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The spin and parity of the 648 keV resonance in Mg26 (p, γ)Al27
N u d e a v P/rys/~ 27 (1961) 112---114; ( ~ No~h-Hollamt l~blishlng Co., ,4m,~tevdam .cot to be reproduced by pbotopflat ~ microfilm without written perml~oa from the publisher
T H E S P I N A N D P A R I T Y OF T H E 648 keY R E S O N A N C E I N MI~,~
(p, 7)AI = ROLF NORDHAGEN
Fysish institutt, Universitetet i Oslo, Blindern, Norway Received 13 April 1961 A b s t r a c t : T h e spin and parity of t h e 648 keV resonance in MgN(p, ~,)AI~ is discussed a n d t e n t a -
tively a s s u m e d to be | - .
The weak648 keV resonance found b y proton bombardment of natural magnesium has recently been positively identified as belonging to the reaction Mg ~ (p, 7) AI ~. This was done b y the same group i) as originally reported the resonance, aided b y the author of the present note. The identification was based on the appearance of the 0.84 MeV 7-my from tl, ~. first excited state of A1~ in the ~-mdiation at 648 keV. This radiation was ,~ot due to the close-lying strong 661 keV resonance. Here also a 1.01 MeV 7-ray from the second excited state can be seen. The latter line is hardly present at 648 keV. In the present note, the spin and parity of the 648 k(~V resonance are discussed. The discussion is entirely based on the careful s t u d y b y Van der Lean of low lying resonances in the mass region 22 ~ A <=35 and a close comparison with his results ~). The complete y-ray spectrum from the 648 keV resonance could not be found, due to the close proxin~Jty of the strong wide F19(p, ~ ~) resonance at 672 keV and the Mg~(p, 7) resonance at 654 keV. The only important feature which could be definitely established was, apart from the mentioned appearance of the 0.84 MeV line, the existence of a strong ground state transition. The total intensity of the resonance was estimated to be close to 10 % of the intensity of the 661 keV resonance. F r o m Van der Leuns' work, a (2Jq-l)~ of 1.4 eV for the latter resonance s) can be deducted, and thus the (2J-F1)7 of the 648 keV resonance is approximately 0.14 eV. Van der Leun finds that in the four Mg~(p, 7)AI~ resonances studied b y him all levels decay b y dipole transitions, and that enhanced E2 transitions are not present. In the odd-mass nucleide A1~ no isobaric spin selection rules hamper the dipole transitions, and it seems safe to conclude, as Van der Leun does, that only dipole trarL~itions actually take part in de-exciting the resonant level. The existence of a transition to the A1= ground state (spin ~+) then limits the resou~mt level spin to ~, ~ and ~. The spin ~ can be ruled out as cascades through the first excited state (~+) are present. 112
648 I ~ V R R S O N A N C ~ IN M~ll(p, 'y) AI I~'
113
We will then test the assumption F 7 < / ' p and thus ( 2 J + 1 ) 7 m ( 2 J + l ) P r ~ 0.14 eV. F o r the t i m e being we assume t h a t the m a j o r i t y of the resonant level de-excitatious takes place via the ground state transition ( ~ 8 . 9 MeV), and then s t u d y the poss~le strengths [M[2 of the probable multipole tmusitions. I n table 1 are listed the two possible values of /'y and the correspon3Jng transition strengths in Weisskopf units. TABLE 1 Possible transition strengths of a 8.9 MeV 100 % ground state transition in the decay of the 648 keV resonance I
Corresponding I ]M]s in W.U. p7 i I E1 : l . l - i O -4 0.035 eV M1 : 2.3 • 10-s
Possible spins | |
I
0.023 eV
I MI E1:7.2-10 -5 : 1.5 • 10-s
Van der Leun finds an average [M[2 of 1.7 o 10 -3 for E1 and of 2.5- 10-2 for M1 transitions, b u t observes a wide distribution ( ~ a factor 10) around these values. The values found are just touching Van der Leun's lower limits and they fall below the d.istributious observed when we take into consideration t h a t the resonance presumably decays only p a r t l y via the ground state transition. The a s s u m p t i o n / ' ~ <=/'p thus does not seem to be in good agreement with the observations. W e then study the reduced proton widths 0ps of a 0.03 e V wide level at 650 keV, using the same curves as V a n der Leun for the barrier penetrability of the proton. For capture or protons with orbital angular m o m e n t u m l = I, we get 0ps = 4 . 5 - 1 0 -5 , l ~ 2 : 0 ~ = 1 . 5 . 1 0 -4 a n d l = 3 : 0 p s - - - 3 - 1 0 -~. Again we compare with Van der Leun's average, this time for 0p~ in the mass region 22 ~ A ~ 35. I t is found t h a t 0p~ has an average of 7 ~/o and lies roughly between 15 ~/o and 1.5 ~/o. I t is seen t h a t the value for l ~- 3 lies well within these limits. The value l ~ 3 will give a spin for the resonant level of ~ - - or ~--s , and the latter is excluded b y previous arguments. The spin ~2 - would give an E l transition to the ground s t a t e whose width m a y be larger t h a n the observed width. We m a y therefore tentatively conclude t h a t the observed resonance at 648 keV in the reaction Mg~(p, 7)Al~ is a f-capture resonance with a spin -~--. The small width m a y be due to a limitation b y the proton width or b y both F r and Pv being of the same order of magnitude. However, the assumption th~.t the observed width is due to the width of a E1 or M1 transition, maldng possible b o t h ~+ or ~+ for the resonance spin, c~nnot be completely excluded.
114
R. I~ORDHAGm~
The author is greatly indebted to his colleagues 1) for permission to publish this note. References I) S. L. Andersen, T. Holtebekk, O. IJ~nsj6 and R. Tangen, Nuclear Physics 13 (1959) 310 ~) C. Van der Leun, thesis, University of Utrecht (1958); C. Van der Leun, P. M. Endt, J. C. Kluyver and L. E. Vrenken, Physica 22 (1956) 12£3 ~) H. E. Cove in Ncclear Reactions, ed. P. M. Endt and M. Demeur (North-Holland Publ. Co., Amsterdam, 19~9) F, 302."
T H E S P I N A N D P A R I T Y OF T H E 648 keY R E S O N A N C E I N MI~,~
(p, 7)AI = ROLF NORDHAGEN
Fysish institutt, Universitetet i Oslo, Blindern, Norway Received 13 April 1961 A b s t r a c t : T h e spin and parity of t h e 648 keV resonance in MgN(p, ~,)AI~ is discussed a n d t e n t a -
tively a s s u m e d to be | - .
The weak648 keV resonance found b y proton bombardment of natural magnesium has recently been positively identified as belonging to the reaction Mg ~ (p, 7) AI ~. This was done b y the same group i) as originally reported the resonance, aided b y the author of the present note. The identification was based on the appearance of the 0.84 MeV 7-my from tl, ~. first excited state of A1~ in the ~-mdiation at 648 keV. This radiation was ,~ot due to the close-lying strong 661 keV resonance. Here also a 1.01 MeV 7-ray from the second excited state can be seen. The latter line is hardly present at 648 keV. In the present note, the spin and parity of the 648 k(~V resonance are discussed. The discussion is entirely based on the careful s t u d y b y Van der Lean of low lying resonances in the mass region 22 ~ A <=35 and a close comparison with his results ~). The complete y-ray spectrum from the 648 keV resonance could not be found, due to the close proxin~Jty of the strong wide F19(p, ~ ~) resonance at 672 keV and the Mg~(p, 7) resonance at 654 keV. The only important feature which could be definitely established was, apart from the mentioned appearance of the 0.84 MeV line, the existence of a strong ground state transition. The total intensity of the resonance was estimated to be close to 10 % of the intensity of the 661 keV resonance. F r o m Van der Leuns' work, a (2Jq-l)~ of 1.4 eV for the latter resonance s) can be deducted, and thus the (2J-F1)7 of the 648 keV resonance is approximately 0.14 eV. Van der Leun finds that in the four Mg~(p, 7)AI~ resonances studied b y him all levels decay b y dipole transitions, and that enhanced E2 transitions are not present. In the odd-mass nucleide A1~ no isobaric spin selection rules hamper the dipole transitions, and it seems safe to conclude, as Van der Leun does, that only dipole trarL~itions actually take part in de-exciting the resonant level. The existence of a transition to the A1= ground state (spin ~+) then limits the resou~mt level spin to ~, ~ and ~. The spin ~ can be ruled out as cascades through the first excited state (~+) are present. 112
648 I ~ V R R S O N A N C ~ IN M~ll(p, 'y) AI I~'
113
We will then test the assumption F 7 < / ' p and thus ( 2 J + 1 ) 7 m ( 2 J + l ) P r ~ 0.14 eV. F o r the t i m e being we assume t h a t the m a j o r i t y of the resonant level de-excitatious takes place via the ground state transition ( ~ 8 . 9 MeV), and then s t u d y the poss~le strengths [M[2 of the probable multipole tmusitions. I n table 1 are listed the two possible values of /'y and the correspon3Jng transition strengths in Weisskopf units. TABLE 1 Possible transition strengths of a 8.9 MeV 100 % ground state transition in the decay of the 648 keV resonance I
Corresponding I ]M]s in W.U. p7 i I E1 : l . l - i O -4 0.035 eV M1 : 2.3 • 10-s
Possible spins | |
I
0.023 eV
I MI E1:7.2-10 -5 : 1.5 • 10-s
Van der Leun finds an average [M[2 of 1.7 o 10 -3 for E1 and of 2.5- 10-2 for M1 transitions, b u t observes a wide distribution ( ~ a factor 10) around these values. The values found are just touching Van der Leun's lower limits and they fall below the d.istributious observed when we take into consideration t h a t the resonance presumably decays only p a r t l y via the ground state transition. The a s s u m p t i o n / ' ~ <=/'p thus does not seem to be in good agreement with the observations. W e then study the reduced proton widths 0ps of a 0.03 e V wide level at 650 keV, using the same curves as V a n der Leun for the barrier penetrability of the proton. For capture or protons with orbital angular m o m e n t u m l = I, we get 0ps = 4 . 5 - 1 0 -5 , l ~ 2 : 0 ~ = 1 . 5 . 1 0 -4 a n d l = 3 : 0 p s - - - 3 - 1 0 -~. Again we compare with Van der Leun's average, this time for 0p~ in the mass region 22 ~ A ~ 35. I t is found t h a t 0p~ has an average of 7 ~/o and lies roughly between 15 ~/o and 1.5 ~/o. I t is seen t h a t the value for l ~- 3 lies well within these limits. The value l ~ 3 will give a spin for the resonant level of ~ - - or ~--s , and the latter is excluded b y previous arguments. The spin ~2 - would give an E l transition to the ground s t a t e whose width m a y be larger t h a n the observed width. We m a y therefore tentatively conclude t h a t the observed resonance at 648 keV in the reaction Mg~(p, 7)Al~ is a f-capture resonance with a spin -~--. The small width m a y be due to a limitation b y the proton width or b y both F r and Pv being of the same order of magnitude. However, the assumption th~.t the observed width is due to the width of a E1 or M1 transition, maldng possible b o t h ~+ or ~+ for the resonance spin, c~nnot be completely excluded.
114
R. I~ORDHAGm~
The author is greatly indebted to his colleagues 1) for permission to publish this note. References I) S. L. Andersen, T. Holtebekk, O. IJ~nsj6 and R. Tangen, Nuclear Physics 13 (1959) 310 ~) C. Van der Leun, thesis, University of Utrecht (1958); C. Van der Leun, P. M. Endt, J. C. Kluyver and L. E. Vrenken, Physica 22 (1956) 12£3 ~) H. E. Cove in Ncclear Reactions, ed. P. M. Endt and M. Demeur (North-Holland Publ. Co., Amsterdam, 19~9) F, 302."