1 2.A.l 1 Madear Physics A155 (1970) 453 -464; @ North-HollantEPublisling Co., Amsterdam~

Not to be repro&wed by pho%wrkzi or miaomm w&ont written per&ss~o~ from ihe publi&er

RESONANCES T.N THE INTERACTION OF 3He SARTLCLES

VVITH r”C AND ‘9%

1. SINGW t Umiversity of Birmingham, England

Received 6 July 1970

Abstract: The ex~iWim f~nctims for the %f3H&, %Ie’)W* (If.11 F&V)* 12C(3He, t)‘%&., and ‘“Bf3He, I#‘N~., reactions have been measured up to 30.6 MeV. 3f3if: energy. The excitation functions show broad resonance structures which indicate the possibility of a compound nucleus mechanism at higher excitation energies. En the 12C(3He, 3He’)12C* (15.1 MeV) reaction, a strong peak is seen at an incident 3He energy of 21.5 MeV.

NUCLEAR REACTIONS 1zC(3He, 3He’)W* (15.1 MeV), E = threshold to 30.6

E MeV, %X~He, t)lzNe.,., E = 22.3 to 30.6 MeV, *%f3He, #*N,,., E = 6.9 to 30.6 MeV; measured b(B). % deduced resommce structure. Natural carbon and em-&bed

boron targets.

1. lntrodwtios

The pi=~dwkion of the lowest T = 1 states of the mass-12 system in the reactions of ‘% with protons and neutrons has been studied by Measday eb a6 ‘) and Rimmer and Fisher ‘). These authors obtained excitation functions for 15.11 MeV gamma radia- tion in the reaction %(p, p’)r2C* and for beta activity in the reactions “C(p, n)12N and “C(n, p)%. They interpreted their results in terms of a final state interaction between the outgoing nucleon and the -final nucleus, and of resonance structure in the ~nte~ed~ate nucleus ofrnass-13 (“N or ’ "C) . In the present work the production of 12C4 (IS.1 1 MeV) and of l2N from carbon by 3He induced reactions has been surveyed with the object of detecting resonance structure in the ’ 5O nucleus at higher regions of excitation than have been previously covered. The reaction l’B(“He, n)l’N was also studied since it provided some overlap with the previous work 1$2) for the nuckzus 1 3N .

2. 33xperimtmtal method

2.1. GENERAL ARRANGEMENT

The general layout of the expernnental area is shown in fig” 1. A beam of 38.6 MeV 3He particles (energy spread x 150 keV) extracted from the Nuffield Axed frequency cyclotron was first focussed by a pair of quadrupole magnets onto aluminnun degrad- ing foils by which the energy available could be varied. The full beam energy was

t Permanenl: address: Panjab University, Chandigarh, India.

453

454 J. SINGH

measured by elastic scattering of 3He+ ’ ions from a melinex (C, a Hs 0,) thickness

6.35 pm. The recoil protons produced by 1H(3He, p)3He reactions were observed in a 1.5 mm surface barrier silicon detector. The calibration of the analyser channels was obtained with the help of standard alpha-sources 210Po (5.30 MeV) and ThC’ (6.06 and 8.776 MeV). The energy losses in the degrading foils (and in the targets used) were calculated using the tables of Williamson “) and the resulting spread in energy was computed with the help of graphs given by Tschalar “). The degraded beam was collimated to a diameter of 6.3 mm and bent through 66” by a magnet

Hi beam

Plastic scintilator

Aluminium foil

Melnex window

Fig. 1. Schematic layout of the experimental area.

which provided energy analysis. The energy analysed beam was focussed by a second quadrupole pair and further collimated to a diameter of 5 mm before striking the target which was mounted at 45” to the beam direction. After passing through the target the beam was collected on a gold foil mounted in a deep Faraday cup. Radia- tions from the target were detected by sodium iodide or plastic scintillators mounted at right angles to the incident beam direction. As observed by Measday et al. ‘) a measurement of the y-ray yield at one angle only is adequate to show important vari- ations of the total cross section for inelastic scattering with energy. In the case of 12C(3He, t)12N and 10B(3He, n)12N reactions leading to the ground state of “N, the positon activity provided the resulting cross section variation with energy.

ICI B, 1zC-3He INTERACTION 455

900-

-800 -

700-

~Koo-

.-500-w’

I

4

Loo-z

5

-300-u

-2oo-

IOO-

Fig. i. Block diagram of counting system for 15.11 MeV gamma radiation.

F&e lnverter and

Stretcher

. - i9(p, Lx.uJ60” x-+“B (p, Y f2i*

3 % I

P f g

I

Fig. 3. Pulse-height spectrum from lgF(p,y)160* and liB(p,y)lzC* reactions. The 5.62 MeV is a single escape peak and the 6.91 MeV peak is a mixture of two lines 6.9 and 7.1 MeV.

456 J. SINGH

24OOL

* 2GOo- I-

5

s 1600-

IZOO-

800 -

400 -

Fig. 4. RCL analyser energy calibration.

l- 15.11 MeV

‘. :

.

l .

.

l .

.

!*C&;~f*C* (151MeV)

------- (a) 19.72MeV

. . . (b) 30.60 MeV

c.s-*~~-Lsx (c) Background

TOTAL INCIOEN? CHARGE 3*79 X&Coulomb

4, . . ’ ,,I I

I

* ..p I

x :‘:-;-;_: l ’ . . --__ b * *‘,:_. a_ *. _ _.s.m.- .

SO 100 I20 140 160 “’ CHAN& N”tfi &R 2zo 2k0

Fig. 5. Pulse-height spectrum for 12C(3He, 3He’)12C* (15.11 MeV) reaction, (a) 19.72 MeV, (b) 39.60 MeV, (c) background.

10 B, 12G3He INTERACTION

2.2. THE 12C(3He, 3He’)12C* (15.11 MeV) REACTI_ON

457

A self-supporting carbon foil of thickness 14.6 mg * crne2 (the 3He energy loss of 2.4OkO.05 MeV at full energy) was used as a target. The gamma radiation from the target was detected in a sodium iodide crystal 12.7 cm long and 12.7 cm in diameter which subtended a solid angle of 0.28 sr at the target centre. The crystal was viewed by an EM1 9530 A photomultiplier and was heavily screened by a conical-shaped lead structure having a wall thickness of 25 cm between the collimators, beam collector and the crystal. The radiation entered the crystal through a 0.5 mm thick stainless steel window. The beam currents of about 3 to 4 nA were used. Pulses from the photo- multiplier were amplified by a main amplifier NE 5259 which was adjusted to produce negative outgoing pulses. These negative pulses were routed to a 256-channel RCL analyser using the electronic circuit shown in fig. 2. This included a linear gate system to prevent the passage of small pulses to the analyser without attenuating pulses corresponding to an energy of about 10.5 MeV. This energy was determined by ad- justing the bias level of the single-channel analyser (see fig. 2) which opened the gate to the multichannel analyser. In order that this gate should be fully operative, the main pulses were delayed by 2.8 ps on their way to the linear gate unit. In the final operation of the system the 256-channel analyser livetime was effectively 100 %.

The energy scale for the detection system was established using monoenergetic y-rays from the suitable proton induced reactions. The pulse-height spectrum from the reactions l’F(p, ay)160* and ‘lB(p, y)%* for a proton energy of about 1 MeV is shown in fig. 3. These reactions provide lines of the energies indicated and the resulting energy calibration of 256-channel analyser is shown in fig. 4, which also includes low-energy points from “Na, 24Na and 6oCo sources. In the experiment, the 24Na lines are usually present because of neutron activation of the scintillator and thus provided a continuous check on the energy calibration. Pulse-height spectra for the 12C(3He, 3He’)12C*(y)12C process taken at incident 3He energies of 30.6 and 19.7 MeV are shown in fig. 5. A background spectrum taken without the carbon target in place is also shown in fig. 5. This spectrum includes a very small contribution due to 11.83 and 12.71 MeV peaks which mostly decay to the final state *Be+ 4He as shown by Prats ‘) and Olsen “). For the 15.11 MeV state of “C the ratio of the radiation width to the ground state of 12C and the cc-particle width to the 2.9 MeV state of *Be is 0.9 [refs. ‘a”)]. Thus the yield h s own in fig. 5 mostly provides a 15.11 MeV peak contribution. On each day, the background and pulse-height spectrum at 19.72 MeV were taken to find the average background and normalization of different runs. This background was taken as the standard value to find the contribution of 15.11 MeV gamma radiation. In analysing the data the efficiency of the sodium iodide crystal for 15.11 MeV quanta radiation was obtained by interpolating the curves given by Marion -“); a value of 1.43 x 10e2 was used. This figure includes the solid angle factor.

2.3. THE 12C(3He, t)12N AND 1oB(3He, n)12N REACTIONS

The positon-emitting nucleus 12N has a half-life of 12 ms and a maximum positon energy of 16.4 MeV [ref. ‘“)I. All the excited states of 12N are particle unstable

458 J. SINGK

against break-up into “I3 +p and thus do not significantly contribute to the ground state beta activity. The branching ratio for this decay is 97 % [refs. loa ““)I* Therefore the “N activity indicates direct formation of the ground state. To detect this activity it was arranged to pulse the incident beam and to aoxrnulate a time spectrum of positon pulses] in a plastic s~intillator 3.1 cm x 5.1 cm cylinder during the periods between beam pulses, A block diagram of the pulsing unit is shown in fig, 6. A digital delay and gating unit (AERE Harwell type 2122 A) controlled driving pulses for the ion source switch of the cyclotron. A beam Iasting for 10 ms was used for the

-+%e**- ____ n Fifament G genera tar

m....: I_____ T=

Positive bias

Negaf ive qm supply

12C(3He, sH)12N reaction and 1 ms for 10B(3He, n)‘“N reaction because of the larger cross section. The acceleration time for the 3He beam was about 15 ,xs but recording was not started until 0.5-Z ms after the end of the beam pulse in order to allow for recovery of the photomuI~~lie~ from the beam burst, For this the triggering pulse from the digital gating unit was delayed by an mount equal to the beam ON period plus the time required between beam OFF and counting to be started. After the pre-determined delay of 0.5-2 ms the counting of positon pulses, obtained from a plastic seintillator coupled to the EM1 6097A, was done in the multiscaling mode of the 256-channel RCL analyser for 128 ms using 128 channels with a time channel width of 1 ms. This automatic operation of the analyser in multiscaler mode was achieved by adjusting the gate widths of the triggering pulse about 40 ps and 5 V positive amplitude. The whole cycle was repeated with a period of 135 - 150 ms until good statistical quality of the results had been reached. Figs. 7 and 8 show decay curves obtained in this way for 12C(3He, t)r2N and ““B(“He, n)12N reactions, with values of the “N half-life obtained from a least-squares Wed line. Control experi-

--- ---/ I’

460 J. SINGH

ment with no carbon or boron target gave no evidence of any decay with the 12N lifetime. From these decay curves the yield of the reaction at corresponding incident 3He energy was obtained after the proper subtraction of the background which was estimated by the inspection of the decay curve over the final channels. This background was fed into a basic Fortran computer programme which calculated the half-life for every 5 channels, the average half-life by the least-square fit method and the yield of the reaction by integration of the time spectrum. In the case of the 10B(3He, n)12N reaction absolute cross sections were obtained by normalizing the observed yields to the results of Peterson 12) and for the 12C(3He, t)12N reaction, relative yields only were obtained.

N’3EXCITAlION ENERGY IN MeV I2 2779 23.33 3086 0 3240 1 3y34 35% I 3701 3855 L Lo08 I 4162 43;16 up9

I 1 I , , I , I , I

6 6 IO 12 14 16 I8 20 22 24 26 2B 30 tNClDEN1 E Hz (LAB) Me’.’

Fig. 9. Activation cross sections for the 10B(3He, II)~‘N,.~. reaction.

3. Results and discussion

3.1. THE 10B(3He, n)“N REACTION

The excitation function for this reaction from 6 to 30.6 MeV is shown in fig. 9. It covers a range of excitation energies in 13N from 27 to 45 MeV, and this is also covered by the work of Rimmer and Fisher “) on the reaction 12C(p, n)12N. The experiments agree in indicating some resonant structures at these high levels of excita- tion and a general decline of cross section for 13N excitation above about 30 MeV. The reaction processes are of course different; but the capture of a pair of protons by 1°B would certainly create a mass-12 system in a T = 1 state as suggested by Rimmer and Fisher for the (p, n) reaction as a preliminary to the formation of fairly long-lived states of 13N. In the present case however there is some evidence for resonances at energies for which the outgoing particle has about 25 MeV energy and for which the picture of the coupling of a slow particle to a T = 1 core would not appear to be valid.

10 B, 12C-3He INTERACTION 461

3.2. THE 1ZC(3He, 3He’)12C* AND 12C(3He, 3H)12N REACTIONS

The excitation function for the production of 15.11 MeV gamma radiation is shown in fig. 10 and that for “N activity from “C ( 3He, t)12N reaction in fig. 11. The error

bars in fig. 11 represent the uncertainty in cross section arising mainly from back- ground subtraction and the uncertainty in energy due to the straggling of the beam

I I I I I I t , , 1 1 , I 19 2u 21 22’ 23 24

I$

25 26 27 26 29 30 31’

INCIDENT ENERGY (Mel’) tNLAB

Fig. 10. Excitation function for 15.11 MeV radiation from 12C(3He, 3He’)i2C* reaction.

160 ,EXCl TATION ENERGY (Met’)

2y67 x$&7 31;27 32.07 1 32;67 33p7 34;47 3727 3qo7 36;87

‘2C(3He,tf2Nc,s.

I I-t, _ F ‘/ I I I I I I I J 21 22 23 24 25 26 27 28

INCIDENT 3He 29 30 31

ENERGV( MeV) IN LA8

Fig. 11. Excitation function for the 1zC(3He, t)12N,,,. reaction.

462 J. SINGH

in the degrading foils. There may be a systematic uncertainty of 2 % in the (3He, 3He’) cross sections because of the lack of knowledge of the “window efficiency”. The distinctive features are (a) the sharp peak in the (3He, 3He’) reaction centred on 21.5 MeV, (b) the gradual rise in both cross sections from 23 MeV to 30 MeV, and (c) the peak in the (3He, t) reaction at 29.5 MeV.

The generally rising cross sections, for these two reactions, in contrast with the falling cross section for the 10B(3He, n) reaction and for the 12C(3He, n)“O reaction which has been studied in a separate experiment ’ “) can be ascribed to the increasing penetrabi~ty of the Coulomb barrier of the residual nucleus for the outgoing charged

Ye = PZOfm R = 4-478fm Reduced mass -i 2.41 amu. Coulomb Bonier Eio = 3=864 MeV Potential Depth = 8,669 MeV

Fig. 12. Barrier penetrabilities for- “He particles i?i,.llC.

particle. Fig. 12 shows barrier penetrability factors 13) as a function of 3He energy and it is clear that a supposition of effects in the I = 0, 1 and 2 partial waves would predict a Cross section rising over the observed energy range. For both reactions the energy of the outgoing particle is about 20 MeV less than that of the incoming 3He, so that the penetrability effects for the latter are less important. A formalism for the calculation of the yield of reactions of the type studied here on the basis of a com- pound nucleus mechanism has been given by Sarantites and Pate 14). Application of their method to the (3He, 3He’) reaction yields smoothly rising cross sections for E(3He) between 20 and 30 MeV in the case of each compound nucleus spin J, = +, 3, 3 and 3.

If the main mechanism operating for the (3He, 3He’) and (3He, t) reactions is that of the compound nucleus then resonant features observed in one excitation function might also be sought in the other; The well marked:peak at 21.5 MeV in (3He, 3Her)

10 B, =G3He INTERACTION 463

is not seen in (3He, t), but it would appear close to the reaction threshold where the yield is very low because of the triton penetrability factor. It is however also not especially marked in the 12C(3He, n)140 cross sections. The peak seen in (3He, t) at 29.5 MeV may possibly account for the irregularity at approximately this energy with the (3He, 3He’) reaction. The cross section in the neighbourhood of the 21.5 MeV peak in the inelastic scattering rises quickly than would be expected solely from the penetrability calculations (see fig. 12). It also falls more rapidly than competition from the (3He, 3H) reaction, which is also governed by a penetrability factor. The width of the peak allowing for beam width and target thickness is about 2.4 MeV. The value found by Measday et al. ‘) for the similar peak observed in the l’C(p, p’)12C* reaction is 4 MeV. These authors interpret the peak in terms of a resonance of the outgoing particle wave in the optical-model potential of the residual nucleus 12C*. Superficially such an explanation is attractive in the present case, but the peak width should be of the order of 2 W where W is the depth of the imaginary part of the optical- model potential for the case of volume absorption. Analysis of low-energy 3He-12C (ground state) scattering, quoted by Hodgson 15), suggest W-values of the order of 20 MeV, which is not consistent with the observed width. It is therefore probable that 21.5 MeV peak is associated with more complex configurations leading to the 15.11 MeV state than in the case discussed by Measday et al.

An effect determined at least to a considerable extent by the outgoing channel would be helpful in explaining the absence of the peak in the 12C(3He, n)140 reaction (Q = -1.15 MeV) f or which the final state is different. The resonance might be expected to appear in the (3He, t) reaction (Q = -17.62 MeV) leading to the 12N member of the T = 1 isobaric triad, but as has already been pointed out, it would be suppressed by penetrability effects. In the reaction 12C(t,3He)12B (Q = - 13.36 MeV) leading to the third member of the T = 1 triad, a similar resonance might well be seen.

The author is indebted to Professor W. E. Burcham and Professor H. S. Hans for their kind interest, encouragement and valuable suggestions. The author wishes to thank Professor J. H. Fremlin for providing a large sodium iodide crystal and pulsing unit for ion source. The author is particularly grateful to Mr. R. Wilson, Dr. N. S. Chen, Mr. E. E. Cartwright and other Nuffield cyclotron operators under the supervison of Mr. W. C. Hardy for the efficient running of the cyclotron. The fiqancial assistance from the University of Birmingham in the form of a Research Assistantship is highly appreciated.

References

1) D. F. Measday et al., Nucl. Phys. 44 (1963) 98 2) E. M. Rimmer and P. S. Fisher, Nucl. Phys. A108 (1968) 561, 567 3) C. F. Williamson et al., Rapport CEA-R 3042 (1966) 4) C. Tschalar, RHEL/R-146 (1967) 5) F. Prats and A. Salyers, Phys. Rev. Lett. 19 (1967) 661

464 J. SINGH

6) W. C. Olsen el aZ., Nucl. Phys. 61 (1965) 625 7) E. G. Fuller and E. Hayward, Phys. Rev. 106 (1957) 991 8) A. Bussiere de Nercy and M. Langevin, J. Phys. Rad. 21 (1960) 293 9) J. B. Marion and F. C. Young, Nuclear reaction analysis (North-Holland, Amsterdam, 1968)

p. 49 10) T. R. Fisher, Phys. Rev. 130 (1963) 2388 11) J. F. Vedder, University of California Radiation Lab. report UCRL-8324 (1958) 12) R. W. Peterson and N. W. Glass, Phys. Rev. 130 (1963) 292 13) Feshbach and Weisskopf, Phys. Rev. 76 (1949) 155 14) D. G. Sarantites and B. D. Pate, Nucl. Phys. A93 (1967) 545 15) P. E. Hodgson, Adv. in Phys. 17 (1968) 563 16) J. Singh, Nucl. Phys. Al55 (1970) 443