The energy levels of 22Na and neutron production in stars

~ I.E.7

NuclearPhysics 51 J

(1964) 427--432; ( ~

North-Holland Publishing Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

T H E E N E R G Y LEVELS OF 22Na AND N E U T R O N P R O D U C T I O N I N STARS s. HINDS, H. MARCHANT and R. MIDDLETON Atomic Weapons Research Establishment, Aldermaston, Berks.

Received 26 August 1963 Abstract: The energy levels of Z2Naup to an excitation of 7.88 MeV have been measured by magnetic analysis of the ~4Mg(d,~)~Na and 23Na(SHe,~)22Nareactions. The assignment of the second T = 1 state is discussed. In connection with the ~lNe(~, n)24Mgreaction as a source of neutrons in stars, it is shown that there are at least two and possibly four resonances in the 21Ne(p, y)Z*Nareaction at stellar temperatures. 1. Imtroduction The energy levels of 22Na have been measured because of their importance to astrophysics in connection with neutron production in stars. Fowler, Burbidge and Burbidge 1) have suggested that the 21Na(~, n)24Mg reaction m a y be a source of neutrons in stars provided that 2~Ne is not destroyed too quickly by proton burning. The latter depends critically on the 2~Ne(p, ~)22Na resonances at proton energies appropriate to stellar temperatures. These energies 2) are from zero up to about 140 keV and so are too low for the resonances to be observed directly, because of the very small cross-section. The 24Mg(d, ~)22Na and 23Na(3He, 0022Na •reactions have therefore been studied to see if there are any levels having excitations between about 6.74 and 6.88 MeV in 22Na, which are the excitations corresponding to the above proton energies. The 23Na(aHe, ct)22Na reaction was studied as well as the 24Mg(d, ~)22Na reaction in case the latter did not excite all the levels, due to the isobaric spin selection rules. I f these rules were obeyed it would be possible to identify the low-lying T = 1 states of 22Na, since these should only be excited by the 23Na(3He, ~)22Na reaction. The low-lying energy levels of 22Na have been measured in several investigations (see Endt and van der Leun 3)). The measurements of Browne 4) covered the largest range of excitation; he observed 15 levels up to an excitation energy of 4.47 MeV. Two resonances in the 2XNe(p, y)22Na reaction are well established 5) but these are above the region of astrophysical interest. 2. Experimental Procedure The 23Na(aHe, 0~)22Nareaction was studied at an incident 3He energy of 8.46 MeV using the Aldermaston 6 MV electrostatic generator, which had been modified, by 427

428

s. HINDSet

al.

the addition o f a gas stripper, to increase the yield of doubly charged helium 6). Thin sodium targets were prepared by vacuum evaporation of sodium metal or sodium hydroxide onto thin carbon films. These were transferred under vacuum to the broadrange magnetic spectrograph in which the reaction products were analysed. We were unable to prepare sodium targets which were durable and also thin enough to give very good resolution. The 24Mg(d, ct)22Na reaction was studied using thin separated isotope 24Mg targets 7) at incident deuteron energies of about 6 MeV and 13.02 MeV. The lower energy was used with the same experimental arrangement as for the (3He, ~) reaction. The higher deuteron energy was obtained from the Aldermaston tandem electrostatic generator, and in this case the reaction products were analysed in the multi-channel magnetic spectrograph 8). ALPHA- PARTICLE ENERGY, He V Z

610

615 Ho (d.~) N,

5,15

24

~ 400

~

4

300 !

i 7.0

;/2

E d - 5 . 7 0 MeV

0

0 - 30" 8

i x~

3

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Fig. 1. Alpha-particle energy spectrum measured from the 2~Mg(d, ~t)mNareaction in the broad-range magnetic spectrograph at an angle of observation of 30° and at an incident energy of 5.70 MeV. The target was a thin layer of separated isotope ~4Mgon a thin carbon backing. 3. R ~ u l ~

Energy spectra from the 24Mg(d, ct)22Na reaction were measured at angles of observation of 30 °, 60 ° and 90 ° at 5.700 MeV incident energy and at 20 °, 30 °, 35 °, 45 ° and 60 ° at 5.912 MeV. One of these spectra is shown in fig. 1, where the groups are labelled by the numerical order of excitation of the corresponding l~=vels in 22Na. These measurements only extend up to the 3.058 MeV excited state. For the 2.56 MeV and higher levels the single 13.02 MeV deuteron energy exposure in the multi-channel spectrograph was used; only the spectra measured at angles of 35 ° , 42½°, 50 ° and 57½° were analysed. The 50 ° spectrum is shown in fig. 2. Strong impurity groups were observed from the 12C(d, ~)I°B and 160(d,~)14N reactions. These groups are labelled by the symbol of the residual nucleus with a subscript to indicate the appropriate excited state. These impurity groups obscured many 2ZNa groups in fig. 2 but these were observed at other angles. G r o u p 50 was only observed

429

ENERGY LEVELS OF ~ZNa

with any appreciable intensity at 27½° and part of this spectrum is inset in fig. 2. The energy levels of 22Na measured from the 24Mg(d, ~) reaction are listed in column 1 of table 1. Alpha-particle energy spectra were measured for the ZaNa(aHe, ~)22Na reaction at angles of observation of 15°, 30 °, 45 ° and 60 ° for an incident aHe energy of 8.46

7:0

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Fig. 2. Alpha-particle energy spectrum measured from the 24Mg(d, ~)2~Nareaction at an incident ener-

gy of 13.02 MeV, in the 50° channel of the multi-channel spectrograph.

I

9

ALPHA- PARTICLE ENERGY, He V r2 Is

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Fig. 3. Energy spectrum of ct-particles measured from the 2aNa(~He, ~)22Na reaction in the broadrange spectrograph at an angle of observation of 45 °. The incident SHe++ energy was 8.46 M e V and the target was a thin layer of sodium metal on a thin carbon backing.

MeV. The 45 ° spectrum is shown in fig. 3. Strong impurity groups were observed from the ground state transitions of the 160(3He, ~)lSO and 12C(3He, ct)11C reactions. Due to the inadequate energy resolution of about 20 keV arising from the poor sodium targets, these measurements were only used as a check on the results from the 2~Mg(d, ~)22Na reaction. They were, for example, of great assistance in confirming the existence of groups 27, 36 and 48. The levels obtained from this reaction are shown in column 2 of table 1.

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s. HINDS e t al.

The weighted mean values of the energy levels of 22Na are shown in column 3. Also shown in column 5 are the values reported by Browne, which are in agreement, within the experimental errors, with the present measurements. In the present method of energy level measurement the accuracy of the measurements decreases as the level excitation increases. A check on the accuracy of the higher excited states in therefore desirable and this was obtained by making measurements on selected impurity groups. For example, in the (d, ~) study the 160(d, ~)14N reaction was used and several determinations were made of the states of 14N. These were found to agree with the previous best values (see Ajzenberg-Selove and Lauritsen 9) and Erskine and Browne lo)) to about _+ 10 keV. TABLE 1 T h e energy levels o f 12Na (MeV)

Group number 1 2 3 4 5 ,6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

a) 0.582 0.655 0.888 1.527 1.933 1.946 1.980 2.210 2.566 2.965 3.055 3.521 3.710 3.943 4.082 4.329 4.365 4.474 4.532 4.593 4.631 4.721 4.786 5.076 5.111 5.139 5.330 5.449 5.615 5.741 5.752 5.842

Present investigation Mean b) value 0.583 0.660 0.890 1.528 1.945 1.980 2.212 2.567 2.967 3.062 3.532 3.712 3.951 4.073 4.322 4.363 4.474 4.531 4.594 4.631 4.732 4.778 5.071 5.126 5.184 5.304 5.444 5.596 5.728 5.739 5.832

0.583 0.658 0.889 1.528 1.933 1.946 1.980 2.211 2.566 2.966 3.058 3.527 3.711 3.947 4.077 4.325 4.364 4.474 4.531 4.594 4.631 4.727 4.782 5.073 5.111 5.132 5.184 5.317 5.446 5.605 5.734 5.745 5.837

Exp error e) 5 5 5 5 7 7 7 7 7 7 7 10 10 10 10 10 10 10 10 10 10 10 10 10 15 10 15 15 10 15 15 15 10

Ref. 4) a) 0.585 0.661 0.893 1.533 1.944 1.990 2.219 2.576 2.975 3.066 3.526 3.713 3.949 4.323 (4.472)

ENERGY LEVELS OF 2~Na

431

TABLE 1 continued

Group number 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 a) b) c) a) e) t)

a) 5.876 5.938 6.001 6.092 6.200 6.251 6.338 6.434 (6.450) 6.533 6.563 6.591 6.643 6.727 6.752 6.863 6.955 7.017 (7.094 7.155 7.212 7.276 7.364 7.409 7.513 7.548 7.573 7.628 7.677 7.822 7.882

Present investigation Mean b) value 5.867 5.953 5.975 6.087 6.180 6.231 6.321 6.436 6.524 6.549 6.637 6.671 6.700 6.857 6.950 6.988

5.871 5.938 5.953 5.988 6.089 6.190 r) 6.241 6.329 6.435 (6.450 6.528 6.556 6.591 6.640 r) 6.671 6.713 6.752 6.860 t) 6.952 7.00 (7.09 7.16 7.21 7.28 7.36 7.41 7.51 7.55 7.57 t) 7.63 7.68 7.82 7.88

Exp error c) 10 15 15 15 10 15 15 15 10 15) 10 15 15 10 15 15 15 10 10 20 20) 20 20 20 20 20 20 20 20 20 20 20 20

Ref. n) e)

7.413 7.476

Measured from the ~Mg(d, ~)2~Na reaction. Measured from the 2aNa(SHe, ~)22Na reaction. On weighted mean (4- keV). Measured from the a4Mg(d, ~)2~Na and 25Mg(p, ~)2~Na reactions. Measured from the alNe(p, 7)22Na resonances. Probable doublet.

A f u r t h e r c h e c k o n t h e a c c u r a c y o f the h i g h e r levels c a n be o b t a i n e d b y c o m p a r i s o n w i t h t h e t w o well e s t a b l i s h e d r e s o n a n c e s in 2 t N e ( p ' y ) 2 2 N a c o r r e s p o n d i n g to levels 5) at 7.413 a n d 7.476. I t m a y be seen f r o m t h e t a b l e t h a t t h e r e is g o o d a g r e e m e n t f o r t h e 7.413 level, b u t t h e a g r e e m e n t is n o t so g o o d f o r t h e 7.476 M e V level. I t is p o s s i b l e t h a t this l a c k o f a g r e e m e n t is d u e to t h e d i f f e r e n t r e a c t i o n s e x c i t i n g d i f f e r e n t levels.

432

s. HINDS et al.

4. Discussion The isobaric spin selection rules are expected to inhibit the formation of the 0.658 MeV (T = 1) excited state of 22Na in the 24Mg(d, ~)22Na reaction. However, this state was consistently observed with an intensity between 1 ~o and 5 ~o of the intensity of the ground state. These limits are slightly lower than those of Browne 4) at incident energies of 6.5 and 7.0 MeV. From comparison with 22Ne the second T = 1 state is expected to occur at about 1.93 MeV. Three states are observed at about this excitation energy in the 24Mg(d, c0 22Na reaction, but unfortunately these were not clearly resolved in the 23Na(aHe, ~)22 Na reaction. One possibility is that there are four states at about this excitation energy and that the T = 1 level is weakly excited in the (d, u) reaction and therefore not observed, also due to poor resolution this level in the (aHe, ~) reaction was not resolved. Alternatively the isobaric spin selection rule may be violated in the (d, ~) reaction, as suggested by Hashimoto and Alford l~) and Erskine and Browne 1o)), and one o f the three observed levels is the T = I state. The latter explanation is possibly more plausible, particularly if account is taken of the higher spin (2 + ) of the second T = 1 state which would suggest that the (d, a) transition to this state should be about five times as intense as the 0 + first T = 1 state. As mentioned in the introduction the astrophysical interest in the energy levels of 22Na lies in the region between 6.74 and 6.88 MeV excitation. In this range there are two or three levels, as may be seen from the table. The 30 keV bound level at 6.713 MeV will also enhance the rate of the 21Ne(p, 7) reaction in the same way 2) that the 26 keV bound level of 21Na enhances the 2°Ne(p, 7) reaction. Since the 2~Ne+p reaction is resonant some of the 2~Ne which would have been available in the later helium burning stage of stellar evolution for the 2~Ne(a, n)24Mg reaction will be destroyed. The amount destroyed will depend on the unknown widths of the 2 1 N e + p resonances. We would like to thank Dr. K. W. Allen for his interest in this work, Professor W. A. Fowler for helpful discussion and Dr. N. Tanner for bringing to our notice the astrophysical problem. We are indebted to Mr. F. A. Howe and Mr. A. H. F. Muggleton for preparing the targets. References l) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11)

w . A. Fowler, E. M. Burbidge and F. R. Burbidge, Astrophys. J. 122 (1955) 271 E. M. Burbidge, G. R. Burbidge, W. A. Fowler and F. Hoyle, Revs. Mod. Phys. 29 (1957) 547 P. M. Endt and C. van der Leun, Nuclear Physics 34 (1962) 1 C. P. Browne, Nuclear Physics 12 (1959) 662 S. E. Arnell and E. Wernbom, Ark. Fys. 23 (1963) 301 S. Hinds and R. Middleton, Proc. Phys. Soc. 75 (1960) 754 S. Hinds, H. Marchant and R. Middleton, Proc. Phys. So¢. 78 (1961) 473 R. Middleton and S. Hinds, Nuclear Physics 34 (1962) 404 F. Ajzenberg-Selove and T. Lauritsen, Nuclear Physics 11 (1959) 1 J. R. Erskine and C. P. Browne, Phys. Rev. 123 (1961) 958 Y. Hashimoto and W. P. Alford, Phys. Rev. 116 (1959) 981