II. The reaction Li7 (p, α)He4

148

w . E . BURCHAM

et

at.

References 1) 2) 3) 4)

W. E. Burcham, W. M. Gibson, D. J, Prowse and J. Rotblat, Nuclear Physics 3 (1957) 217 P. Ajzenberg and T. Lauritsen, Rev. Mod. Phys. 27 (1955) 77 D. R. Inglis, Phys. Rev. 74 (1948) 21 J. B. B l a t t and V. F. Weisskopf, Theoretical Nuclear Physics (John Wiley, New York, 1953) pp. 315, 320, 377 5) N. P. Heydenburg, C. M. Hudson, D. R. Inglis and W. D. Whitehead, Phys. Rev. 73 (1948) 241; 74 (1948) 405 6) R. F. Christy and R. Latter, Rev. Mod. Phys. 20 (1948) 185 7) D. Walker, W. T. Link and W. I. B. Smith, Proc. Phys. Soc. A 65 (1952) 861

II. T H E REACTION Li T (p, ~)He ~ JOAN M. F R E E M A N , R. C. H A N N A and J. H. M O N T A G U ~ Atomic Energy Research Establishment, Harwell Received 12 July 1957 The absolute differential cross section for the Li 7 (p, c~) He t reaction a t 90 ° has been measured for a n u m b e r of proton energies between 1 and 1.5 MeV for comparison with results on the inverse reaction He 4 (~, p) Li 7.

Abstract:

Although a number of measurements have been made of the excitation function and angular distributions of the ~¢-particles emitted in the Li T (p, a) He 4 reaction 1), no precise data on absolute yields are available. We have made some absolute measurements of the cross section at 90 °, in the proton energy range 1.0 to 1.5 MeV, in order to provide data required for use in connection with experiments on the inverse reaction He*(~, p)Li ~ (described in Part I of this paper). Thin targets of lithium fluoride (2 to 3 keV thick for 1.5 MeV protons) were prepared b y evaporation on to 0.005 mm thick aluminium backings, the mass of LiF per unit area being determined b y weighing. These were bombarded b y a collimated beam of protons from the Harwell Van de Graaff machine and the reaction products emitted at an angle of 90 ° to the beam direction were detected b y a 2.5 mm thick CsI (T1) crystal mounted on an E.M.I. photomultiplier, the crystal being shielded from light b y a thin aluminium foil. The solid angle subtended b y the crystal at the beam spot on the target was estimated (i) from the geometry, (ii) b y counting the a-particles from calibrated Am 2a sources placed at the position occupied b y the target spot. The direction of the target normal was set to an angle of 45 ° with respect to the proton beam. The number of protons bombarding the target was measured b y collecting the beam on a tantalum plate mounted behind the target and connecting the target and collector to a current

THE NUCLEAR REACTION He i (~X,p) Li ~

149

integrator; secondary emission was suppressed by applying suitable potentials to the target and a grid placed in front of the target. The spectra of pulses obtained in the CsI (T1) crystal were displayed on a hundred-channel pulse-height aualyser. Alpha-particle groups due to the reactions Li T (p, =) He 4 and F 19 (p, %) Ole (ground-state transitions) were completely resolved provided the beam intensity was kept low enough to prevent build-up on the small pulses due to scattered protons. With the beam intensities used, not more than 3 ~o of the fluorine =-pulses were displaced into the peak of lithium ~-pulses; a correction was applied by assuming the peak shapes of the two ~-particle groups to be the same. The number of =-particles from each reaction was measured for a given proton charge as a function of proton energy at small intervals in the range 1.3 to 1.5 MeV. In this way a measurement of the ~-particle yield lrom the F 19(p, %) reaction at the peak of the 1.36 MeV resonance was obtained simultaneously with the yield from the lithium reaction. A value for the cross-section of the fluorine reaction has already been obtained by Clarke and Paul ~), so this procedure provided a check on our absolute measurements. Our estimate of the fluorine crosssection at 90 ° at the 1.36 MeV resonance is 2.71±0.27 millibarns per steradian. The value calculated from Clarke and Paul's results is 2 . 1 6 i 0 . 3 2 millibarns per steradian, which is in fair agreement with our value. The cross-section for the Li T (p, ~) He 4 reaction was calculated for four proton energies (i) from our absolute measurements and (ii) by comparison with the F19(p, %) cross-section at the 1.36 MeV resonance, for which the value of Clarke and Paul was assumed (account was taken of the fact that in the lithium reaction two ~-particles are emitted per disintegration). The means of these two sets of results are given in table 1. The errors shown refer to the absolute values; the errors on the relative values at different proton energies are about + 4 ~o. TABLE 1. P r o t o n e n e r g y (MeV) Cross-section of Li (p, ~) a t 90 ° (lab. s y s t e m ) m b • s t e r a d -1

1.01

1.28

1.36

1.47

0.67+0.06

0.79+0.07

0.944-0.08

1.164-0.10

Values of the absolute cross-section can be obtained over a wider range of energy and at angles other than 90 ° by combining the results given in table 1 with the relative cross-section values of Heydenburg, Hudson, Inglis and Whitehead 3).

References 1) F. A j z e n b e r g a n d T. L a u r i t s e n , Rev. Mod. P h y s . 27 (1955) 77 2) R. L. Clarke a n d E. B. P a u l , C a n a d . J. P h y s . 35 (1957) 155 3) N. P. H e y d e n b u r g , C. M. H u d s o n , D. R. Inglis a n d W . D. W h i t e h e a d , P h y s . Rev. 73

0948) 241