nuclear Physics A329

ACTIVATION AND MASS SPECTROMETRIC STUDY OF 3He PARTICLE EMISSION IN THE INTERACTIONS OF FAST NEUTRONS WITH MEDIUM MASS NUCLEI C. H. WU, R. WOLFLE anti S. M. QAIM

Institut für Chemie der Kernforschungsanlage

Allich GmbH, Institut D-S17 Jülich, Federal Republic of Germany

1:

Nuklearchemie,

Received 2 April 1979

Absbtct: Cross sections for some (n, 3 He), (n, a), (n, 2a) and (n, n'a) reactions induced by fast neutrons produced via breakup of 53 MeV deuterons on a Be target (E. = 4-50 MeV ; I. at 22.5. MeV; FWHM = 15 .8 MeV) were measured for isotopes of the elements Al, P, K, Sc, V, Mn, Co, Zn, As, Nb, Mo and In by the activation technique using high-resolution 7-ray spectroscopy, wherever necessary chemical separation, and in several cases enriched isotopes as targets. Furthermore, the relative 3He/4 He emission cross sections were measured for Al, Ca, Sc, V, Mn, Fe, Cu, Zn, As, Nb and Ag using a quadrupole mass spectrometer. A comparison of the two sets of data shows that in the medium mass region the emission of a bound 'He particle is more probable than the emission of three single nucleons (2pn). The emission of 3 He particles reltive to 'He particles increases with increasing Z of the target element. In terms of absolute magnitude, however, even at relatively high excitation energy the emission of 3 He particles constitutes a relatively weak reaction channel.

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NUCLEAR REACTIONS 3'Al, 31p, 41K , 41Sc, 1I V, ISM , s9C o~ 67,6sZn, 75A ., 93 Nb, iis 1n(n, 3He) ; 27ALt 3i P. 4sSc, si V, ssMn, s9co, se zM , 75A  93Nb, i 1 sln (n , a) ; s'P , 39y , s9CO, 66,682!14 .93 Nb, 92Mo(n, 2a) ; s'V, I I-In(n, n'a) ; Al, Ca, Sc, V, Mn, Fe, Cu, s5 Mn Zn, As, Nb, Ag [(a, x3He)/(n, x 4Hef; E=4-50 MeV [from Be(d, n), E= 53 MeV], measured a. Activation and mass spectrometry . Natural and enriched targets in diverse chemical forms; Ge(Li) detector, quadrupole mass spectrometer . Investigated ( 3 He) emission relative to (2pn) emission .

1. Intrnduction Whereas the emission of neutrons, protons and a-particles in the interactions of fast neutrons with medium and heavy mass nuclei has been rather extensively investigated, the available information on the emission of some complex particles like ZH, 3H, 3He and 'Be is relatively small (df. ref. t )) . The latter reactions have generally low cross sections and are more difficult to investigate. Systematic studies in the medium and heavy mass regions on trinucleon emission reactions, i.e. (n, t) and (n, 3He), carried out at Jülich Z -6) and elsewhere 7) have shown that at 14.6 MeV the (n, t) and (n, 3He) reaction cross sections lie in the pb region and follow somewhat similar systematic trends a . s) ; in terms of absolute values, the (n, t) cross section is by anorder of magnitude higher than the respective (n, 3He) cross section s). 63

64

C. H. WU et al.

Our studies 6) showed further that at higher excitation energies though the (n, t) cross section is considerably higher than with 14.6 MeV neutrons and reaches the mb level, its contribution to the nonelastic cross section 6) does not exceed 0.25 %. It was also concluded that for target nuclei with A > 40 the emission of three nucleons (lp2n) is favoured over the emission of a bound triton. Now we report on our investigations on (n, 'He) reactions at relatively high excitation energies . Besides activation measurements, mass spectrometric technique has been applied. Mass spectrometric measurements have been reported previously (cf. ref. s)) for the determination of "He gas produced in fast neutron induced (n, a) reactions on reactor materials. The present work describes the first application of this technique to the study of (n, 3He) reactions on medium mass nuclei . 2. Experimental methods Integral cross-section measurements were carried out for a deuteron breakul neutron spectrum . Many of the experimental methods were similar to those toi studies on (n, t) reactions Z-4 .6) ; here only the newer information is given in detail 2.1 . NEUTRON SPECTRUM AND IRRADIATIONS

Irradiations were carried out with fast neutrons produced by bombarding a 1 crr thick Be target with 53 MeV deuterons at the Jiilich isochronous cyclotron (JULIO) The irradiation setup has already been described 3, 6) . For defining the neutron spectrum, we had originally 3 , 6) adopted Schweimer's data 9) extending over neutron energies of 11 .5 to 43 .5 MeV. However, recently Meulders et al. 1°) have reported more extensive measurements covering the neutron energy region of 4-50 MeV . Using those recent data we constructed the shape of the neutron spectrum for 53 MeV deuterons on Be and obtained an integrated neutron yield (E > 4 MeV) of 6.377 x 101 ' neutrons - uC ' - sr- ' . The constancy in the shape of the spectrum at various irradiation geometries used was tested using the 27Al(n, a)24Na, 197Au(n, 2n)'96Au, 197 Au(n, 3n)' 95Au and 197 Au(n, 4n) 194Au reactions. In the forward direction the maximum intensity of the neutrons (I.=) occurs at 22 .5 MeV and the FWHM of the spectrum is 15.8 MeV. The energy region below 4 MeV is unexplored . Though the uncertainty in the low-energy part of the spectrum would affect the absolute values of the cross sections reported here, the ratios of the (n, 3He) to (n, 4He) cross sections should remain unaffected . The neutron flux densities at the irradiation positions were calculated from the integrated charge at the Be converter using a Faraday cup ; additionally the reactions 27Al(n, a) 24 Na and 197 Au(n, 2n)'96 Au were used as flux monitors . The reaction 197Au(n, y)19sAu served as a useful check on the relative contribution of thermal neutrons which was small. In general, neutron flux densities of 6 x 101° cm- 2. sec -1 were available at the irradiation geometries used.

(n, 3 He) CROSS SECTIONS

!65

Targets for irradiations were prepared in two different ways. For measurements involving ß - counting or y-ray spectroscopic analysis of the activation products, 0.1 to 0.3 g of the high-purity target material (cf. table 1), in several cases as highly enriched target isotope, was packed in a polyethylene foil, sandwiched between monitor fbils, and irradiated for periods ranging between a few minutes to several hours, depending on the product nucleide to be studied. For mass spectrometric measurements, however, 5-10 g of the high-purity target metal (cf. table 2) was degassed and sealed under vacuum in a quartz ampoule and irradiated for 15 h. 2.2 . CROSS-SECTION MEASUREMENTS VIA ACTIVATION TECHNIQUE

Except for a few cases where, due to the absence of suitable y- or X-rays ßproportional counting was employed, the radioactivity of the reaction product was determined by high-precision y-ray spectroscopy using a co-axial 35 cm-' Ge(Li) detector or, in case of low energy y- and X-ray emitters, a Ge detector with an active area of 2 cm2 having a thin beryllium window. The y-ray spectra were analysed using a 24 K ND System 4420 . In addition to the identification of the characteristic y-ray peaks, a check of the half-lives was also carried out. Whenever necessary, especially in those cases where ß - counting was applied, radiochemical separations 11) of the product elements were performed. Cross sections were calculated by applying the usual corrections 2-1 .12) like those for decay, y-ray branching, counting efficiency, geometry, absorption etc. The decay data used were taken from the literature 13,1 4) and are given in table 1 together with other data . In general, only strong y-transitions with well-defined abundances were used; however, in those cases where two activation products emitted the same y-lines and their half-lives were more or less similar (e.g. some (n, 4n) and (n, 'He) products), less abundant but more characteristic y-rays were employed. The total errors in cross sections were estimated as described earlier 2-4,12) and 'amount to about 20 % for (n, a) and (n, n' a) reactions, 30 % for (n, 3He) reactions and 50 for (n, 2a) reactions. 2.3 . RELATIVE CROSS-SECTION MEASUREMENTS VIA MASS SPECTROMETRY

The apparatus used for mass spectrometric measurements is shown schematically in fig. 1 . The associated ultra-high vacuum system has already been described 1 s). A quadrupole mass spectrometer (Quad 250 B, EAI, Calif.) was used to analyse the' composition of the gas released from the samples. The irradiated sample was loaded into a molybdenum capsule (inner diameter 12 mm, length 19 mm) which was degassed at 800 °C for several hours under a vacuum of - 10- e Torr. During opening of the irradiated ampoule and transfer of the irradiated material to the capsule some of the formed gas might have escaped. This possible loss, however, should not introduce any extra errors in the present work

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Fig . 1 . Schematic diagram of the apparatus used for mass spectrometric measurements of the light-mass gaseous products formed in the interaction of fast neutrons with nuclei.

since only relative cross sections were measured. The capsule was heated by an r.f. generator (6 kW, 0.5 MHz) which was held at a constant power output by means of an alternating current stabilizer . An installed shutter served to distinguish the gas evaporated out of the capsule from the residual gas: The temperature of the capsule was measured with an optical pyrometer (Pyrowerk, Hannover) and a Pt Rh/Pt thermocouple, calibrated at the triple points of lithium, silver and gold . A capsule temperature stability of ± 1 .5° at 950 K was achieved. It was to be expected that on heating an irradiated sample the gaseous species 1H, 2H, 3H, 3He and 4He formed via (n, charged particle) reactions would be present. The amounts of those atoms should be dependent on the cross sections of the respective contributing nuclear reactions. If it is assumed that the diffusion coefficients of 3 He and'He in the irradiated sample (matrix), theirrates ofdesorption and solubilities are equal, the concentrations of 3 He and 'He over the whole sample should be directly proportional to the cross sections of the 3He and 4He producing nuclear reactions (n, oc a,) .

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From the relation

69

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I oc AEan,

where I is the relative ion intensity, AE the effective electron energy for electron impact ionization, a the area of vaporization and n the number of particles in unit volume of the gas, one obtains hHe oc n,H. and I,H . oc n,H., since during the measurements dE and a are the same for both 'He and "He. One therefore gets the expression It cc ar On heating the irradiated samples, due to the presence of hydrogen and helium isotopes, the following mass numbers (mle) could possibly be expected : 2 l( 1H+ ), 2(H2 ~ H+ ), 3(3He+ , 3H +, 2H'H+ ), 2 1H3H 4(4He + , H+, +), 5(3H2H +), 6(3H+) .

The aim ofthe present measurements was to determine accurately the relative intensities of'He and 'He. For this purpose, at first the ion intensity as a function of electron energy was determined for 2H2 and 2H + . A constant pressure of ZHZ in the vacuum chamber was maintained by means of a gas delivery system which permits admission of-2 x 10-9 mole ZHZ with an accuracy of f 1 % with the help ofa Barocel Type E523 (Wilmington, Ma) membrane micromanometer . A similar ion intensity measurement was then carried out for 'He. In those two measurements it was assumed that 3H+ and 3 H2 have the same ionization efficiency as 2H+ and 2H2, and 3He the same as 4He. The ionization potentials of 3H (13.54 eV) and 3H2 (15.46 eV) are much lower than that of 3He and 4He (24.59 eV). Therefore, in order to distinguish the contributions from 3H and 3He to mass 3 and from ZHZ and 4He to mass 4, two sets of electron energies were used. Evaluations of the relative ion intensities for 3He and 4He were carried out using the following relations I(3 He') = I(3He++3H++1H2H+)4oev-I(3H++1H2H+)2oevx2.5, I(4He + ) = I(4He++ZH2 + 1 H3H +)4o ev-I(2H2 + 1 H3H +)2o ev x 2.5,

where I4o ev is the total relative ion intensity of mass number 3 or 4 at 40 eV and I2oev is the relative ion intensity of 3H + and ZHZ at 20 eV; the constant 2.5 was obtained by means of the ionization efficiency curve of ZHZ, J(2 H+) J(3 H%oev g 40 @V , - 2.5 I( H2920 ev I( H )2o e v

The quadrupole mass spectrometric system used has a high sensitivity and ion currents of - 3.5 x 10 - ' 6 A can be detected . This detection limit corresponds to an effusion rate of - 2.1 x 108 particles - sec-1. Furthermore, the dynamic range of the system is 107. This means that the intensity ratios of 1 : 107 for neighbouring masses can be well distinguished . The estimated errors in the measured 3He/4He ratios amount to f 20 %. Measurements were carried out within one month after

C. H. wu er al .

70

the end of the irradiations so that 'He produced in the samples via the decay of tritium was < 1 %. The ratios therefore depict the 'He to 4 He formation reaction cross sections. 3. Results

The nuclear reactions investigated by the activation technique, their Q-values (calculated using the binding energies given in ref. t')) and the measured cross sections are given in table 1 together with other data . A total of 13 (n, 3 He),11(n, a), 8 (n, 2a) and 2 (n, n' a) reactions were investigated on isotopes of the target elements between aluminium and indium . Each cross-section value is based on at least three independent measurements and the given errors include both statistical and systematic errors . The contribution to each activation product from decay of precursors as well as from interfering nuclear reactions on target impurities was invariably subtracted. In the case of (n, 3 He) reactions, the measured activation cross section may also entail some contributions from (n, 2pn), (n, n' 2p) and (n, dp) processes, all ofwhich are energetically possible and lead to thesameproduct nucleus. Tear£ 2

3He to 'He emission cross-section ratios measured via mass spectrometry Target Al Ca Sc

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Mn Fe Cu Zn As Nb Ag

Chemical purity (/)

I( 3He) _ a(n, x3He) I e) a(n, He) H

99.99 99 .3 99 .8 99 .8 "specpure" 99 .99 99 .99 99 .99 99A 99 .99 99 .999

0.085 0.112 0.168 0.156 0.222 0222 0.250 0.270 0.286 0.294 0.417

The ratios of 3 He to 'He emission cross sections for 11 target elements between aluminium and silver, obtained mass spectrometrically, are given in table 2. The ratio refers to the target element as a whole and not to any specific stable isotope of the element; it depicts the ratio of the cross section for the emission of 3 He particles to that for 'He particles averaged over all the stable isotopes of the particular element investigated.

(n, 'He) CROSS SECTIONS

71

4. Diwomioo The (n, 'He) cross-section data for the "breakup" neutron spectrum reported here are by a factor ofabout 2 x 102 higher than those at 14.6 MeV [ref. s )], evidently due to the much higher excitation energy encountered in this work . In absolute terms, however, the (n, 'He) cross section is small and constitutes only a very small fraction ofthe nonelastic cross section. In spite of the rather high excitation energies involved, the emission of a 3 He particle from medium mass nuclei thus remains a relatively rare process. Mass spectrometric measurement of the emitted a-particles gives a sum of the cross sections of all the a-emitting processes, i.e. a sum of the cross sections of reactions like (n, a), (n, n' a), (n, 2a) etc. Activation measurements given in table 1 show that the major contribution to the a-emission process is furnished by (n, a) and (n,n' a) reactions, the cross section for the (n, 2a) reactions being negligible .

PROTON NL4EER OF THE TARGET ELE4ENT UI

Fig . 2 . 'He to 'He emission cross-section ratios as a function of Z of the target dement . The activation data describe the ratio of af(n, 'He)+(minor contributions from (n, 2pn) and (n, dp) processes)] to a(n, `He) ;'the mass spectrometric data give a(n, x'He)/a(n, x4He).

The ratios of 'He to 'He emission cross sections determined mass spectrometrically are shown in fig. 2 together with the ratios u(n, 'He)/a(n, `He) obtained from the activation cross-section data as a function of proton number of the target element (Z). Two conclusions can be drawn (a) The cross-section ratios obtained by the activation technique are identical with those determined via mass spectrometry. (b) The emission of 'He particles relative to "He particles increases with the increasing Z of the target nucleus.

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C. H. WU et al.

The identity of the 3He/4He ratios, obtained by the activation and mass spectrometric methods, is as yet difficult to explain. On possible explanation may be that similar to a-emission in the case of 'He emission as well (n, 'He) and, to a much lesser extent, (n, n' 3He) processes are involved ; furthermore, the contribution of the processes like (n, dp), (n, 2pn) and (n, n' 2p), which lead to the same activation product as the(n, 3He) reaction, is relatively small. This explanation would, however, imply that over the energy region considered here the emission of a bound 3He particle is favoured over that of three single nucleons (2pn). This observation. is in contrast to that for the (n, t) reaction where the emission of three single nucleons (1p2n) is favoured over that of abound triton 6). Presumably the reaction mechanisms involved in 3H and 3He emission are different. Our Hauser-Feshbach calculations at incident neutron energies of 14.6 MeV tend to show that in the medium mass region, whereas the (n, t) reaction has appreciable contributions from the statistical process, in the case of the (n, 3He) reaction more direct interactions are involved . Further experimental work and detailed calculations are underway to confirm this . The apparent increase in the 3He particle emission cross section relative to 4He particle emission cross section as a function of Z originates from a sharper decrease in the (n, a) cross section with increasing Z as compared to that in the case of (n, 3 He) cross section. This may also indicate higher contributions from direct processes in the case of (n, 3He) reaction than in the (n, a) reaction . We thank Prof. G. St6cklin for his active support of this research programme, the staff of the Jflich Isochronous Cyclotron (JULIO) for carrying out the irradiations, and Mr. H. Ollig, Mr. F. Fr6schen and Mrs. A. Schleuter for experimental assistance . References 1) S. M. Qaim, Proc . Int . Conf. on neutron physics and nuclear data for reactors and other applied purposes, Harwell, September 1978 (NEA, Paris, 1979) p. 1088 2) S. M. Qaim and G. St8cklin, J. Inorg. Nucl . Chem . 35 (1973) 19 3) S. M. Qaim, R. W61fle and G. St5cklin, J. Inorg. Nucl. Chan . 36 (1974) 3639 4) S. M. Qaim and G. St6cklin, Nucl. Phys . A257 (1976) 233 5) S. M. Qaim, Radiochim. Acta 25 (1978) 13 6) S. M. Qaim and R. W61fle, Nucl . Phys. A295 (1978) 150 7) T. Biro, S. Sudar, Z. Miligy, Z. Denso and J. Csikai, J. Inorg. Nucl . Chum. 37 (1975) 1583 8) H. Farrar IV, W. N. McElroy and E. P. Lippincott, Nucl . Techn. 25 (1975) 305 9) G. W. Schweimer, Nucl. Phys. A100 (1967) 537 10) J. P. Moulders, P. Leleux, P. C. Macq and C. Pirart, Phys. Med. Biol. 20 (1975) 235 11) S. M. Qaim, R. W61fle and G. St6cklin, J. Radioanalyt. Chem . 30 (1976) 35 12) S. M. Qaim, Nucl . Phys. A224 (1974) 319 13) Nucl. Data Sheets ORNL-USA (Academic Press, NY) 14) G. Erdtmann and W. Soyka, J. Radioanalyt . Chan . 26 (1975) 375 15) H. R. We and C. H. Wu, J. Chem . Phys. 63 (1975) 1605 16) C. H. Wu, J. Chem . Phys . 66 (1977) 4400 17) A. H. Wapstra and N. B. Gave, Nucl . Data Tables A9 (1971) 265