Coincidence spectra from d + d → d + p + n and p + d → p + p + n reactions with neutron detection at zero degrees

Nuclear Physics Not

to be

COINCIDENCE REACTIONS

A182 (1912) 225-233;

reproduced

by photoprint

@

North-Holland

Publishing

or microfilm without written permission

SPECTRA FROM d+d+d+p+n WITH NEUTRON DETECTION

Co., Amsterdam from the publisher

AND p+d+p+p+n AT ZERO DEGREES

V. VALKOVIC t, N. GABITZSCH, D. RENDIC rt, I. DUCK and G. C. PHILLIPS T. W. Bonner Nuclear Laboratories, Rice University, Houston, Texas 77001 ftt Received

22 November

1971

Abstract: Proton-neutron

reaction were measured coincidence spectra from the p+d + p+p+n for 8, = 0” and 8, = 15”-40” at Ep = 10.0 MeV. Deuteron-neutron coincidence spectra from the d+d + d+n+p reaction were measured for 8, = 0” and 0, = 15”-35” at Ed = 10.0 MeV. Coincidence Spectra were analyzed in terms of a quasi-free scattering model. The possibility of using these reactions as a source of monoenergetic neutrons is discussed.

1. Introduction Nuclear reactions with three particles in the final state have been studied by many research groups. Most of the recent measurements involve coincidence detection of two outgoing particles. Usually two charged particles are detected (when possible) due to numerous experimental difficulties connected with the detection of neutrons. However, some useful information about the p + d + p + p + n reaction ’ - “) has been obtained by the detection of neutron-proton coincidences. Only a few measurements of (d, pn) and (p, pn) reactions, involving the detection of both outgoing proton and neutron, have been reported 4- “). N one of the reported measurements include the measurement of outgoing neutrons at zero degrees. In contrast to coincidence measurements, some measurements of neutron spectra at zero degrees from the charge-particle-induced reaction have been reported in kinematically incomplete measurements. Neutron energy spectra from the ‘H(p, n)2p reaction have been extensively studied at various proton energies ‘). The use of this reaction as a possible source of “monoenergetic” neutrons for the bombarding energies above 100 MeV has been discussed in refs. 8*“). In the measurements done by Larsen lo), it was found that 740 MeV proton beam bombardment of a deuterium target produced a neutron beam which could be represented by a Gaussian distribution with a standard deviation of 8 MeV and a most probable energy of 710+8 MeV. Measurements performed by Bowen et al. I’, 12) at E, = 143 MeV indicated that the neutron production proceeds mainly by simple proton-neutron quasi-free scattering. However, the experimentally derived value for the forward neutron production cross section ‘I) of 40 mb/sr disagrees with the impulse approximation calculation (22-28 mb/sr) of Castillejo and Singh 1“). f On leave from Institute “Ruder BoSkoviC”, Zagreb, Yugoslavia. tt Present address: Institute “Ruder BoSkoviC”, Zagreb. trt Work supported in part by the US Atomic Energy Commission. 225 March

1972

226

V. VALKOVIC et al.

Measurements performed at bombarding energies of 30 and 50 MeV [ref. ‘“)I show also a peak in the neutron spectra at 6 = 0” for high neutron energies (w 6 mb/sr MeV). The impulse approximation as formulated in ref. ’ 5)gives a satisfactory fit to this measurement. Measurements of neutron spectra at zero degrees from the p + d + ./TO

Fig. 1. Schematic layout of the experimental

BEAM SW’

arrangement.

p+ p+ n reaction for various low proton bombarding energies have usually been interpreted as indicating the strong influence of the proton-proton final-state interaction. Neutron spectra at en = 0” have been measured for E,, = 4-7 MeV [ref. r6)], E, = 8.9 MeV [ref. I’)], E, = 6.06,7.15, 8.90, 13.5 MeV [ref. ‘s)]and E, = 14.1 MeV [ref. ’ 9)]. Wong et al. “) reported reasonable agreement of measured neutron spectra with an impulse approximation calculation even at such low bombarding energies. Neutron spectrum from deuteron break-up in the ‘H(d, n)dp reaction have not been studied in as much detail. However, it was found ’ “) that the zero-degree neutron spectrum differed from the phase-space prediction considerably. The most prominent features of the neutron spectrum at zero degrees were found to be a shift to higher neutron energies (the most probable neutron energy slightly higher than half of the maximum possible neutron energy), and a lack of neutrons near zero energy. In order to obtain more information on reaction mechanisms of the neutron production at zero degrees from proton and deuteron induced deuteron break-up, the neutron-charged particle coincidences were measured in the p + d --, p + p + n and d + d + d + p + n reactions for a bombarding energy of 10.0 MeV and with neutron detection at 0, = 0”. 2. Experimental

procedure

A schematic layout of the experimental arrangement is indicated in fig. 1. Proton and deuteron beams from the Rice University Tandem Van de Graaff accelerator were used to bombard a deuterated polyethylene target. After passing through the scattering chamber the beam was deflected by approximately 20” and led to a beam stop. Such a geometry allows the neutron detector to be placed at small angles, 8,. In all the measurements to be reported in this paper, the neutron detector was placed at 8, = 0”. Inside the scattering chamber two silicon surface-barrier counter telescopes were mounted when the d + d --f d+ p+ n reaction was studied, but AE detec-

d+d

tors were removed used are described

AND p+d

227

REACTIONS

during the study of the p+ d + p+p+ in some detail in ref. ‘I).

n reaction.

The electronics

Three pairs of coincidences were measured simultaneously: (i) charged particles were detected at 0, and a neutron detected at 8, = 0”. (ii) charged particles were detected at 8, and a neutron at 0, = O”, and (iii) a charged particle at 8, and a charged particle at o2 were detected. In order to reduce background in charged particle-neutron coincidences, pulse-shape discrimination was employed (measurement of pulse shape, PSD, and pulse height, DYN) for rejection of y-ray events in the neutron detector. The data were written on magnetic tape as multi-parameter spectra using the Bonner, Nuclear Laboratories computer-analyser system “), The data record for each coincident pair of detectors was identified by a tag word. Data recorded were: tag 1: E,, AE,, E,, AE,; tag 2: E,, AE,, PSD, DYN, TOFl; tag 3: E2, AE,, PSD, DYN, TOF2; and tag 4: E, monitor. El, AE, and E2, AE, are energy and energy-loss pulses measured at 8, and 8,; PSD and DYN are pulses from the pulse-shape discrimination circuit for y-ray rejection; while TOFl and TOF2 are time-to-amplitude converter pulses using charged-particle pulses as start and neutron pulse as stop. Neutron counter efficiency as a function of neutron energy was determined using the reaction ‘H(d, n)3He as described in ref. ‘). In order to obtain additional information neutron counter efficiency was measured with the neutron counter placed at I!I,, = 0” and by measuring the neutron yield from the “C(p, n)’ 3N reaction. Neutron spectra were obtained using the pulsed and bunched beam 23). Both efficiency curves are found to be in very good agreement. The absolute cross sections were then determined by normalization to the elastic scattering cross section 24).

3. Experimental 3.1. THE

p+d

+ p+p+n

results and analyses

REACTION

Proton-neutron coincidences from the p + d + p +p + n reaction were measured for a proton bombarding energy of E,, = 10.0 MeV and detector angle settings: 8, = 0” and 8, = 15”, 20”, 25”, 30”, 35” and 40”. Measured coincidence spectra corrected for the neutron counter efficiency are shown in fig. 2 as projections onto the proton energy axes. The relative energy of the detected neutron-proton pair is indicated by the dashed curves, and the spectator proton energy is indicated by solid curves. Spectra measured at 8, = 15”, 20” and 25” show a peak corresponding to the minimum neutron-proton relative energy. The peak slowly disappears with increasing 8,. For 8, = 35” and 40”, the measured spectra are flat, although the neutron-proton relative energy, E12, goes through a minimum. However, the values of El2 are such (0.600 MeV for 8, = 35” and 0.750 MeV for 0, = 40”; see table 1) that the neutronproton final-state interaction does not contribute significantly. The peak cross sections corresponding to minimum neutron-proton energies are listed in table 1. The spectator proton energy rises almost monotonically with increasing detected proton energy for every 8,. The influence of the quasi-free scattering process is indicated by an increase

228

V. VALKOVIC et al. ptd-n+ptp ED = IO MeV

I

I

I

I

2

I

3

4

5

PROTON

6

I

I

ENERGY

I

I

2

3

4

I

,

5

6

I

(MeVl

Fig. 2. Proton-neutron coincidence spectra from the pfd --f p+p+n reaction for Ep = 10.0 MeV and 0, = 0”, shown as projections onto the proton energy axes. The solid curves are spectator proton lab energy, while the dashed curves represent the relative energy of detected neutron-proton pair.

TABLE 1

Peak cross section for the p+d + p+p+n (degree)

Peak cross section (mb/sra * MeV)

15 20 25 30 35 40

10.2kO.4 8.4+0.5 5.5 +0.5 4.ozto.3 2.1 i-o.2 1.8*0.2

0,

reaction at

Ep =

10.0 MeV and 8, = 0”

E 12,rnlrl

(MW 0.12 0.21 0.33 0.46 0.60 0.75

(MEV) 0.57 0.67 0.79 0.95 1.14 1.41

d+d

AND p+d

REACTIONS

229

of the measured yield for very small values of the detected proton energies. Tn conclusion it is obvious that neither final-state interaction processes nor quasi-free seattering processes are predominant for any pair of angles B,,, when 6, = 0.

reaction for Ed = 10.0 MeV Fig. 3. Impulse approximation calculations for the d-j-d -+ d+n+p and 6, = 25”, 0. = 0”. The m&eon-deuteron QFS processes are those discussed in ref. 21). 3.2. THE d+d + dip+n

REACTION

Quasi-free scattering processes are expected to be much more important in the d+ d + d-t-p + n reaction with neutron detection at 8, = 0”. As it was pointed out [ref. “‘)I, bath target and beam nucleons can act as the spectator particle. For 0, = 0 one expects a signi&zant contribution of the QFS process where a neutron from the beam deuteron acts as the spectator particle. Calculations performed using the nuclean-deuteron scattering model in the d+d + d + p + n reaction ‘I) show that this is indeed so. Fig. 3 shows impulse approximation predictions for 0, = 0” and e, =25’ at a bombarding energy of 10.0 MeV. Two processes have dominant contributions: (i) The process in which the neutron in the beam deuteron acts as the spectator particle, (CP); and (ii) the process in which a proton in the target deuteron acts as the spectator particle (A). A very strong interference contribution, (A. CP), is also present. It should be noticed that the peak of the resulting total spectrum does not coincide with either the peak of process (A) or the peak of process (CP). The shift in peak posi-

Fig. 4. Beuteron-neutron coincidence spectra from the d+d -+ d+p+n reaction for .& = 10.0 MeV and 0, =i 0”. Data are shown as projections on the deuteron energy axes. The solid line is an im~u~s~a~~r~x~mat~on calculation.

d+d

AND p+d

231

REACTIONS

d+d--n+p+d E.-DO

Mev

OWTERON

EMRGY(MeVl

reaction for Ed = 10.0 MeV Fig. 5. Deuteron-proton coincidence spectra from the d+d --, d;p+n and asymmetric angle settings. Data are shown as projections on the deuteron energy axes. TABLE 2 Peak cross section for the d+d

+

d+p+n

reaction at

ED = 10.0 MeV and 0, = 0”

Peak cross section (mb/sr2 . MeV)

NdII

(degree) 15 20 25 30 35

55.0&4.0 27.0f2.0 13.0*2.0 5.5fl.O 4.5*1.0

0.0891 0.0536 0.0289 0.0183 0.0182

%

232

V. VALKOVIC

et al.

tion is due to comparable intensities of (A) and (CP) as well as the intensity of the iI~terferen~~ term (A * CP), Deuteron-neutron coincidence spectra from the d+d -+ d+p+n reaction were measured for deuteron bombarding energy Ed = 10.0 MeV and detector angle settings 8, = O”, 0, I= 15”, 20”, 25”, 30” and 35”. The experimental set-up allowed simultaneous measurements of, for example, d-n coincidences for 8, = O”, Bd = 15” and A,, = O”, 8, = 20”. At the same time the eharg~d-partic2e_charged-particJe coincideace spectra for til = 15” and dz = 20” were measured also, The measured deuteronneutron coincidence spectra are shown in fig. 4 as projections onto the deuteron energy axes. Data have been corrected for the neutron counter efficiency. Fig. 5 shows deuteron-proton coincidence spectra as projections onto the deuteron energy axes. The solid curves on each figure are impulse approximation ~~~~lat~~ns predictions. The calculations reproduce reasonably well the spectral shapes when normalized to the peak cross sections. It should be noted that the impulse approximation calculation curves are always broader than the measured spectra. No effort to obtain the best fit was put into the calculation and no effects of energy and angular smearing were considered, ~e~tero~-neuron coincidence spectra with neutron detection at 8, = 0” show strong peaks for 0, = lS’, 20” and 25”. For 0, = 30” and 35” the peak cross section is an order of magnitude smaller than for 61d= 15” and the peaks are not very well defined. Table 2 lists the measured d-n peak cross sections, The ratios of the measured peak cross sections and calculated ones are also shown. This ratio varies very strongly with the angle Bd, indicating the limitations of the simple nucleon-deuteron quasi-free scattering model. 4. Conclusions Proton-neutron coincidence spectra from the p +d + p +p+ n reaction for 0, = 0” and bombarding energy of 10.0 MeV show peaks ~orres~n~~~g to ~inim~rn relative energy of the detected neutron-proton pair, The bighest peak cross section is for small t?, indicating that the yield is due to the neutron-proton final state interaction. The observed peak slowly disappears with increasing 8,. Deuteron-neutron coincidence spectra from the d + d -+ d-!-p+ n reaction for 8, = 0” are dominated by a strong peak (d3#~d~~d~~d~~ % 55 mbjsr” * MeV at S, = 1Se>for small Bd. The position of the peak and its shape are reasonably well explained by the deuteron-neutron quasi-free scattering model, although the calculated curves are always broader than the measured spectra and do not peak at precisely the measured energy. Tn both reactions the final state contains two charged particles and a neutron. fn nuclear reactions with three particles in the final state there are five independent kinematical variables. Therefore, if the energy and direction of one of the outgoing charged particles (El and B,, 41) are measured, the coincident neutron has a definite energy, E,, at a given angle I!?,,4,. In other words, measurement of the energy and

d+d

AND p+d

REACTIONS

233

direction of one of the outgoing charged particles and of the neutron direction (O,, &,) fixes the neutron energy with an energy uncertainty that is determined by the spread in the beam energy and the energy resolution of the detector used to determine E,. The fact that the E, versus En relation for given angles and bombarding energy is a quadratic function should be taken into account by imposing additional restrictions on E, and/or E,,. The reaction d f d + d+n+p appears to be more convenient for such an attempt due to the higher cross section, The short-comings due to low intensity of the neutron beams obtained in such a way could be overcome by the use of large solid-angle position sensitive counters (i.e., multi-wire proportional counters). The feasibility of such an approach is currentiy being studied. References 1) \I. ValkoviC,D. Rendid, V. A. Qtte, W. von Witsch and G. C. Phillips, Nucl. Phys, Al66 (1971) 547

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