Nuclear Physics 45 (1963) 123--128; ( ~ North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
INVESTIGATION OF THE REACTION Al~(p, =o)Mg~t IN THE PROTON ENERGY RANGE 1-2.5 MeV M. A. ABUZEID, F. M. ALY, Y. P, ANTOUFIEV, A. T. BARANIK, T. M. N O W E R and P. V. SOROKIN
Atomic Energy Establishment, Cairo, Egypt, U.A.R. Received 30 August 1962
Abstract: The excitation function o f the reaction AlST(p, ae) Mg st has been measured in the proton energy range 1-2.5 MeV. Angular distributions are measured for most of the resonances. Resonance strengths have been determined and the possible values of spins and parities of resonance levels of compound nucleus Siss are given.
1. Introduction The reaction A127(p, =o)Mg2. with Q = 1.59 MeV has been investigated at different energies of the incident protons 1-5). Shoemaker 2) in 1951 has determined the excitation function through the range 1-4 MeV; Anderson s) has determined resonance strengths and parities of some levels which appear pronounced at proton energies of 800-1400 keV. The AlZ7(p, C¢o) reaction at higher energies is not sufficiently investigated. In the present work, the excitation function for A127(p, go)Mg 2+ was measured together with the angular distribution of ~o- particles for 13 resonances in the proton energy range 1-2.5 MeV. Information about the cross-sections resonance strengths, spins and parities for most of the corresponding levels in the Si2s nucleus were obtained from the analysis of the experimental data. 2. Equipment Accelerated protons were obtained from the electrostatic accelerator type ~F-2.5-II of the Atomic Energy Establishment of the U.A.R. The energy of the accelerated protons is measured by a magnetic analysis. The magnetic field of the analyser was measured by the proton resonance method. The energy spread in the incident beam was 0.25 ~o. The scattering chamber used for measurements is shown in fig. 1. The proton beam after 90° deflection by the magnetic analyser passed through collimator diaphragms 1, 2 and 3 of 3 mm diameter and hits the target 4. The beam current was collected by Faraday cup 5 fed to a current integrator of the type Elcor Model-A-30-9A. Secondary electrons were suppressed by means of a guard at the entrance of the Faraday cup. 123
M . A . ABUZEID £t a/.
Self supporting targets A127 of 20-50#g/cm2 thickness were prepared by evaporation in vacuum. The energy loss of such targets for 1 MeV of protons is 3-7 keV. The reaction products were recorded by two semiconductor detectors of the type ORTEC-100A-40. One of them with solid angle 2.1 x I0-3 sr, was placed at the angle 135° with respect to the beam and used as a monitor. The other detector (7) could be rotated around the target without disturbing the vacuum in the range of scattering angles 30°-150 ° in the laboratory system. The solid angle of the rotating counter was 1.9 x 10 -3 sr. Pulses from the semiconductor detectors were fed to two pre-amplifiers placed on the scattering chamber and were further amplified by two linear amplifiers of the type 1430 Dynatron Radio L.T.D. $
Fig. 1. Scattering chamber used in the measurements.
The pulse spectrum was analysed using integral discriminators of the type 100 9E Dynatron Radio L.T.D. or by 100-channel pulse-height analyser At~-100. The energy resolution of the recording devices was 2 9/0 for ~-particles emitted from a Po 21o source. 3. Exlmrimental Results
3.1 EXCITATION FUNCTION The excitation function was measured at the angle of 150° with respect to the incident beam. Since the ~-peaks could be very well separated from the elastic scattering peaks and the background under c~-peakwas negligible the integral discriminator was used in measuring the excitation function. The discrimination level was chosen so that all ~-particles corresponding to the ground state of Mg 24 could be detected. Measurements were carried out by increasing the proton energy in steps of 4 keV each, while in the resonance region steps of 2 keV were used. The alpha particle yield was measured per 50/~C charge accumulated on the target.
THE Al~(p,~o)Mg~ R~,C'nON
Fig. 2. The relation between the energy o f the incident proton beam and the cross-section for ~ particles from the reaction Al~7(p, ~0)Mgu at scattering angle 150 °.
TABLE 1 Experimental data 1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
1.184 1.319 1.363 1.391 1.437 1.569 1.583 1.650 1.726 1.899 1.985 2.031 2.135 2.165 2.175 2.206 2.311 2.368 2.380 2.450 2 484
12.772 12.907 12.957 12.979 13.025 13.157 13.171 13.238 13.314 13.487 13.573 13.619 13.723 13.753 13.763 13.794 13.899 13.956 13.968 14.038 14.072
( 2 J + 1)Fp/'~/F (keY)
1.2 0.1 2.1 0.2 1.0 0.I 0.25 0.2 2.0
a, 4- A as
0 +, 2 +, 3-
--0.49 4-0.27 --0.88 4-0.06
3.2 0.6 1.3 0.5
0.06 1.93 --0.24 0.99 2.85
4-0.05 4-0.05 4-0.05 4-0.09 4-0.21
7.3 5.3 5.2
1.87 4-0.004 0.11 4-0.07 --0.08 4-0.04
ABUZEID et aL
Determining the ratio between the area under the go-peak and that of proton peak from the A127(p, go) and A127(p, p) reactions at Ep = 1.184 MeV, it was possible to obtain the absolute cross-section for the AIZ~(p, ~o) reaction. The elastic scattering cross-section for the reaction A127 (p, p) was calculated assuming it to be equal to the Coulomb scattering cross-section. Fig. 2 shows cross-section for the reaction A127(P, =o) at the angle 150° as a function of proton energy. The values of resonance energies and the corresponding excitation energies of Si 2s levels are tabulated in columns 2 and 3 of table 1. The resonance 2.380 MeV, which has been determined by Shoemaker z), has been resolved in our measurements into two resonances with energies 2.368 and 2.380 MeV. 3.2. A N G U L A R
Angular distributions of u-particles were measured for isolated resonances with considerable intensity. Angular distributions were also measured for the resonances 2.311 and 2.380 MeV, which do not appear isolated in the excitation function. At every angle the pulse spectrum has been measured using a 100- channel pulseheight analyser. The area under the g-peak is normalised to a constant number of counts recorded by the monitor with discrimination level chosen so that it could record go-particles from the reaction AlZT(p, go) only. The measured results are shown in fig. 3 by dots. Lines are made in accordance with the least-squares method. The values of a2 coefficients in the angular distributions 1 +02 cos20 are given in column 5 of table 1. £p=1.363
_, Ep : 1.650 MeV
Ep =L/.~ MeV • - , , T .... ' .....
Ep• 2.20 MeV
Epq . ~
. d /
Ep =2.F/~ MeV
60 ;o ,~o ,~o ,8o'
120 150 180
L £p - 2,45 MeV
Figs. 3a and 3b. The angular distribution for the ~-resonances.
90 120 150 180
THE AI|V(p,~o)MgS" REACTION
Using the data obtained on the cross-section of the reaction A127(p,~o) and angular distributions, the resonance strengths (2./+ have been calculated. To determine the resonance strength the total cross-section for each resonance and the area under it was calculated. The area under the resonance which corresponds to the total cross-section is related to the resonance strength as follows 6):
f a(E)dE= ~(n~)2(2j+ 1)r
where £ is the wave length of the incident proton; J is the level spin of the compound nucleus SiZS; Fp, F~ and F are proton width, ,,-particle width and the total width of the level. 1 Clearly, resonance strengths, calculated in this manner, do not depen~l on the thickness of the target or on the energy speard in the incident beam. Assuming the angular distribution of ~-particles to be isotropic, resonance strength for weak resonances were also estimated. The calculated resonance strengths are tabulated in column 4 of table 1. In the investigated energy region, the resonance strengths in the reaction A127 (P, ~0) are much bigger than those, which appear in the reaction AlZT(p, 7).\The average value of resonance strengths investigated in this work ~ 2000 eV, while the average value of resonance strengths in the reaction AlZ7(p, 7) ~ 8 eV in the same energy region 7). To determine the possible values of spins and parities of the compound nucleus Si 2s levels the measured angular distributions were compared with the calculated ones s). When calculating angular distributions it was assumed that 1) the observed resonance is related to one level only. 2) The level is formed and decays only with the minimum value of the orbital momentum, which is allowed by the conservation laws of the total angular momentum and parity. TABLE 2 J~
0+ 01+ 12+ 23÷
isotropic forbidden forbidden 1 + 0 . 3 3 cos 2 0 isotropic forbidden forbidden 4--5t I + ~ S t c°s2 0
M . x . ~UZmD et al.
3) The orbital momentum of the ~o-particle emitted does not exceed 3. The results of the calculation are tabulated in table 2. Here t is the ratio of the probability that the reaction takes place with entrance channel spin 3 to the probability of its taking place with the entrance spin 2. Values of spins and parities of the levels which have been determined as a result of the comparison of experimental angular distributions with the calculated ones are tabulated in column 6 of table 1. Comparison of the results of the present work with the data obtained from the A127(P, T) reaction makes it possible to determine spins and parities of two levels (Ep = 1.184 and 1.363 MeV) unambiguously. The analysis of the T-ray spectrum for the resonance 1.184 MeV shows that the corresponding level decays to the ground and the first excited state (J -- 0 + and 2 +, respectively) of the nucleus Si2s with approximately the same probability. Angular distributions for both lines are isotropic. Since the transition 0-0 is forbidden and the transition 3--0 is little probable the spin of the corresponding level must be 2 +. For the 1.363 MeV resonance, T-transition to the ground state is absent. The angular distribution for the T-ray corresponding to the first excited state of Si2s is not isotropic. Thus the values 0 + and 2 + are excluded, and the level should have spin and parity 3-. It is to be noted that the values of the mixture parameters of entrance channel spins, which are obtained from both the reactions (p, ~o) and (p, T), are approximately the same if the spin and parity of the 1.363 MeV level is 3-. The resonance strengths obtained in the present work agree well with the results of Anderson 5). The average difference of 2 0 ~ is obviously due to inaccuracy in determination of the cross-section in the reaction A127(p, ~o)Mg24. The same value of spin and parity 2 + as in ref. 2) is obtained for resonance level 1.183 MeV. Spin and parity value 3- for resonance level 1.363 MeV also agrees well with ref. 2), where two possible values (2 + and 3-) are given for the corresponding level. We express our sincere gratitude to Professor M. El-Nadi for his interest and encouragement during the course of this work. We are also indebted to the Van de Graaff group for their extensive assistance in the work. References 1) Rutherglen and Smith, Proc. Phys. Soc. 6A (1953) 101 2) F. C. Shoemaker, J. E. Faulkner, G. M. Bouricius, S. C. Koufman and F. P. Mooring, Phys. Rev. 83 (1951) 1011 3) Sukeaki Yamashita, J. Phys. Soc. Japan 16 (1961) 2378 4) G. E. Fisher, V. K. Fisher, E, A. Rember and M. D. Tather, Phys. Rev. 110 (1958) 286 5) Anderson, Haug, Holtebekk, Lonsjz and Nordhagen, Nuclear Physics 34 (1962) 121 6) Nuclear reactions, ed. by P. M. Endt and M. Demeur (North-Holland Pubi. Co., Amsterdam, 1959) p. 502 7) Y. P. Antoufiev, D. A. Darwish, L. M. E1-Nadi, O. E. Badawi and P. V. Sorokin, private communication (1962) 8) M. Abdel-Moaty, Report of the Atomic Energy Establishment U.A.R. (1962)