Pramana. Vol. 8, No. 5, pp 9771, 478-488. © Printed in India.
High resolution gamma-ray spectroscopy of the ~TAI (p, y)~sSi resonance reaction
M A R A H M A N , M A AWAL, M R A H M A N , H M SENGUPTA*
and S K GUPTA#
Atomic Energy Centre, Ramna, Dacca, Bangladesh
* Department of Physics, University of Dacca, Dacca, Bangladesh t Bhabha Atomic Research Centre, Bombay 400085, India
MS received 20 December 1976; in revised form 28 February 1977
Abstract. High resolution gamma-ray spectra have been measured from the ~TAI (p, y) ~Si reaction for the resonances at Ep = 2"482, 2"511 and 2.735 MeV at
0p~ --- 0 °, 30 °, 55 ° and 90 ° using a Ge (Li) gamma spectrometer. From the spectra and the angular distributions the properties of the resonance states have been obtained. These states are the isobaric analogues of the levels at 4.69, 4.75 and 4-93MeV levels respectively i n the parent nucleus A~l.
Keyword& (p, y) reaction ; resonances; angular distributions; partial gamma transition strengths.
1. Introduction
The properties of unbound levels of ~sSi have been studied by many authors through proton capture reactions on 27A1 (Antoufiev et al 1964, Nordhagan and Teeter 1965, Gibson et al 1968, Huang and McDaniels 1971, Meyer and Wolmarans 1969, Meyer et al 1970) and an up-to-date information on the resonance levels upto an excitation of 13-483 MeV has been compiled by Endt and Van der Lcun (1973). This corresponds to E~ ~ 2 MeV. Several other works have since been reported (Neal and Leon 1973, Dalmas et al 1974).
Analogue states have been identified by means of the 27A1 (p, y) reaction in the energy range E , = 0 . 8 5 - 3 . 0 MeV, as discussed in a previous communi- cation (Ahmed et al 1972). The present work is concerned with the properties o f some of these resonances given by the high resolution gamma-ray spectra taken on the resonances and the angular distribution measurements of the emitted gamma rays. For this purpose three strong and isolated resonances were selected, namely at E , = 2.482, 2-511 and 2-735 MeV.
2. Experimental Procedure
The experimental measurements have been performed by using the proton beam from the 5-5 MoV Van de Graaff accelerator of the Bhabha Atomic Research Centre, 478
Gamma rays o f (P,7) reaction 479 Bombay. The beam was collimated by using a number of tantalum collimators of various diameters inside the beam tube. The beam energy resolution was found to be less than 1 keV by measuring the width of the 992 k e y resonance having a natural width of 100 eV. The targets were prepared by vacuum evaporation of natural aluminium on tantalum backings. The target was nominally 20/zgm/cm z thick and was kept at an angle of 45 ° to the beam direction. A coaxial Ge(Li) detector (Princeton Gamma Tech) of active volume 20 cc has been used for the measurements reported here. The signal output of the detector was amplified by a Fet preamplifier and an active Filter amplifier model PA manufactured by the Electronics Corporation of India Ltd. and was analyzed by a Nuclear Data 4096 channel pulse height analyzer. The gain of the main amplifier was adjusted so as to obtain an energy dispersion (keV/channel) so that the full scale of either 2048 or 4096 channels in the pulse height analyzer covered a possible ground state transi- tion. The low energy '/-ray backglound increases with proton energy and it becomes difficult to keep the dead time down to acceptable limits by adjusting only the beam current. These low energy y-rays were attenuated by placing thin lead sheets either on the face of the detector or on the outer surface of the target chamber. The energy calibration of the Ge(Li) detector was obtained by taking a "/-ray spectrum at the 992 keV resonance. The energy dispersion was obtained from the 511 keV energy difference between the appropriate pairs of full energy, single and the double escape peaks associated with the high energy '/-rays in the spectrum of the 992 keV resonance. The relative efficiency of the detector was also obtained using the branching ratios for gamma transitions at the 992 k e y resonance.
The resonances in the ~TAI (p, 3') reaction as determined by Ahmed et al (1972) at E, = 2-482, 2-511 and 2-735 MeV are shown in figure 1. In the present experiment, these resonances were located by using a NaI(T1) detector of size 12.7 cm × 15.2 cm at 90 ° with respect to the incident beam direction. The reso- nances were scanned automatically in an energy range of 100 keV using the tech- nique described by Bhalerao et al (1974). In this technique (illustrated in figure 2) the reference voltage for the analyzing magnet of the accelerator is varied by adding a ramp and accordingly the accelerator energy varies with the magnet current due to the corona feed-back control of the accelerator. For the (p, '/) excitation function only one detector, NaI(TI), was used and was recorded directly on the 4096 channel pulse height analyzer. The automatic scanning preceded the manual scanning of the excitation functions and saved machine time by a quick location of the resonances under study. Setting the accelerator energy at the desired resonances, the high energy gamma spectra were obtained at the angles 0 °, 30 °, 55 ° and 90 ° using the Ge(Li) detector. A gamma spectrum at E~ = 2.482 MeV is shown in figure 3. For the angular distributions, the NaI(T1) detector served as the monitor.
3. Data Analysis
From the measured spectra the branching ratios for various transitions from the resonance levels at 13.980, 14-007 and 14.223 MeV in 28Si corresponding to the resonances at 2-482, 2-511 and 2.735 M e g respectively have been obtained after correcting for the detector efficiency. The gamma ray strengths (2J-l- 1) p--li
480 M A Rahman et al
25 3 0 -
2.735
D
2 , 8 2
o
x 20
¢ J o o
3
Z : D
o 15
U
s -
I l I
0 I I I I
2./.0 2.60 2.80 3.00
PROTON ENERGY (MeV)
Figare 1. Excitation function for the ~TAI (P,7) reaction; the resonances studied in the present work are labelled.
F~F,~/F for many resonances in the ~TA1 (P,7) reaction have been listed by Endt and Van der Leun (1973) upto g~ = 1.968 MeV. Using the excitation function data of Ahmed et al (1972) and the above compilation of Endt and Van der Leun, the strengths could also be determined for the three resonances discussed in the present work. The resonance strength at E~ --- 2.482 MeV is in agreement with that given by Lyons et al (1969).
The angular distributions were analyzed using the expression W (O) = Ao + A~ Q~P2 (cos O) + A , Q , P , (cos O)
G a m m a rays o f ( p , ? ) reaction 481
F
I L
F •
ANALY'SINOMAONET r..,Ib TO CORONA
Ill M _.
I~ ~ ~ / /-'~SWflCHING ~ = r ~ 3 FARADAY
• ~J 4 0 9 6 CHANNEL R H ANALYSER
SCANNING UNIT
Figm'e 5. Schematic representation of the automatic excitation function set-up.
where W (0) is the intensity observed at an angle 0 relative to the incident beam and Q2 and Q4 are the attenuation factors due to the finite size of the detector (H~tuser e t a l 1966). The ratios .4JAo = as and A d A o = a4 depend on the spins of the initial and final states J~ and Jt and on the multipole mixing ratio 8. The analyses of the data were done in the usual way by varying arctan 8 in the range - - 90 ° to + 90 ° for the assumed value of J~ (J/ being known). The quality of each fit is given by the quantity X ~ defined by
x~ = n---i] ~ [Y' -- w,l'/z,'
t l X
where n is the number of data points, Y~ and W~ are respectively the experimental and theoretical yields and E, is the error of the experimental yield. The phase convention o f Rose and Brink (1967) has been used.
The coefficients A0, a-z and a4 are summarized in table 1. The Go(Li) detector has a low efficiency at the high energy gamma rays involved in the present work.
The large errors in the coefficients thus arise primarily from the uncertainty in the subtraction o f backgrounds for the gamma ray peak areas. The coefficients alone cannot therefore be used conclusively for the assignment of spin-parity of the resonance states studied. We have therefore used the data on the resonance strengths and the identification of the levels as isobaric analogues of the levels in
~SAl as well to assign the J~-values and the reduced transition strengths. The reduced transition strengths were obtained by assuming F~ ~ / " >~ F t.
P--12
25.000
0 .j40C U; (J u~ I,-- Z (~20( (.J 0 4OO 00m ""-.. -. ~.---. _ --.../':
) :o I 100 200
I I I I I I I I I 30(
, | 27Ai(p, ¥) 28Si RESONANCE SPECTRUM ~' Ep = 2.482 MeV - I )
} ,
IX2.$ I -" • I I 400 500 600 70( ,.. . ~`e~:~'-*~'~;-~'~r~.'.L`'~'2~- ~'~ ~ "~'~'K~~ ~~~a~~:~>~~ ~ 1 800 ... " 9 LO0 I ~ I I ~oo Tpoo 1:]oo ~200 " - t300 t40~ c 0 "~::'~'~"~'~"~"~"~'~":~"~>~'~::"~"~,'-~5~'~~'~ ':~e:~'~'*j~-'~'~'~'~ 2.800 2.900 3.000 3.100 3.200 3"300 CHANNEL NUMBER~..'~"., ..~.,~: ,':-'..'.-':~;.~,,*,,,;.,,'~;".,~,-..~,:..~ .~',.',:.,j:~.-~:.,,~t~.~':.~., ..¢.¢...=.-'v~.,;.,.'.'.:.~#..~,~,.,:,7=¢,~,.',,,..~...,,.., ,,,.: ,.,;,~','x" "~,'.,~7..'7,.',~.',,-. ", ,~ ,- ~, - • .~ ~ • : ' "~ .~ "" ,v z~ I I I /~/ I I I I I l ... I . I ... "*~'~".',-,,,~,,-',I~,~.,~'.,..~ .... 100 2200 2300 2,400 2500 2~:o0 ~ 2.800 I ~00 3r~O0
t~ Figaro 3. The gamma ray spectrum observed at E~ -- 2.482 MeV using the 20cc Ge(Li) detector. The single and double asterisk marks over the gamma peaks indicate respectively the first and second escape peaks.
G a m m a r a y s o f (P,V) reaction 483 Table 1. Sum~niry of the ~,-ray angular distribution coefficients
i
Primaty
E~ (lab) (MeV) transition A o as = AJAo a 4 = A J A s E.~ (MeV)
2"482 2"511
2"735
I
4. Results and Discussion
9.363 21824-52 0.35=1=0-50 0-104-0"10 12"200 46=/=8 0 " 5 0 4 - 0 " 3 3 0"20=1=0"65 9-390 28554-54 --0"114-0"04 0"054-0"07 5"275 2284-30 --0"494-0"26 0"014-0"49 7-335 1224-20 --0.494-0"33 --0"144-0"63 9"606 4224-24 --0"134-0"13 0-134-0"24 12-442 404-7 --0.334-0"31 0"084-0"70
The angular distributions of y-rays which are emitted by the resonance levels 13.980, 14.007 and 14.223 MoV of 2sSi at E~ - 2 . 4 8 2 , 2.511 and 2.735 MoV respectively are discussed below. Some of the angular distributions with the theo- retical fits and the X ~ as a function of arctan 8 are shown in figures 4 and 5.
The results are summarized in table 2.
The E~ = 2"482 M e V resonance
The resonance level 13.980 MoV of ~aSi emits the 9.363 and 12"200 MoV
~,-rays populating the 4.618 (J~" ---- 4 +) and the 1.779 (J~ = 2 +) MoV levels respec- tively. The gamma ray spectra at E, = 2.482 MoV wore studied by Antouliev et al (1972) and the level was found to bo de-excited through the 4"618 MoV level without branching. On the basis of the angulm distributions of the 7-rays, the j T = 4- is the best assignment. But this assignmg~t leads to an M2 multi- polarity for the 12-200 MoV transition. If the transition is assumed to be such a character, the reduced gamma transition strength for it is ,~19 W.u., which is rather unreasonably high. Hence adopting the next best J'~-valuc of 3-, we have calculated the transition strengths and found them to be reasonable. This assign- merit is ~dso consistent with that of the 4.685 MoV level in the parent nucleus
~SA1 [ J ~ = (1-4)-] (Endt and Van dot Lean 1973).
The E, = 2.511 M e V resonance
The 4.618 MeV level (J'~=4 +) of 2ssi is populated by the do-excitation o f the 9-390 MoV y-rays from the resonance level 14.007 MoV. The spin of this level is found to be 4, which is again consistent with that of the parent level in 2SA1 leo --- 4.741 MeV, J ~ = (0--5) +, Endt and Van der Leun 1973].
The E,, = 2.735 M e V resonance
The de-excitation of the resonance level 14.223 MoV takes place through the omis- sion of the 5.275, 7.335, 9-606 and 12.442 McV 7-rays and the levels 8.945, 6"889 (J~" ~- 4+), 4.618 (J'~ = 4 +) and 1.779 (J~ ---- 2 +) MeV of ~s Si aro rospcc-
d
1£• /1 ,, .q \ \ \ ~ ... t~.-. I .i \ / -.-.( r~4\ i '1 \ / I- "'z'"-- . ~ d'l" y I I 1.01 - is-gm-.-F--Ji
I o.3l
i , i-90 -60
-3060 90
0.3 i I i l I I i i I I i
-90 -60 -30 0 30 60 90 0 30 -30 0 30 60 90 an:ton 6 arcton 6 arcton 6
ii ,c
_~., / \
_...~...~> / . c"/ %,,-~-, / \, 10
' "XI
/" II ,VL
... 2 ...1.(j - ,4.617-" ~4 0.3 i -90 -60
3 4 A IOC- o 33 c 3-4 --- r3~2/2~2
]4-2-~ ] 3-4 ~.~.
-- ~5--4 .~.~.~"_...I.2..:.4~.~.~ -~'~ 50 29, ~f
I I I I I C I l I f f 2 I I f I0 0.25 0.50 0,75 1.0 0 0.25 0.50 0.75 1.0 0 0.25 0.50 0.75 1.0 COS 2 e cos 2e cos2e
Figure 4. The an~ular distribution and the X 2 plots for the (A) 9.363 and (B) 12"200 MeV gamma rays produced at the Ej = 2.482 McV resonance and (C) 9.390 MeV gamma ray produced at the Ep = 2.511 MeV resonance.4~ O0
lol i¢ 1
IOZ 10 2
, ".. \ i I ",.L....~__... ix. ,o~.. 'U_'_,L~t_ D, r,~', ... ,"-~ ... /- -" I,~,: ... 7 7--\-H- ".~'-' i' \ /~ ',,' Ji/ ~lii~/l'"..i ~J 11 T
I I 14-223 Ji I 4. 620 --v'-- 40.1. "60J L J, t j 0.II i i i i -3,, o 3o 6o ~ -9o-6o -3o 6 3o 6o ~o arctan
arctan ~ ,/k- " m_ ~-" ~-, I
o ~oot- L___~ ~-= ~/'2 -~ ,-=~oL; o.'~s o!= o:~,.~ j~°°r
co;~ ' .o~ ., , , ,
oo.~ = o.~ ,.o cos2e
102 I [ r-4- Z C
I01L-,,,,,,~3- i \\ f
"\\ ~" i~- \ ... 77"- - -.,-'~ "-'~./-;7 l - 2",_,,,-..---.x. / ./ " ,.:,__L ~ b °-9~ 4o-:1o b 3b 6!o 90 arctan 6 o•10C
L) 0 z 0 i I ,I I .... I o o~s o so o.~ locos2o
Figure 5. The angular distribution and the X 2 plots for the (A) 7.355, (]3) 9.606 and (13) 12.442 MeV gamma rays produced at the Ep = 2.735 MeV resonance.l'
,K486 M A Rahman et al Table 2. Reduced transition strengths
Reduced
Primary transi- (2J+l) transi-
Ep (lab) tion energy Branching Spin sequence I~,P,,/F Multi- tion (MeV) (MeV) ratio (~) j~r j~r ±267o polarity strength
(eV) 4-20~
(m.W.u.)
2"482 2"511 2"735
9.363 95 3- 4 + 40 E 1 33-9
12-200 5 3- 2 + 2 E1 0.8
9-390 100 4 + 4 + 15 MI 37.5
5.275 13 3+ 4 + 14.6 M1 690
7.335 7 3+ 4 + 7-8 M1 136
9.606 66 3+ 4 + 73.9 M1 570
12.442 14 3+ 2 + 16.0 M1 57
tively p o p u l a t e d by them. The spin o f the resonance level has been assigned t o be 3, (2) f r o m the latter three angular distributions; the spin 3 is favoured because 66"7~o branching o f the 9.606 MoV y-rays takes place by decaying to t h e 4-618 M c V level (d= = 4+). T h e spin value is consistent with that o f the parent
level in 2SAl.
T h e angular distribution o f the 5.275 MeV y-rays, populating the 8.945 MeV level o f 2ssi, was analysed with the above spin assignment (d = 3); this leads t o the d = 4 for the latter level. This level decays t o the J~" = 4 + level at E ,
= 4-618 MeV without branching which is consistent with the above spin assign- ment. T h e other spin value (2) does not lead t o any satisfactory fit. T h e data o n the r e d u c e d gamma transition strengths are listed in table 2.
5. C o n c l u s i o n
T h e decay scheme o f the resonance levels 13-980, t 4 . 0 0 7 a n d 14-223 MeV as assigned in this experiment is shown in figure, 6. Thes~ resonance levels are identified as analogues f r o m a consideration o f the (3oulomb displacement energy in the pair o f nuclei 2SA1JsSi. A c o m p a r i s o n with the properties o f the corres- p o n d i n g p a r e n t levels derived f r o m the (d, p) stripping reaction on 2~A1 ((3arola a n d Van dor Baan 1971, (3hen et al 1972) is made in table 3. T h e spin values are f o u n d to he consistant with those o f the p a r e n t levels in 2SA1. Sinc~ the g r o u n d state spin o f the target nucleus is 5/2 +, the possible channel spins are 2 and 3.
This fact was t a k e n into account in the process o f analyses o f the data. Large M1 strengths as carried out b y t h e 5.275 a n d 9.606 MeV transitions h e m the 14.223 M e V level are noteworthy.
T h e m o s t likely p r o t o n configurations o f the resonance levels are suggested as follows:
13-980 MeV level ( J ~ = 3-, T ~ 1): [(dsc~) -1 (f7/2) 1]
14-007 MeV level (J~" = 4 +, T ~ 1): ['(dst2) -x (d3u) 1]
14-223 MeV level ( j r . : _ 3 +, T = 1): [(d~t~ -a (ds/~) ~]
Gamma rays of (p,~) reaction
4873 " ( 2 ~ - - ~ . ~ ~ oi ~ oi r< ~i 1 4 . 2 2 3
14. 0 0 7
A ~ - ~" . ' - - 1 3 . 9 B 0
i l II tit
9 5 5 1 0 0 1 4 6 6 7 1 3
4"
OI r-,
o
pt'~t p,.
r ~
,/
t ~ . 9 4 5
6 . 6 6 9
4 . 6 1 B
1 . 7 7 9
Table 3.
Ep (lab) (MeV)
O ¢ v
Figure 6. The decay scheme of the levels measured in the present experiment for the nucleus 2sSi.
Properties of the analogue resonances
~TA1Co,7)2sSi
Eo (MeV) j,r E, (MeV)
, , , | |
2~AI (d,p)2SA1
I, j,r
Carola Chen et al Carola Chen et al
and (1972) and (1972)
Van der Van der
Baan Baan
(1971) (1971)
2 . 4 8 2 1 3 . 9 8 0 3 - 4- 685 1 +3 1 + 3 ( 1 - 4 ) - ( 1 - 4 ) -
2-511 14.007 4+ 4.741 (0)+2 2 ( 0 - 5 ) + ( 0 - 5 ) + 2.735 14.223 3+, ( 2 ) + 4.928 (0+2) (0-5)+
I t m a y be m e n t i o n e d here t h a t the energy levels a n d the electromagnetic transi- tion rates in 2ssi have boon calculated by Farris a n d Eisonberg (1966) in t h e f r a m e w o r k o f the particle-hole description a n d a n u m b e r o f o d d p a r i t y levels w i t h b o t h T = 0 a n d T = 1 h a v e been predicted a t Eo > 10 M o V ; for the l a c k o f experimental d a t a t h e oven parity levels wore n o t calculated. I t would b e o f
488 M .4 Rahman et al
interest to calculate such levels with the availability of more data (present work;
Huang and McDaniels 1970, Noal and Loon 1973, Dalmas et al 1974).
Acknowledgements
The authors express their thanks to M K Mehta and (3 V K Baba for useful discussions during the experiment. The help of M Musa and the computer personnel of the Atomic Energy C~ntre, Dacca, is appreciated. Two of the authors (MAR and MAA) thank the Ministry of Education and Social Works, Government of India, for financial support and the Nuclear Physics Division of Bhabha Atomic Research Centre, Bombay, for providing the facilities at their disposal and one of the authors (HMSG) appreciates an associateship provided by the Atomic Energy Centre, Dacca.
References
Ahmed H et al 1972 Z. Phys. 257 380 Antoufiev Y P et al 1964 NucL Phys. 56 401
Bhalerao P J, Vaje M Y, Gupta S K and Baba C V K 1974 Nucl. Phys. Solid State Phys.
Symp. B17 285
Carola T G P and Van der Baan J G 1971 Nucl. Phys. A173 414
Chen S, Rapaport J, Enge H and Buechner W W 1972 Nucl. Phys. A197 97 Dalmas J, Leccia E and Ale~nard M M 1974 Phys. Rev. C9 2200
Eudt P M a n d Van der Leun C 1 9 7 3 Hurl. Phys. A214 194 Farris S A and Eisonberg J M 1966 Nucl. Phys. 88 241
Gibson E F, Battleson g and McDaniels D K 1968 Phys. Rev. 172 1004
I ~ u s e r O, Lopes J S, Rose H J and Gill R D 1966 Hurl. Phys. Lab. Rep. (Oxford).
Hossain D 1972 D.Phil thesis (Oxford University) (unpublished) Huang F C P and McDaniels D K 1971 Phys. Rev. C2 1342 Lyons P B, Toevs J W and Sargood D G 1969 NucL Phys. A130 1 Meyer M A and Wolmarans N S 1969 NucL Phys. A136 663
Meyer M A, Wolmarans N S and Reitman 1970 NucL Phys. A144 261 Heal G F and Leon S T 1973 Phys. Lett. B45 127
Hordhagan R and Tvetor A 1965 Nucl. Phys. 63 529 Rose H J and Brink D M 1967 Rev. Mod. Phys. 39 306
