Proton decay measurement with RIPS for astrophysical interest

Nuclear Instruments and Methods in Physics Research B70 (1992) 583-587 North-Holland

Beam InteraCtions with Materials Atoms

a

Proton decay measurement with RIPS for astrophysical interest S . Kubono, Y. Funatsu, N . Ikeda, M.H . Tanaka and T . Nomura Institute for Nuclear Study, University of Tokyo, Tanashi, Tokyo 188, Japan

H. Orihara

The Cyclotron and Radioisotope Center, Tohoku University, Sendai 980, Japan

S . Kato

Department of Physics, Yamagata University, Yamagata 990, Japan

M . Ohurs, T. Kubo, N. Insbe, T. Ishihara, M . Ishihsra and 1. Tanihata RIKEN. Wako, Saitama 351-01, Japan

H. Okuno, T . Nakamura and S. Shimoura

Department of Physics, University of Tokyo, Bunkyo, Tokyo 113, Japan

H. Toyokawa

Tandem Accelerator Center, University of Tsukuba, Tsukuba, Ibaraki 305, Japan

C .C . Yun and H. Ohnuma

Department of Physics, Tokyo Institute of Technology, Megro, Tokyo 152, Japan

K. Asahi

Department of Applied Physics, Tokyo Institute of Technology, Megro, Tokyo 152, Japan

A . Chakrabarti and T. Mukhopadhyay Bhabha Institute, Calcutta, India

T. Ksjino

Department of Physics, Tokyo Metropolitan University, Hachiohj4 Tokyo 192, Japan

The decay property of 20Na was studied using a 2° Mg beam to obtain a better understanding of the onset mechanism of the rapid proton process. The delayed protons were successfully measured, which correspond to one of the known I + states . There is no clear beta decay to the first excited I + state above the proton threshold in 20Na, suggesting that this state would not be the s-wave resonance as was expected before. The present result is preliminary; the final result will be published elsewhere.

1 . Introduction An explosive hydrogen burning process, which is called the rapid proton (rp) process [1), begins by breaking out from the hot-CNO cycle through the

reaction sequence [1-6] : '50(a, y)t9 Ne(p, y)2°Na (p, y) 21 Mg . . . . which is shown in fig. 1 . In this sequence the most crucial reaction is t9Ne(p, y)'Na, as it is not well understood. Specifically, the nuclear properties of the states near and just above the proton

0168583X/92/505.00 0 1992 - Elsevier Science Publishers B .V. All rights reserved

IX. RELATED TECHNIQUES

584

S. Kubono et al. /Proton decaymeasurement with RIPS

20~

I(P. Y 19 -, Ne*

15

(p . Y 18 F

S

HCNO Cycle

20 NeNa Cycle ß 19 F

(P.

11)

Fig. 1 . Breakout from the hot-CNO cycle and onset of the rapid proton process. threshold are important . The first excited state above the threshold in 20 Na was found at 2.637 MeV with J'= I+ by using ('He, t) and (p, n) reactions [3,4], which is shown in fig . 2. These results were confirmed experimentally [5]. Since the excitation energy of this state is so close to the Gamow r°sk for the temperature of interest, it enhances the reaction rate of ' 9Ne (p, y)2° Na roughly by two orders of magnitude, and predicts a reduction of about 50% in the onset temperature of the breakout from the hot-CNO cycle [4,5] . However, the absolute onset temperature has not yet been determined experimentally. This depends entirely on the nuclear property of the state, i .e ., the decay property, and the total width. Langanke [2] estimated the Thomas-Ehrman shift for the 1+ states in 20Na. The 3.488 MeV 1 + state in F, 20 which has a large st/2 component, was predicted to come down to 2.876 MeV in 2ONa, and thus this state would enhance the reaction rate considerably . Ex-E t h(1 9Ne+p ) 5 .s9 (MeV) ~- 3 5.20

2 1

The natural expectation was that the 2.637 MeV 1+ state was the one predicted by Langanke, but the level shift is too large . Brown [5] discussed the possibility of another ar,alogue state for this, suggesting that the entrance channel of t9 Ne + p could be a d-wave resonance rather than an s-wave resonance. In the experiments of charge exchange reactions [3-5] mentioned above, the nuclear property of 'Na was not clarified . An experiment to study these quantities was performed by measuring the decay particle and gamma rays using the projectile fragment of 2° Mg from Z4Mg at the RIKEN ring cyclotron facility . 2. Experiment A 100 MeV/u Z4 Mg primary beam of about 10-20 nA was used to produce 20 Mg particles with a Be target, giving 10-20 particles of 20 Mg per second using RIPS (RIKEN projectile fragment separator [7]) . Fig. 3 illustrates the experimental setup. The secondarybeam particles were identified by time-of-flight measurement just before the detector system, and stopped mostly in

a (2,3 .4) * ((3,45)+)

4.62

a

4.16

w(4,5,6)_

3.84 3.644 ((2,3,4) - ) 330 n (4.5,6)+ .0!6 =(1,2,3) 2.96717.02!!! 2.9673 2.842-. (3) + -----------------

2.196

1.832

~2_ 3

0.00

2

20

Na

Fig. 2. Nuclear levels of 2° Na near theproton threshold .

Fig. 3. Experimental setup.

S. Kubono et aL / Proton decay measurement with RIPS

the third silicon detector of the five silicon detector array (4 x 100 gm + 500 ltm) . This was checked by measuring the energy of the arrival particles . This array was surrounded by a beta-ray spectrometer made of three sets of DE-E telescopes of plastic scintillalots, which covers about 60% of the whole solid angle. Two sets of Ge detectors were also placed perpendicularly to the beam axis to try to measure the gamma decays . However, we did not see any specific lines related to the decay of 2° Na because of the secondary beam intensity used . The primary beam was stopped for 200 ms each time 2°Mgwas detected on the silicon detector array. The purity of the secondary beam of 2°Mg was about 1% on the focal plane where the detectors were set. The dominant particles were X60, 17 F and 18 Ne . Fortunately, there were no particle stable nuclei adjacent to 2° Mg, i .e., ' 9Na and 2'AI are particle unstable. The proton decay measurement did not suffer from severe beam contamination, since there is no other nuclear species which produces a delayed particle emission.

58 5

3. Result and discussion The delayed particle lines were clearly observed in the Si spectra, as shown in fig. 4, where only a low energy part is shown. There are four strong lines observed at 857, 1740, 2836 and 5892 keV. The last two lines are the delayed a-particles via the 7.424 and 10.274 MeV states in 20 Ne, leading to the ground state in 160, and their yield ratio is consistent with the known ratio of the a-decays. Furthermore, the range of these particles in the silicon detectors is short enough to be that of a-particles . The other low energy peaks correspond to the proton decays of the 2° Na states to i9 Ne . There is no such low energy a-decay in 20 Ne . The decay time was also measured by stopping the primary beam for a 200 ms duration just after 2° Mg detection . Fig. 5 shows the decay curves of the 1740 keV state, and those of 2' Mg in the lower part . Here, the secondary beam of 21 Mg was tuned to set up and check the whole detection system at the beginning of the experiment . The upper half of the figure shows a decay of about 110 ms, which is consistent with the known half-life of 2° Mg [7]. The lowest energy peak of

m c c as

t U N

C

O

U

Fig. 4. Delayed particle spectra obtained with the third silicon detector while the beam was off. The upper spectrum is the singles spectrum, and the bottom one is the coincidence spectrum with the beta rays detected. IX. RELATEDTECHNIQUES

S. Kubono et al. /Proton decay measurement with RIPS

586 IOz

Decay of

the latter state is that of 3.175 MeV. This 3.175 MeV state was not fed in the beta decayof 200 [9], and there is no 1= 0 strength for this state in the spectroscopic

20Mg

factor of the "F(d, p) 2° F reaction [10] . These data clearly suggest that the 2.637 MeV state has little component of (sd) 4. A shell model calculation [11] predicts an intruder 1+ state of 2h(u in this excitation energy region, which has a configuration of (p)-2(sd)6. The present data seems to indicate that the mixing of

id

the (sd) 4 configuration is very small in this state. If one takes this 2.637 MeV 1+ state to be the

analogue of the 3.175 MeV state in 2017, the gamma iCP

0

15

30

45

60

103

width may be similar to that of the analogue, which is 9.3 meV. Since Fy = 15 meV was used in the previous estimate [3,4] of the 19 Ne(p, y)2° Na reaction rate, this

rate will be reduced roughly by a factor of 2 since the rate is roughly proportional to the gamma width. Therefore, the main conclusion in the previous esti-

Decay of 21Mg

mate for the rate would not change if the assumption from the analogue state is correct. Although it seems quite difficult to determine the gamma width of the 2.637 MeV state by the present method, it is still very interesting to measure it experimentally for the study

id

of the onset mechanism of the rp process.

1000

500 TIME

(full

1500

2000

20mg 0+

range = 200 ms)

Fig. 5. Decay curves obtained for the peak of 1740 keV (top), and that forall states of 21 Mg (bottom) .

3.930 p 11

,,

(- .3) +

857 keV, which is 3.056 MeV in the excitation energy of 'Na, corresponds well to the previously known state, the 3.046 MeV (1,2,3) + state [3,4]. This could be

, 3.056 . . i; (2 .199) si "1.6-37119 Ne+p (1/2+) 0.9901+

MeV, in "Na. There is no peak observed at 438 keV for the 2,637 MeV 1 + state in the spectra. There is only the trail of the background due to the beta decay .

,-20 N a

a

s 14 <2

the second 1 + state above the proton threshold, 2.199

The upper limit of the branching ratio estimated for this state is about 2% . The experimental results of the branching ratios are summarized in fig. 6, where the branching ratios for the decay of 2° Na were obtained from ref. [8]. There are three 1+ states below 3.5 MeV in the

minor nucleus 2017, which has been well studied. The first 1+ state at 0.990 MeV corresponds to the 1.057 MeV 1+ state in 2OF, and is dominantly fed in the beta

decay as it has a configuration of (sd) 4. There is a relatively strong beta decay to the third 1+ state at 3.045 MeV, whereas almost no feeding was observed to the second 1 + state at 2.637 MeV. The former state is likely the analogue of the 3.488 MeV state in 70 F, and

0.02+ 10.274

1 al

16 0+,,

2.9

r

7.424

16

1.634

79

0.00

J

20 N e Fig. 6. The beta decay branching ratio obtained from the present experiment.

S. Kubono et al. / Proton decay measurement withMPS References [1]

[2] [3] [4] [5]

R. Wallace and S.E. Woosley, Astrophys . J. Suppl. 45 (1981) 389. K. Langanke et al ., Astrophys. J. 301 (1986) 629. S. Kubono et al., in : Heavy Ion Physics and Nuclear Astrophysical Problems, eds. S. Kubono et al. (World Scientific, 1988) p . 83 . S . Kubono et al ., Astrophys . J. 344 (1989) 460. L .O. Lamm et al ., Nucl . Phys . A510 (1990) 503.

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[6] S . Kubono et al ., Z. Phys. A334 (1989) 512 . [7] T. Kubo et al., in : Radioactive Nuclear Beams, eds . W.D. Myers et al. (World Scientific, 1989) p. 563 . [8] F. A]zenberg-Selove, Nucl. Phys. A475 (1987) 1 . [9] D .E. Alburger et al., Phys. Rev . C35 (1987) 1479 . [10] H .T. Fortune and R.R. Betts, Phys . Rev . C10 (1974) 1292. [11] J .B. McGrory and B.M . Wildenthal, Phys. Rev . C7 (1973) 974.

IX. RELATED TECHNIQUES