Neutron spectrum from the T+T reaction

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Nuclear Physics 71 (1965) 106--112; (~) North-Holland Publishing Co., Amsterdam

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Not to be reproduced by photoprint or mlerofilmwithout written permissionfrom the publisher

NEUTRON SPECTRUM FROM THE T+T

REACTION

C. WONG, J. D. ANDERSON and J. W. McCLURE

Lawrence Radiation Laboratory, University of California, Livermore, California Received 15 March 1965 Abstract: The neutron spectrum from the T ÷ T reaction has been measured by time-of-flight techniques using the swept and bunched 0.5 MeV tritium beam from the Cockcroft-Walton accelerator. The 90° neutron spectrum shows evidence for the following reaction modes: (a) direct breakup into 2 neutrons plus alpha particle (T÷T ~ n - r n + H e 4, Q = 11.33 MeV), the three-body breakup shape being modified by the presence of the neutron-neutron interaction; (b) sequential decay proceeding via the He 5 ground state; and (c) sequential decay proceeding via a broad He5 excited state. The branching ratios for the T-kT reaction at 90° are approximately 70 ~ for (a), 20 ~ for (b) and 10 ~ for (c). The neutron group leading to the He ~ ground state, estimated width 0.74±0.18 MeV, is isotropic to within an accuracy of ±10%.

El

NUCLEAR REACTIONS aH(SH, n), (3H, nn), E = 0.5 MeV; measured a(En, On). Deduced branching ratios for 3H +3H reactions.

1. Introduction

Recently, there has been m u c h interest in m e a s u r i n g the n e u t r o n - n e u t r o n interaction by m e a n s of n e u t r o n - n e u t r o n scattering 1), the rr- + d ~ n + n + 7 reaction z, a) a n d the n + d ~ n + n + p r e a c t i o n 4 ) . L a c i n a et al. 5) have investigated the effect o f the n e u t r o n - n e u t r o n i n t e r a c t i o n o n the n e u t r o n s p e c t r u m from the T + T ~ n + n + H e 4 reaction. T h e i r calculations 5) show that the three-body b r e a k u p n e u t r o n spectrum is significantly modified by the presence of the n e u t r o n - n e u t r o n final state interaction. Previous m e a s u r e m e n t s o f the T + T n e u t r o n spectrum were o b t a i n e d with p h o t o g r a p h i c emulsions 6, 7) a n d p r o t o n recoil telescope s); the statistics 6, 7) a n d energy resolution 8) were sufficiently p o o r that n o definitive j u d g m e n t could be m a d e a b o u t the presence or absence of effects due to the n e u t r o n - n e u t r o n final state interaction. The n e u t r o n spectrum from the T + T reaction has been m e a s u r e d by time-of-flight techniques using the swept a n d b u n c h e d 0.5 M e V t r i t i u m b e a m f r o m the CockcroftW a l t o n accelerator. W i t h i m p r o v e d statistics, the n e u t r o n s p e c t r u m shows evidence for a d i s t o r t i o n in the three-body b r e a k u p shape in a g r e e m e n t with that calculated 5) f r o m the n e u t r o n - n e u t r o n final state interaction. W i t h i m p r o v e d energy resolution, a width estimate of 0.74___0.18 MeV is o b t a i n e d for the He 5 g r o u n d state. 106

T+T REACTION

107

2. Experimental Method The time-of-flight electronics on the Cockcroft-Walton accelerator has been described previously 9). The only essential difference was that the stop pulse into the time-to-height converter was generated from the sweeper-buncher voltage instead o f the target cup pulse. In this manner, a double display was generated, i.e., one stop pulse for every two beam pulses. In order to expand the time spectrum, the 256-channel analyser was replaced with an 800-channel analyser. The neutron detector was a 5.08 cm long by 5.08 cm diam. Pilot B plastic scintillator viewed by an RCA 6655A photomultiplier. The 90 ° long flight path measurements (l = 7.5 m) were made with the detector outside the target pit. The tritium-loaded titanium target was viewed through a 30 cm diam. hole in the 1.5 m thick target pit shielding wall. The short flight path measurements were made inside the target pit. Angular distributions were obtained by mounting the detector on a remotely controlled angle changer. Background measurements were obtained by inserting a 50 cm copper absorber between target and detector. The yield from the T + T reaction was monitored by counting the neutrons with a BF3 long counter and by counting the alphas with a solid state detector. To conserve the tritium and reduce machine contamination, the rf ion source was fed with a mixture of 25 % tritium and 75 % helium l o).

3. Results 3.1. N E U T R O N SPECTRUM

Fig. 1 is a time-of-flight spectrum obtained at 90 ° for a flight path of 7.5 m. The time between ~ rays is 4fiB, where JB is the klystron buncher 9) frequency t, which was measured to be 9.692 MHz. In the region around channel 460, a 14 MeV neutron peak is observed. This peak is attributed to deuterium contamination of the tritium target and/or to a small amount of (HD) + in the magnetically analysed 9) mass three beam. At 7.5 m, the D(t, no)He* neutron peak is well separated from the neutron spectrum from the T + T reaction. The peak around channel 420 is due to a neutron group leaving He 5 in its ground state. The background between channel 200 and 450 was assumed flat as shown. This background was measured by inserting a 50.8 cm copper absorber between tritium target and detector. These measurements showed that the background in the above region of interest was indeed flat within statistics. Conversion of counts per channel into centre-of-mass neutron energy spectra (see fig. 2) is straightforward and is described in a previous publication it). The time per channel was not assumed constant, but was measured 12) with a radioactive source. t The buncher R.F. is divided by 8 and amplified before being applied to the sweeper plates. Since the pulsed beam rate is twice the sweeper frequency, the pulsed beam frequency is ¼fB.

108

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Fig. 1. Time-of-flight spectrum from the T + T reaction at 90 ° lab for a flight path of 7.5 m. Time calibration of the system is 1.05 nsec/channel, and increasing time-of-flight is towards the left. T h e detector bias was 0.34 M e V electrons or 1.6 M e V neutrons. > 4r

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MeV

Fig. 2. Centre-of-mass neutron spectrum from the T + T reaction at 90 ° for a flight path of 7 • 5 m . The spectrum has been d e c o m p o s e d into its various reaction and decay modes• See text for significance of the solid lines. At the lower neutron energies, the data have been averaged over energy.

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14

Fig. 3. Centre-of-mass neutron spectrum from the T - F T reaction at 70 ° lab for a flight path of 2 . 1 5 m . At the lower neutron energies, the data have been averaged over energy. The detector bias was 0.17 M e V electrons or 1.1 M e V neutrons. The solid line represents a smooth curve through the data.

T+T

REACTION

109

Since a "thick" tritium target was employed and since the bombarding energy of 0.5 MeV is small compared to the Q values of interest ((Q = 17.6 MeV for D(t, no) He 4 and Q = 10.37 for T(t, no)HeS)), these reactions were assumed to proceed at zero energy. The calculated energies of 14.1 MeV for the D(t, no)He 4 and 8.64 MeV for the T(t, no)He 5 neutron groups are in good agreement with the measurements shown in fig. 2. The solid line labelled (1) represents a smooth curve through the measurements. Solid line (2) is the calculated three-body breakup shape of Lacina et al. 5) and includes the distortion effects due to the neutron-neutron final state interaction t. As pointed out by Lacina et al., the calculated spectrum peaks at ½Q, in agreement with that expected from breakup of a dineutron with zero binding energy. [Q is the Q value for the three-body breakup reaction (T + T --, n + n + He 4, Q = 11.33 MeV)]. Without any final state interactions, the three-body shape peaks at one-half the maximum neutron energy or ½ . 5 0 = ~2Q (see solid line (3)). A subtraction of (2) from (1) yields the triangles as shown. The triangles have further been decomposed into (4) - neutron group leading to the He 5 ground state, (5) neutron group leading to a broad He 5 state at approximately 2.0 MeV excitation energy, and (6) - neutrons from the decay of the He 5 2.0 MeV level. Neutrons from the latter decay are expected to start contributing in the region around 4.4 MeV neutron energy. Neutrons from the decay of the He 5 ground state start contributing around 2 MeV neutron energy. These decay neutrons are evident in the neutron spectrum of fig. 3, which represents a poor resolution short flight path measurement with a lower detector bias. At 2.15 m, the resolution is insufficient to resolve the He 5 ground state neutron group. However, the spectral shape in the region of ~Q is reproduced. In fig. 1, the time spread as measured by the full width at half-maximum of the rays is 5___1 channels or about 5 _ 1 nsec. This time spread introduces an energy spread of 1.0__+0.2 MeV for 14 MeV neutrons in the laboratory system (also centreof-mass system since the angle of observation is 90 ° in the laboratory system). Since the measured full width at half-maximum of the 14 MeV peak is 1.1 MeV (see fig. 2), it is clear that the observed energy spread can be fully accounted for by the time spread. This time spread applied to the He 5 ground state neutron group yields an energy spread of 0.5+0.1 MeV. Since the measured width is 0.8+0.1 MeV, this yields an intrinsic width for the He 5 ground state neutrons of 0.62+0.15 MeV. Neutron energy spread *t due to triton energy loss in the tritium target is neglected since it reduces the intrinsic width by, at most, 30 keV. Multiplication of the neutron width by 6 yields a He S ground state width of 0.74+__0.18 MeV. t Modification of the calculated spectrum due to energy resolution effects has been ignored since the correction effects are less than 10 ~ in the region of 7 MeV in the breakup spectrum. tt The neutron energy spread introduced by a 0 and 0.5 MeV incident energy triton is 0.2 MeV in the centre-of-mass system. Compounding this spread in the usual manner (total width is the square root of the sum of the squares of the individual widths) reduces the intrinsic width by 30 keV, which is a factor of 5 smaller than the error on the intrinsic width determination.

110

c. worqo e t al.

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Fig. 4. C e n t r e - o f - m a s s n e u t r o n s p e c t r u m f r o m the T + T reaction at 38 ° lab for a flight p a t h o f 3.98 m. A t the lower n e u t r o n e n e r g i e s the d a t a have been averaged over energy. T h e detector bias w a s 1.0 M e V electrons or 3.5 M e V neutrons. T h e g r o u n d state yield was a p p r o x i m a t e d by the product o f the peak height (p) a n d o n e - h a l f the full width at h a l f - m a x i m u m (b). 16

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140

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T + T REACTION

I 11

Integration of the areas represented by (2), (4) and (5) yields the following branching ratios for the T + T reaction *: (2)

T+T~n+n+He

4,

(4)

T + T - - + n o + H e 5,

Q = 10.37 MeV ( 2 0 % )

(5)

T+T~nl+He

Q = 8.4 MeV (10%).

5,

Q = 11.33 MeV (70%)

3.2. ANGULAR DISTRIBUTIONS Fig. 4 represents the neutron spectrum at 38 ° observed inside the target pit for a flight path of 3.98 m. The ground state yield was estimated from the product of the peak height (P) and one-half the full width at half-maximum (b in fig. 4). The ground state neutron angular distribution is shown in fig. 5. Within an accuracy of + 10 %, the ground state neutron group is isotropic in the centre-of-mass system. The angular distribution of the continuum was also measured for a flight path of 2.98 m. A typical spectrum observed at 68 ° lab is shown in fig. 6. F r o m 0L = 4 to 100 °, the spectrum between 2 and 7.5 MeV is isotropic to within an accuracy of

+ 20700. 4. Discussion The T + T reaction proceeds predominantly via direct three-body breakup, the three-body shape being modified by the presence of the neutron-neutron final state interaction. A b o u t 20 % of the time the two neutrons are emitted in a two-stage process proceeding via the He s ground state. The measured width of 0.74+0.18 MeV for the He 5 ground state is in fair agreement with the value of 0.55+__0.03 quoted in the compilation by Ajzenberg-Selove and Lauritsen 13). The observed isotropy of the neutron group leading to the He 5 ground state is consistent with the reaction proceeding via s wave capture of the incident triton. Assuming the calculations of Lacina e t al. for three-body breakup with final state neutron-neutron interaction, the spectrum shows evidence for a broad neutron group leaving He s in an excited state at approximately 2.0 MeV. The branching ratio of 10 %, width of about 2.4 MeV and excitation energy of about 2 MeV are, of course, rough estimates since these are dependent upon both the correctness of the breakup calculations in this energy region and the absolute normalization of the breakup spectrum. For example, normalizing the calculated breakup shape (2) in fig. 2 to a slightly lower value would increase the branching ratio, increase the width, and increase the excitation energy. However, our rough estimates of the position and width of the He 5 excited state are in fair agreement with the values of Fesscnden and Maxson 14) who quote an excitation of 2.6+0.4 MeV and width of 4 . 0 + 1.0 MeV. t The area enclosed by (2) is divided by 2 since two neutrons are emitted. In the framework of the calculations by Lacina et al., the formation of a dineutron (neutron-neutron final state interaction) cannot be separated from the direct three-body breakup. The 70 % figure therefore includes both the formation of a dineutron and direct three-body breakup based solely upon phase space arguments.

112

c. WONQet aL

The observation of distortion effects due to the final state neutron-neutron interaction is difficult because of the presence of many reaction and decay modes from the T + T reaction. All one can say is that the assumption of the calculations by Lacina e t a L produces a reasonable decomposition of the spectrum into its various reaction and decay modes. For example, curve (6), fig. 2 is approximately the expected shape from neutron decay of the 2.0 MeV level in He 5. The peaking of the spectrum at ½Q rather than ~ - Q is undoubtedly due to both the neutron-neutron interaction and decay neutrons from the He 5 2.0 MeV level. We wish to thank D. Dorn and B. Lacina for interesting discussions and for making available their calculations before publication. Thanks are also due R. Cedarlund and D. Rawles for solving many of the problems associated with accelerating tritium on the Cockcroft-Walton machine. This work was performed under the auspices of the U.S. Atomic Energy Commission.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)

Michaerl J. Moravcsik, Phys. Rev. 136 (1964) B624 J. W. Ryan, Phys. Rev. Lett. 12 (1964) 564 R. P. Haddock et al., Bull. Am. Phys. Soc. 9 (1964) 443 Ivo Slaus, Progress in fast neutron physics (The Universityof Chicago Press, Chicago, 1963) p. 61 B. Lacina, J. Ingley and D. W. Dorn, UCRL-7769 (1964), to be published Allen et al., Phys. Rev. 82 (1951) 262 W. T. Leland and H. M. Agnew, Phys. Rev. 82 (1951) 559 S. J. Bame, Jr., and W. T. Leland, Phys. Rev. 106 (1957) 1257 Anderson et al., Phys. Rev. 110 (1958) 160 Jarmie et al., Rev. Sci. Instr. 34 (1963) 936 Wong et al., Phys. Rev. 116 (1959) 164 Anderson et al., Nuclear Physics 36 (1962) 161 F. Ajzenberg-Seloveand T. Lauritsen, Nuclear Physics 11 (1959) 1 P. Fessenden and D. R. Maxson, Phys. Rev. 133 (1964) B71