The excitation function of the 29Si(n, α0)26Mg reaction for neutron energies around 14 MeV

Nuclear Physics 79 (1966) 108--112; (~) North-Holland Pablishin9 Co., Amsterdam Not to be reproduced by photoprint or m i c r o f i l m without written permission from the p u b l i s h e r

T H E E X C I T A T I O N F U N C T I O N OF T H E 29Si(n, O~o)26Mg R E A C T I O N FOR N E U T R O N E N E R G I E S A R O U N D 14 M e V M. JASK6LA, L M. TURKIEWICZ, J. TURKIEWICZ and F. GRADZKI Institute of Nuclear Research, Polish Academy of Sciences, Warsaw, Poland

Received 26 October 1965 Abstract: The nuclear reaction 29Si(n, ~)26Mg has been studied by bombarding a silicon detector with fast neutrons. The excitation function for the ground state transition has been measured in the neutron energy range 13.4-15.0 MeV. E

NUCLEAR REACTION 29Si(n,%), E ~ 13.4-15.0 MeV; measured ~(E).

i I

1. Introduction The measurement of excitation functions with high resolution offers a possibility to determine the mechanism of nuclear reactions 1). At energies sufficiently high t o form a compound nucleus in the region of overlapping levels (F >> D), the characteristic Ericson fluctuations appear in the excitation function. The width of these fluctuations is related to the mean life of the compound nucleus. The Ericson fluctuations exist if the compound nucleus mechanism contributes considerably to the reaction and if the number of uncorrelated reaction channels is small a). In the case of pure direct interactions the Ericson fluctuations do not appear. Other mechanisms which can cause these fluctuations are the formation of the socalled "doorway states" a) and the existence of partial equilibrium in intermediate resonances 8, 9). At present the importance of these mechanisms cannot be clearly evaluated. We have measured the excitation function of the z9Si(n, e)a6Mg reaction for neutron energies corresponding to excitation energies in the compound nucleus a 0Si o f about 25 MeV. This should be well within the region of overlapping levels. 2. Experimental M e t h o d The reaction z9si(n, c~)26Mg can be conveniently studied by bombarding a silicon detector with fast neutrons. The detector used in this experiment was an n-type surface barrier detector of 12 m m diam. and 1200 f2 • cm resistivity. The voltage applied was 200 V which corresponds to 2.5 ranges of the most energetic alpha particles. The energy resolution for ThC alpha particles was better than 1%. The high resistivity o f the silicon was a guarantee of the high purity of the target. The pulses from the detector were fed to a low noise charge sensitive preamplifier,. then amplified linearly and analysed with a 128-channel amplitude analyser. 108

29Si(n, ~)26Mg g.s. REACTION

109

Neutrons from the all(d, n)4He reaction were produced by deuterons, accelerated up to 400 keV in the Van de Graaff accelerator, striking a thin (0.36 mg/cm 2) T - T i target. Between the emission angles of 126 ° and 56 ° the neutron energy varied from 13.41 to 14.96 MeV.

/

J5 \

/ /

Fig. 1. Schematic diagram of the experimental arrangement. 1) silicon detector, 2) HV power unit, 3) charge sensitive preamplifier, 4) linear amplifier, 5) multichannel analyser, 6) dE/dx counter, 7) E counter, 8) cathode follower, 9) linear amplifier, 10) trigger, 11) coincidence unit, 12) gate, 13) discriminator, 14) scaler, 15) T-Ti target and 16) direction of the incident deuterons. The target detector was placed at a distance of 6 cm from the neutron source, parallel to the direction of the incident neutrons. The neutron energy spread, due to the thickness of the tritium target and the geometrical conditions, depended on the neutron energy. For energies up to 14.4 MeV the energy spread was less than 50 keV and for those above 14.4 MeV less than 100 keV. The neutron flux at the solid state detector was monitored by a proton recoil telescope consisting of two scintillation counters. Both counters were placed in tandem along the direction of the incident neutrons. The use of thin polyethylene foil and the coincidence technique ensured a good discrimination of the proton group against the background. The corrections resulting from the energy dependence of the monitor efficiency were taken into account. The experimental arrangement is shown in fig. 1.

llO

M.

JASK6LAet al. 3. Results

A typical spectrum of pulses from the silicon detector is shown in fig. 2. All spectra show a peak corresponding to alpha particles from the 29Si(n, ~0)26Mg reaction. We also observed transitions to the well-known 4) first excited state of 26Mg, but not to a 1330 keV level suggested by Dearnaley and Ferguson s).

N 15000

B ii

]

,oooI

;

p

gg0

850

E

,/i,OOOo

D

., o

11.30

A

14.10

1270

E~ (Me V)

Fig. 2. Particle energy spectrum from a silicon detector bombarded with 14.2 MeV neutrons. The peaks A and B correspond to the ~gSi(n,~.)261Vfgreaction, P to the ~sSi(n, p)-°SA] reaction and the remaining peaks to the ~sSi(n, ~)~Mg reaction. 0"i

800

Sg°{'oc)Mg n 2eGs.

] I .k

600 [

,

i

400

200

i

13.5

i

i

i

140

/4.5

150

Fig. 3. Excitation function for the ~°Si(n,~0)~6Mg reaction.

~ag~(n, cz)ggMg g.s. REACTION

111

From the areas under the ground-state alpha peaks we obtained the excitation function for the ~9Si(n, ~o)26Mg reaction. It exhibits fluctuations with energy (fig. 3).

;-(E)

] gO00

!0000

5000

4~o

6 ff
-5000

Fig. 4. The correlation function.

F[~ev;' 0

208

•

0

0

100

o 0

ee

e@

J O

•O

o

I)

2~

3?

~

Jo

55

Fig. 5. Plot of coherence energies versus mass number A; open circles from reactions induced by neutrons, full circles from reactions induced by charged particles.

112

M. JASKdLA et al.

From the excitation function we constructed a correlation function (fig. 4) defined as 6):

F(e) = @ ( E + e ) ~ ( E ) ) - ( o ) 2, where the brackets denote averaging over the whole energy interval. The coherence ene~ gy for the compound nucleus 3 osi obtained from the correlation function is about 60 keV. In fig. 5 this coherence energy F is compared with those obtained for other nuclei 7) for excitations between 18 and 25 MeV. The coherence energies obtained in the reactions induced by neutrons and charged particles differ considerably. This feature is difficult to explain with the Ericson fluctuation theory. The coherence energies obtained from neutron induced reactions might be overestimated due to an underestimated neutron energy spread. If this is not the case, it seems reasonable to suppose that another mechanism exists, which causes the fluctuations. The authors wish to thank Professor Wilhelmi for stimulating discussions and comments and Dr. J. Chwaszczewska for preparing the detectors. Thanks are also due to the Van de Graaff accelerator crew for helpful cooperation during the experiment. References 1) 2) 3) 4) 5) 6) 7) 8) 9)

T. R. B. P. G. T. T. K. K.

Ericson. Phys. Rev. Lett. 5 (1960) 430 O. Stephen, Clarendon Laboratory Report (unpublished) Block, H. Feshbach, Ann. of Phys. 23 (1963) 47 IVL Endt and C. van der Leun, Nuclear Physics 34 (1962) 1 Dearnaley, A. T. Ferguson, Phys. Lett. 1 (1962) 196 Ericson, Ann. of Phys. 23 (1963) 390 Mayer-Kuckuk, Hereegnovi Lectures (1964) Izumo, Prog. Theor. Phys. 26 (1961) 807 lzumo, Nuclear Physics 62 (1965) 673