LIM

ACTIVATION AND MASS SPECTROMETRIC STUDY OF 3He PARTICLEEMISSION IN THE INTERACTIONS OF FAST NEUTRONS

WITH MEDIUM MASS NUCLEI

E

Nuclear Physics A329 (1979) 63-72; © North-Holland Publishing Co ., AmsterdamNot to be reproduced by phOtoprint or microfilm without written permission from the publisher

C. H. WU, R. WOLFLE anti S. M. QAIMInstitut für Chemie der Kernforschungsanlage Allich GmbH, Institut 1: Nuklearchemie,

D-S17 Jülich, Federal Republic of Germany

Received 2 April 1979

Absbtct: Cross sections for some (n, 3He), (n, a), (n, 2a) and (n, n'a) reactions induced by fast neutronsproduced 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, wherevernecessary chemical separation, and in several cases enriched isotopes as targets. Furthermore,the relative 3He/4He emission cross sectionswere 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 showsthat in the medium mass region the emission of a bound 'He particle is more probable than theemission of three single nucleons (2pn). The emission of 3He particles reltive to 'He particlesincreases with increasing Z of the target element . In terms of absolute magnitude, however, evenat relatively high excitation energy the emission of 3He particles constitutes a relatively weakreaction channel.

NUCLEAR REACTIONS 3'Al, 31p, 41K , 41Sc, 1I V, ISM , s9Co~ 67,6sZn, 75A ., 93Nb,iis1n(n, 3He) ; 27ALt 3iP. 4sSc, siV, ssMn, s9co, se Mz , 75A � 93Nb, i 1 sln (n , a) ; s'P, 39ys5Mn, s9CO, 66,682!14 .93Nb, 92Mo(n, 2a) ; s'V, I I-In(n, n'a) ; Al, Ca, Sc, V, Mn, Fe, Cu,Zn, As, Nb, Ag [(a, x3He)/(n, x4Hef; E=4-50 MeV [from Be(d, n), E= 53 MeV], mea-sured a. Activation and mass spectrometry . Natural and enriched targets in diverse chemicalforms; Ge(Li) detector, quadrupole mass spectrometer . Investigated (3He) emission relative

to (2pn) emission .

1. Intrnduction

Whereas the emission of neutrons, protons and a-particles in the interactions offast neutrons with medium and heavy mass nuclei has been rather extensivelyinvestigated, the available information on the emission of some complex particleslike ZH, 3H, 3He and 'Be is relatively small (df. ref. t)) . The latter reactions havegenerally low cross sections and are more difficult to investigate. Systematic studiesin 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 14.6MeV the (n, t) and (n, 3He) reaction cross sections lie in the pb region and followsomewhat similar systematic trends a. s); in terms of absolute values, the (n, t) crosssection is by anorder ofmagnitude higherthan 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) crosssection is considerably higher than with 14.6 MeV neutrons and reaches themb level,its contribution to the nonelastic cross section 6) does not exceed 0.25 %. It was alsoconcluded that for target nuclei with A > 40 the emission of three nucleons (lp2n)is favoured over the emission ofa bound triton. Nowwe report on our investigationson (n, 'He) reactions at relatively high excitation energies . Besides activationmeasurements, mass spectrometric technique has been applied. Mass spectro-metric measurements have been reported previously (cf. ref. s)) for the determinationof "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 breakulneutron spectrum . Many of the experimental methods were similar to those toistudies 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 crrthick Be target with 53 MeV deuterons at the Jiilich isochronous cyclotron (JULIO)The irradiation setup has already been described 3, 6) . For defining the neutronspectrum, we had originally 3, 6) adopted Schweimer's data 9) extending over neutronenergies of 11 .5 to 43.5 MeV. However, recently Meulders et al . 1°) have reportedmore extensive measurements covering the neutron energy region of 4-50 MeV.Using those recent data we constructed the shape ofthe neutron spectrum for 53 MeVdeuterons on Be and obtained an integrated neutron yield (E� > 4 MeV) of6.377 x 10 1 ' neutrons - uC- ' - sr- ' . The constancy in the shape of the spectrumat various irradiation geometries used was tested using the 27Al(n, a)24Na, 197Au(n,2n)'96Au, 197Au(n, 3n)' 95Au and 197Au(n, 4n)194Au reactions . In the forwarddirection the maximum intensity of the neutrons (I.=) occurs at 22.5 MeV and theFWHM of the spectrum is 15.8 MeV. Theenergy region below 4 MeV is unexplored .Though the uncertainty in the low-energy part of the spectrum would affect theabsolute 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 aFaraday cup ; additionally the reactions27Al(n, a)24Na and 197Au(n, 2n)'96Au were used as flux monitors . The reaction197Au(n, y)19sAu served as a useful check on the relative contribution of thermalneutrons which was small. In general, neutron flux densities of 6 x 10 1° cm- 2. sec -1were available at the irradiation geometries used .

(n, 3He) CROSS SECTIONS

!65

Targets for irradiations were prepared in two different ways . For measurementsinvolving ß - counting or y-ray spectroscopic analysis of the activation products, 0.1to 0.3 g of the high-purity target material (cf. table 1), in several cases as highlyenriched target isotope, was packed in a polyethylene foil, sandwiched betweenmonitor fbils, and irradiated for periods ranging between a few minutes to severalhours, depending on the product nucleide to be studied . For mass spectrometricmeasurements, however, 5-10 g of the high-purity target metal (cf. table 2) wasdegassed 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 ß-proportional counting was employed, the radioactivity of the reaction product wasdetermined 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 activearea of2 cm2 having a thin beryllium window . The y-ray spectra were analysed usinga 24 KND System 4420 . In addition to the identification of the characteristic y-raypeaks, a check of the half-lives was also carried out. Whenever necessary, especiallyin those cases where ß - counting was applied, radiochemical separations 11) of theproduct elements were performed .

Cross sections were calculated by applying the usual corrections 2-1.12) like thosefor decay, y-ray branching, counting efficiency, geometry, absorption etc. The decaydata used were taken from the literature 13,14) andare given in table 1 together withother data . In general, only strong y-transitions with well-defined abundances wereused ; however, in those cases where two activation products emitted the samey-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 totalerrors in cross sections were estimated as described earlier 2-4,12) and 'amountto about 20% for (n, a) and (n, n' a) reactions, 30 %for (n, 3He) reactions and 50for (n, 2a) reactions .

2.3 . RELATIVE CROSS-SECTION MEASUREMENTS VIA MASS SPECTROMETRY

The apparatus used for mass spectrometric measurements is shown schematicallyin 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 avacuum of - 10- e Torr. During opening of the irradiated ampoule and transferof 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 ofthe light-massgaseous 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) whichwas held at a constant power output by means ofan alternating current stabilizer . An installed shutter served to distinguish the gasevaporated out of the capsule from the residual gas: The temperature ofthe capsulewas measured with an optical pyrometer (Pyrowerk, Hannover) and a Pt Rh/Ptthermocouple, calibrated at the triple points of lithium, silver and gold . A capsuletemperature 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 re-spective contributing nuclear reactions . If it is assumed that the diffusion coefficientsof3He and'He in the irradiated sample(matrix), theirrates ofdesorption and solubili-ties are equal, the concentrations of 3He and 'He over the whole sample should bedirectly proportional to the cross sections of the 3He and 4He producing nuclearreactions (n, oc a,).

From the relation 16)

(n, 'He) CROSS SECTIONS

69

I oc AEan,

where I is the relative ion intensity, AE the effective electron energy for electronimpact ionization, a the area of vaporization and n the number of particles in unitvolume ofthe gas, one obtains hHe oc n,H. and I,H. oc n,H., since during the measure-mentsdE and a are the same for both 'Heand"He. One therefore gets the expressionIt cc arOn heating the irradiated samples, due to the presence of hydrogen and helium

isotopes, the following mass numbers (mle) could possibly be expected :

l( 1H+ ), 2(H2~ 2H+ ), 3( 3He+ , 3H+, 2H'H+ ),4(4He + , 2H+, 1H3H+), 5(3H2H +), 6(3H+) .

Theaimofthe present measurements was to determine accurately the relative intensi-ties of'Heand'He. For this purpose, at first the ion intensity as a function ofelectronenergy wasdetermined for 2H2 and 2H+ . Aconstant pressure of ZHZ in the vacuumchamber wasmaintained by meansofa gas delivery system which permits admissionof-2 x 10-9mole ZHZ with an accuracy of f1%with the help ofa Barocel Type E523(Wilmington, Ma) membrane micromanometer. A similar ion intensity measure-ment was then carried out for 'He. In those two measurements it was assumed that3H+ and 3H2 have the same ionization efficiency as 2H+ and 2H2, and 3He the sameas 4He.The ionization potentials of3H(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 contributionsfrom 3H and 3He to mass 3 and from ZHZ and 4He to mass 4, two sets of electronenergies were used . Evaluations of the relative ion intensities for 3He and 4He werecarried out using the following relations

I(3He') = I(3He++3H++1H2H+)4oev-I(3H++1H2H+)2oevx2.5,I(4He+ ) = I(4He++ZH2+ 1H3H+)4o ev-I(2H2 + 1H3H+)2o ev x 2.5,

where I4o ev is the total relative ion intensity of mass number 3 or 4 at 40 eV andI2oev is the relative ion intensity of 3H+ and ZHZ at 20 eV; the constant 2.5 wasobtained by means of the ionization efficiency curve of ZHZ,

J(2H+)

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40 @V

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- 2.5H2920 ev

I( H )2o evThe 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 aneffusion rate of - 2.1 x 108 particles - sec-1 . Furthermore, the dynamic range ofthe system is 107. This means that the intensity ratios of 1 : 107 for neighbouringmasses can be well distinguished . The estimated errors in the measured 3He/4Heratios amount to f20 %. Measurements were carried out within one month after

70

C. H. wu er al .

the end of the irradiations so that 'He produced in the samples via the decay oftritium was < 1 %. The ratios therefore depict the 'He to 4He formation reactioncross sections .

The nuclear reactions investigated by the activation technique, their Q-values(calculated using the binding energies given in ref. t')) and the measured crosssections are given in table 1 together with other data . Atotal of 13 (n, 3He),11(n, a),8 (n, 2a) and2 (n, n' a) reactions were investigated on isotopes ofthe target elementsbetween aluminium and indium . Each cross-section value is based on at least threeindependent measurements and the given errors include both statistical andsystematic errors . The contribution to each activation product from decay ofprecursors as well as from interfering nuclear reactions on target impurities wasinvariably subtracted . In the case of (n, 3He) reactions, the measured activation crosssection 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.

3He to 'He emission cross-section ratios measured via mass spectrometry

3. Results

Tear£ 2

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

Target Chemical purity(/)

I( 3He) a(n, x3He)_IHe) a(n, He)

Al 99.99 0.085Ca 99 .3 0.112Sc 99 .8 0.168v 99 .8 0.156Mn "specpure" 0.222Fe 99 .99 0222Cu 99.99 0.250Zn 99 .99 0.270As 99A 0.286Nb 99.99 0.294Ag 99.999 0.417

(n, 'He) CROSS SECTIONS

71

4. Diwomioo

The (n, 'He) cross-section data for the "breakup" neutron spectrum reportedhere are by a factor ofabout 2 x 102 higher than those at 14.6 MeV[ref. s )], evidentlydue 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 fractionofthe nonelastic cross section. In spite ofthe rather high excitation energies involved,the emission of a 3He particle from medium mass nuclei thus remains a relativelyrare 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 ofreac-tions like (n, a), (n, n' a), (n, 2a) etc. Activation measurements given in table 1 showthat 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 ofZ of the target dement . The activationdata 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 sectionsdetermined mass spectrometricallyare shown in fig. 2 together with the ratios u(n, 'He)/a(n, `He) obtained from theactivation 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 withthose determined via mass spectrometry.

(b) The emission of 'He particles relative to "He particles increases with the in-creasing Z of the target nucleus.

72

C. H. WU et al.

The identity of the 3He/4He ratios, obtained by the activation and mass spectro-metric methods, is as yet difficult to explain. On possible explanation may be thatsimilar to a-emission in the case of 'He emission as well (n, 'He) and, to a muchlesser extent, (n, n' 3He) processes are involved ; furthermore, the contribution of theprocesses like (n, dp), (n, 2pn) and (n, n' 2p), which lead to the same activationproduct as the(n, 3He) reaction, is relatively small. This explanation would, however,imply that over the energy region considered here the emission of a bound3He 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 ofabound triton 6). Presumably the reaction mechanismsinvolved in 3H and 3He emission are different . Our Hauser-Feshbach calculations atincident 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 . Furtherexperimental 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 decreasein the (n, a) cross section with increasing Zas compared to that in the case of(n, 3He)cross section. This mayalso indicate higher contributions from direct processes in thecase 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 irradia-tions, and Mr. H. Ollig, Mr. F. Fr6schen and Mrs. A. Schleuter for experimentalassistance .

References1) 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. 10882) S. M. Qaim and G. St8cklin, J. Inorg. Nucl . Chem . 35 (1973) 193) S. M. Qaim, R. W61fle and G. St5cklin, J. Inorg. Nucl. Chan . 36 (1974) 36394) S. M. Qaim andG. St6cklin, Nucl. Phys . A257 (1976) 2335) S. M. Qaim, Radiochim. Acta 25 (1978) 136) S. M. Qaim and R. W61fle, Nucl . Phys. A295 (1978) 1507) T. Biro, S. Sudar, Z. Miligy, Z. Denso and J. Csikai, J. Inorg. Nucl . Chum. 37 (1975) 15838) H. Farrar IV, W. N. McElroy and E. P. Lippincott, Nucl . Techn. 25 (1975) 3059) G. W. Schweimer, Nucl. Phys. A100 (1967) 53710) J. P. Moulders, P. Leleux, P. C. Macq and C. Pirart, Phys. Med. Biol. 20 (1975) 23511) S. M. Qaim, R. W61fle and G. St6cklin, J. Radioanalyt. Chem . 30 (1976) 3512) S. M. Qaim, Nucl . Phys. A224 (1974) 31913) Nucl. Data Sheets ORNL-USA (Academic Press, NY)14) G. Erdtmann and W. Soyka, J. Radioanalyt . Chan . 26 (1975) 37515) H. R. We andC. H. Wu, J. Chem. Phys. 63 (1975) 160516) C. H. Wu, J. Chem . Phys . 66 (1977) 440017) A. H. Wapstra and N. B. Gave, Nucl . Data Tables A9 (1971) 265