Excitation functions and angular distributions for the reaction C12(He3, n)O14

Nuclear Physics 50 (1964) 267--272; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

EXCITATION FUNCTIONS AND ANGULAR DISTRIBUTIONS FOR THE REACTION C12(He 3, n)O 1. G. U. DIN, H. M. K U A N and T. W. BONNER 1" T. W. Bonnet Nuclear Laboratories, Rice University, Houston, Texas tt Received 20 August 1963

Abstract: Excitation functions at 0 ° and 90 ° and the total yield from the reaction CXS(Hes, no)O x' have been measured for HO bombarding energies from 1.8 to 5.5 MeV. Angular distributions were measured at 1.87, 2.33, 2.66, 2.87, 3.11, 3.93, 4.49, 4.75 and 5.19 MeV. The excitation curves and the total yield show considerable resonance structure. The angular distributions vary from isotropic to backward peaking at low energy and above 4 MeV bombarding energy are peaked in the forward direction.

1. Introduction From two viewpoints (He 3, n) reactions are of interest. The reaction mechanism is of interest, since direct interaction and compound nucleus formation may both be expected to play a substantial role. Also, the high charge-to-mass ratio permits the study of the proton-rich members of isobaric multiplets. The (He 3, p) and (He 3, n) reactions on a given target lead simultaneously to different members of the same isobaric multiplet and thus give direct information regarding the charge independence of nuclear forces without complication for differing reaction mechanisms. Several experiments on (He 3, p) reactions reported by Hinds and Middleton 1) and by Johnston et al. 2) give strong evidence of a double stripping reaction mechanism, particularly when the energy of the incident He 3 particle is above 5 MeV. This paper reports measurements designed to provide information on the (He 3, n) reaction. The C12(I-Ie3, n)O 14 reaction has been studied for the bombarding energies in the range of 1.8 to 5.5 MeV. The excitation functions, angular distributions and total yield were measured. This reaction has also been studied in some detail in the energy range 1.3 MeV to 3 MeV by Bromley et al. 3). Towle and Macefield 4) have measured the excitation functions between 2 and 5.7 MeV and the angular distributions at 4.65, 4.98 and 5.26 MeV. Gale et al. s) have reported the measurement of angular distributions at 4.09, 4.62, 5.16 and 5.7 MeV. Angular distributions and excitation functions have also been measured at energies between 6.5 and 11 MeV by Fulbright et al.6). t Deceased. tt Work supported in part by the U. S. Atomic Energy Commission. 267

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2. Experimental Method Singly charged He 3 ions were accelerated by the Rice University 5.5 MeV Van de Graaff accelerator and were analysed by a 90 ° deflecting magnet. In the reaction C lz (He 3, n)O 14, only the neutron group which leaves O t4 in the ground state is present for He 3 bombarding energies below about 6 MeV. Therefore, the neutrons were detected by a B 1° enriched BFa counter (modified long counter). The counter tube was 2.5 cm in diameter and 15 cm in length and was embedded in a paraffin cylinder 15 cm in diameter and 12.2 cm long. The carbon target used for the excitation functions and for the total yield was made by cracking carbon from methyl iodide (CH3I) on a tungsten blank o f 1.905 cm diameter and 0.025 cm thickness. The thickness o f the carbon target was about 60 keV for 3 MeV He 3 particles. For the study of the angular distributions, a carbon target was made in the same way as mentioned above on a thic tantalum backing. It was about 300 keV thick for 3 MeV He 3 ions. The excitation functions at 0 ° and 90 ° were measured with the modified long counter. The distance between the front part of the paraffin cylinder and the target was 28 cm for the 0 ° yield curve and 14 cm for the 90 ° yield curve. The angular distributions were also measured using the long counter, which could be rotated about the target. The plane of the target was at 45 ° to the incident beam and the distance between the front part of the paraffin cylinder and the target was 42.5 om. The acceptance angle was about _+ 10°. A detection efficiency ~) correction, depending on the energy of neutrons, was applied for the neutrons counted at various He 3 bombarding energies and angles o f detection. The total yield of the reaction C12(He 3, n)O 14 was determined by measuring the yield of positons (maximum energy s) o f 1.83 MeV) from the radioactive O 14 whose half life s) is 72.1 sec. The total yield was also obtained by measuring the yield of gamma rays of 2.31 MeV which accompany the decay 014 --, N t4* 2.31 MeV state. A cylindrical plastic scintillator (Pilot B) 2.2 cm in diameter and 1.8 cm long was used to detect the positons and a 2.54 c m x 2.54 cm NaI(TI) crystal for the detection o f 2.31 MeV gamma rays. The thin carbon target on a 0.025 cm tungsten backing was mounted on a 0.035 cm thick aluminium plate. The positons passed through the tungsten backing as well as the aluminium holder and lost about 250 keV of energy. The carbon target was bombarded for 100 sec with a steady He 3 beam. Ten seconds after the beam was turned off, the positons and 2.31 MeV gamma rays were counted for 70 sec. The data, plotted on semi-logarithmic graph paper, gave a slope that agreed with the known half life of O 14 of 72 see. The cross section for the CtZ(He 3, no)O 14 reaction was obtained by comparison with the known cross section for the ClZ(d, no)N x3 reaction 9). The same carbon target and neutron detector set-up was used for the two reactions Cl2(He 3, no)Ot4and ClZ(d ' no)N 13. The deuteron bombarding energy was set to give outgoing neutrons with the same energy as from the (He 3, n) reaction to avoid the correction due to the

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change in detection efficiency with energy. The error of this neutron cross section determination is estimated less than 30 %. 3. Results

The 0 ° and 90 ° yield curves and the total curve measured between 1.8 and 5.5 MeV are shown in fig. 1. The excitation curves at 0 ° and 90 ° are quite different for He 3 bombarding energies in the neighbourhood of 2.5 MeV. In the 0 ° yield curve there are two separate anomalies, one at 2.33 MeV andthe other at 2.87 MeV, whereas the 90 ° yield curve shows a broad peak centred at 2.5 MeV He 3 energy. In the total yield curve there is a prominent peak in the region from 2 to 3 MeV which does not have a simple resonance shape, but is complex. Marked resonances at 4.6 and 5.0 MeV are seen in both the 0 ° and 90 ° excitation curves. The shape of the total cross section curve obtained from integrating the angular distributions agrees very well with the one obtained from the measurements of yields of positons and gamma rays. The angular distributions taken at 1.87, 2.33, 2.66, 2.87, 3.11, 3.93, 4.49, 4.75 and 5.19 MeV He 3 energy were converted to the centre-of-mass system and are shown in fig. 2. The angular distributions obtained at 1.87, 2.33, 2.66, 2.87 and 3.11 MeV correspond to peaks and valleys in the 0 ° yield curve. At the lowest energy (1.87 MeV) the i

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angular distribution is isotropic, while at the energies o f two 0 ° resonances (2.33 and 2.87 MeV) there is a small forward peak as well as a strong backward peak. The angular distribution at 2.66 MeV also gives a very strong backward peak, while that at 3.11 MeV indicates only a small variation of cross section with maxima at 0 ° and 90 °. All the angular distributions at higher energies (4.49, 4.75, and 5.19 MeV) are very similar and indicate strong forward peaking with relatively small yields at the backward

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The excitation functions at 0 ° and 90 ° as shown in fig. 1 agree in shape as well as in cross-section value with the results of Towle and Macefield 4). There is also a good agreement in shape with the results of Bromleyetal. 3) but their cross section values are one and half times higher than ours. At lower energies the angular distributions agree in shape with the angular distributions measured by Bromley eta/. 3) and at higher energies the agreement is very good with the results of Towle and Maceticld 4). Our angular distributions agree with those of Gale et al. 5) at the forward angles but not at the backward adgles. 4. D b e u ~ o n Theexcitationfunctions at0 ° and 90 ° show a strong energy dependence in the differential cross section, indicatingalarge contribution from compound nucleus formation. Below 3.3 MeV the total yield shows a broad resonance where the 0 ° excitation has two prominent resonances and that at 90 ° a broad one. Resonance structure is again noticeable in the present work in the 0 ° and 90 ° excitation curves above 4 MeV and also above 6 McV He 3 bombarding energy as shown by Fulbright et ale).

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THB Clt(lk I, n)O14 RllACTION

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It is of interest to compare Ct2(He 3, pt)N t¢ (first excited state) with CtZ(He3, n) O t* since the two final states are members of an isobaric spin triplet. The excitation functions for CtZ(He 3, pt)N 14 at 7° and 90° from 2 to 5 MeV He s bombarding energy measured by Johnston et al. 2) are very similar in shape to those of Ci2(He 3, n)O t't at 0 ° and 90°. The similarity in the excitation functions for the two reactions is also seen from 6.0 MeV to 11.0 MeV He 3 bombarding energies as shown in the extended work of Fulbright et ale). Further the Ct2(He 3, n)O t* cross section is roughly twice the C12(He 3, pt)N t't* cross section for excitation of the analogue state; this result is expected from a consideration of the isobaric spin multiplicity 3). The shape of the total yield curve for Ct2(He 3, n)O t* agrees very well with the total yield curve for Ct2(He3, pt)N t*. The total yield curve for the proton group was obtained from the results of Johnston et a[.2). The angular distributions were plotted from the six excitation functions measured at 7° through 150°. These deduced angular distributions and the angular distributions actually measured were then integrated to give the total yield curve. The rapidly changing neutron angular distributions of CtZ(He 3, n)O t* in the region of bombarding energy from 2 to 3 MeV (cf. fig. 2) may be due to the overlapping of several states in the compound nucleus. At 4.49, 4.75 and 5.19 MeV bombarding energy the angular distributions show forward peaking and relatively small yields in the backward direction. The forward peaking is also observed in the angular distributions taken in the region 6-10 MeV bombarding energy as indicated in the results of Fulbright et al.6). They have tried to fit these angular distributions by Newns" double stripping theory to). These fits are quite successful in the forward direction. The angular distributions for Ct2(Hc 3, pt)N t4* between 2 MeV and 4.2 MeV He 3 energy also show variation in shape in the results of Johnston et al.2). Above 4.2 MeV He 3 energy the angular distributions for the Pt group show peaking in the forward direction and a small contribution at backward angles. This is seen in the results of Fulbright et aL e) where angular distributions at 6.82, 7.60, 8.80, 9.50 and 10.65 MeV He 3 bombarding energy have been taken. The forward peak has been fitted successfully by a Newns to) double stripping calculations.

5. Conclus/om The reactions CtZ(He 3, n)O 14 and C12(He 3, pt)N 14 which have been studied from 1.8 to 11.0 MeV He 3 bombarding energy bring to light the following conclusions. The two analogue reactions CI2(He 3, n)O 14 and CXZ(He3, pl)N 14 exhibit similar excitation functions, total yield curves and angular distributions in the region of 1.8 to 1I. 0 MeV He 3 energy. The similarity in the experimental results in the two analogue reactions indicates charge independence of nuclear forces. The compound nucleus mode of reactions seems to be present at all energies. The angular distributions at higher He 3 energies indicate the presence of the direct interaction mode.

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The authors w o u l d like to t h a n k Dr. Jesse Weft for valuable discussion a n d for a critical reading o f the m a n u s c r i p t a n d wish to t h a n k Professor G . C. Phillips for his interest in the work.

Refel~mees 1) S. Hinds and R. Middleton, Proc. Phys. Soc. 74 (1959) 196, 75 (1960) 745 2) R. L. Johnston, H. D. Holmgren, E. A. Wolicki and E. Geer Illsley, Phys. Rev. 109 (1958) 884 3) D. A. Bromley, E. Almqvist, H. E. Gore, A. E. Litherland, E. B. Paul and A. J. Ferguson, Phys. Rev. 105 (1957) 957 4) J. H. Towle and B. E. F. Macefield, Proc. Phys. Soc. 77 (1961) 399 5) N. H. Gale, J. B. Garg, J. M. Calvert and K. Ramavataram, Nuclear Physics 20 (1960) 313 6) H. W. Fulbright, W. Parker Alford, O. M. Bilaniuk, V. K. Deshpande and J. W. Verba, University of Rochester Report NYO-10034 (Feb. 5, 1962) 7) T. W. Bonner, in Nuclear spectroscopy, Part. A, ed. by F. Ajzenherg-Selove (Academic Press, New York, 1960) p. 477 8) F. Ajzenberg-Selove and T. Lauritsen, Nuclear Physics 11 (1959) 1 9) J. B. Marion, T. W. Bonner and C. F. Cook, Phys. Rev. 100 (1955) 847 10) H. C. Newns, Proc. Phys. Soc. 76 (1960) 489