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Resonance States in 22Mg, 26Si for Reaction Rates in the RP-Process Y. Shimizu

Resonance States in 22Mg, 26Si for Reaction Rates in the RP-Process Y. Shimizu

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368

cesses during these explosive events by the measurement of reaction rates

and structure of unstable, proton rich nuclei4.

Even though new facilities are now able to directly measure relevant

reaction rates by using radioactive beams and reversed kinematics, measurement.s will also benefit from level structure information with excitation

energies in t,he resonance regions of interest. In order to study the precise level structme of relevant nuclei: a good energy resolut,ion is required.

The advantage of t.he (4He,6He) react.ion is its possibility to achieve high

resolution.



Figure 1. Detailed configuration of new Faraday cups installed inside, near the exit of

the first dipole magnet of the Grand Raiden spectrometer.



2. Experiment



The measurements were performed at the Research Center for Nuclear

Physics (RCNP): Osaka University by using a 205 MeV *He beam from

the K = 400 RCNP ring cyclotron and the Grand Raiden spectrometer5

placed at, 0". The extracted beam from t,he ring cyclotron was transported

to the West Experimental Hall via the WS beaniline6. In order to stop

and integrate the current of the beam, a new Faraday cups were inst.alled

inside, near the exit of the first dipole magnet of the Grand Raiden spectrometer (see Figure 1). The reaction product,s continue to travel through

the spectrometer and are measured in a focal plane detector syst,eni, consisting of two vertical drift chambers (VDC) that allow the determination



369



of the horizontal and verbical positions and angles in the focal plane. Three

plastic scintillators, 1 mn,10 nini, and 10 inni thick, mounted behind the

VDC system allowed particle identification, time of flight measuremnent.s,

and light. particle rejection. Typical beam particle currents were 200 enA.

For best vertical scattering angle definition the spectroineter was used in

overfocus mode which allows reconstruction of the vertical component of

the scattering angle from the measured vertical position in t.he focal plane.

For good resolution in momentum and horizontal scattering angle c.oiiiponent, full dispersion matching7 was applied. While these are newly developed standard proc.edures, t,he energy spread in the 0.7 nig/cni2 target,

the large Q-value, and 1iniitat.ionsin the angle definition have limited the

best. resolut.ion to 60 keV so far.

50



40

30



%



20

10

50



$



40

30



.-I



20



fi

9



(a) ?dg



target



6



300



"



'OC

.



0.0



2.5



3



5.0



7.5



Excitation energy in "Mg



10.0



12.5



[MeV]



Figure 2. Measured 22Mg>loC; and Mylar spectrum with angle cut 0 O - lo and resolution of 60 keV



3. Data Analysis

In all of the runs we observed 6He and other light particles in the detector.

The energy lost in the scintillator, and the time of flight informabion built

by the scintillator and the rf signal of the AVF cyclotron were used for

particle identification. The horizontal position and angle in focal plane were



370

also used to correct, time of flight information. Since most of background

particles didn't come from the target, two-diniensional gates in the time of

flight signal and t'he horizontal position or angle in focal plane allowed us

to separate each part.icle group clearly. The final 6He spect.runi is shown in

Figure 2(a).

Thin self-supporting enriched 24h1g and 28Si foils with a thickness of

0.7 iiig/cm2 were used as the target. Target contaminations of I2C and

l6O were observed. In order to subtract contaiiiinant yields? mylar and

carbon target were used. Figure 2 shows the 221vlg>I2C, and Mylar spectra

measured at t.he same spectrometer sett.ing. The l4O yields was obt.ained

by subtracting "C from the (4He: 'He) spectruni on the Mylar target. The

background was estimated by normalizing

and l0C ground state yields.



-500



-250



0

'focal



Figure 3.



plane



I



4



250



500



[mml



Energy calibration in whole the focal plane; fit (above), and residuals (below).



p is calculated from kinematics and magnetic field.



In order to calibrate the particle momentum, all well separated known

low-lying levels8 in 22Mg labeled g s . and 1-9 were used at two magnetic

field settings to calibrate whole the focal plane. We observed twenty isolated levels in the focal plane. Figure 3(a) shows the relation between the

focal pale position and the bending radius p which was calculated from kinematics and magnetic field. The calibration is linear with a small quadratic



371



term. Energy of the particles was calculated by bhis relation. Alniost all of

these states have excitation energies that are known to better than 10 keV

(see Figure 3(b)). The standard deviation of t,he calibration is 9 keV.



4. Excitation energies in 22Mg

Figure 4 shows t.he measured energy spectra of t,he 2'Mg.(4He:6He)221\,Ig

reaction at several scattered angles around a-threshold. The resolution of

ineasurenient was 56,64, and 80 keV at 0"-1", 1"-2", and 2"-3", respect.ively.

Our measured 22Mg excitation energies above G MeV are suinmarized in

Table 1 and compared to results of the previous ~neasurement~.

Errors

shown in Table 1 resulted from uncert.ainties in t.he peak fitting procedure.

For most. levels our results agree wit.h previously report'ed excitat,ion energies. Energy levels above 8.14 MeV are relevant for rat.e calculations of the

" N e ( ~ , p ) ~ ' N areaction which controls the break-out of the hot. CNO cycles

above 0.8 GK.



200



0

200



0

100



n

6



7



8



Excitation energy in ''Mg



9



10



[MeV]



Figure 4. Excitation energy of 22Mg spectrum at several scattered angles. The dashed

line shows a-threshold (Ex= 8.14 MeV).



372

Table 1. Energy levels of 22Mg7in units of keV

this work



Ex9



this work



Ex9



6046f 4



6041 f 11



8384f 8



8396 % 15



6246f 7



6255 f 10



8521f 5



8 5 4 7 f 18



10165f 4



10090 f 29



6605% 3



6606 f 11



8685f 6



8613 f 20



10262f 3



10297 f 25



6774f 2



6767 f 20



8791f 5



8754f 15



10404f 7



10429 f 26



6876% 6



6889 f 10



8947f 4



8925% 19



10609f 5



10570 f 25



9 0 6 6 f 18



10711f 4



7070% 8

2



7169 f 11



7604f 7



7674 f 11



7224f



9154f 3



10078 f 24



( 9172 f 23 )

( 92485 20 )



7402 5 13



this work



9329



10660 f 31

10750 f31



* 18



10872f 4



10844 f 38



11003f 5



10980 i31

11135



7784 f 18



9 5 2 6 f 10



9533 f 24



11125f 5



7931% 5



7964 f 16



9748f 6



9712 f 21



11236f 5



8006*



8



8062 f 16



9861f 6



9827 f 44



11347f 5



8191f 8



8203 f 23



9972f 6



9924 rir 28



11540* 5



40



5. Excitation energies in 26Si



Figure 5 shows the measured energy spect.ra of the 28Si(4He,6He)26Si

reaction at several scatt.ered angles around pthreshold. The resolution of measurement. was GO, 64, and 64 keV at 0"-1", 1"-2": and 2"-3", respectively.

Our measured 26Siexcitation energies are summarized in Table 2 and compared to results of the previous nieasurement". Errors shown in Table 2

resulted from uncertainties in the peak fitting procedure. For most levels

our results agree with previously reported excitation energies. However, we

cannot see the st,ate at 5.515 MeV reported by ( p , t ) measurements". This

suggests that the st,atehas an unnatural parity, because (4He,6He)reaction

cannot excite a state with unnatural parity, but ( p , t ) reaction can excite

it. Energy levels above 5.518 MeV are relevant to t.he product'ion rate of

26A1 in its ground stat.e which provides a valuable constraint on models

used to understand the explosive hydrogen burning process in novae and

supernovae.



6. Summary and Conclusion

Excitation energies of 22Mgand 26Siwere measured by (4He, 6He) reaction

on 24Mgand "Si targets, respectively, at an incident energy of E, = 205

MeV. Excibation energies were deternlined up to 12.5 MeV. The lateral



373



0

m

"

m



.-



-



5



4



-1



7



6



8



Excitation energy in "Si [MeV]

Figure 5. Excitation energy of "Si spectrum at several scattered angles. The dashed

line shows pthreshold (Ex= 5.518 MeV).



Table 2.



Energy levels of 26Si, in units of keV



ExlO



this work



1793f 3



1795.9



5892f 4



59165 2



8570f 4



2785f



6292f 8



6300f 4



8806% 5



6380f 4



8952f 7



this work



2



2783.5



3337% 3



3332.5



41385 11



4138f 1



3756f 2

4211 f 16



68105 8



7199f 6



E,10



this work



G787f 4



9067f 5



7019 f 10



9247+ 8



7160% 5



9374f 7



4447*



4



4445f 3



7447i 2



7498*



4819*



3



4805+ 2



7705*



7687



5151*



4



5145f 2



7921h 3



5312*



2



5291f 3



8282i 6



10070f 8



5515f 5



8431f 6



10294f 7



3



4



ExlO



96072~ 9



* 22



9802f 7



7900 5 22



9917f 2



and angular dispersion matching was applied to the beam optics. The

spect,ronieter was used in the overfocus mode. For 'Mg excit.ation energies,

we observed new levels around a-threshold. Several of these levels are



374

important for radiative proton capture reactions on 21Na occurring in a

classic nova stellar explosion. For 26Siexcitation energies, we observed new

levels around the a-threshold. We cannot see the state at, 5.515 MeV which

reported by the previous ( p , t ) measurement''. Our result, suggest.s that.

this state has an unnatural parity.

In future works, the spin and parity will be assigned to the nieasured

levels which are necessary by t.he Network calculation. We have nieasured

angular distributions of these levels. They will be compared with a DWBA

calculat.ion. Now Network calculation based on our results is in progress.



7. Acknowledgments

We thank the RCNP staff for their support during the experiment. ?Ve

also wish to thank Professor H. Toki for his encouragenients throughout

the work. This experiment was performed under Program No. El63 and

El87 at, the RCNP.

References

1. S. Starrfield et al., Ap. J . Suppl. 127,(2000) 485.

2. R.K. Wallace and S.E. Woosley, Ap. J. Suppl. 45>(1981) 389.

3. H. Schatz, L. Bildsten, A. Cumming, and WI. Wiescher, Ap. J. 524, (1999)

1014.

4. H. Schatz, J. Gorres, H. Herndl, N.I. Kaloskamis, E. Stech, P. Tischhauser,

M . Wischer, A. Bacher, G.P.A. Berg, T.C. Black, S Choi, C.C. Foster,

K. Jiang, and E.J. Stephenson, Phys. Rev. Let. 79, (1997) 3845.

5. M. F'ujiwara, H. Akimune, I. daito, H. Fujimura, Y. Fujita, K. Hatanaka,

H. Ikegami, I. Katayama: K. Nagayama, N. Matsuoka, S. Morinobu, T. Noro,

&I. Yoshimura, H. Sakaguchi, Y. Sakemi, A. Tamii, and M. Yosoi, Nucl.

Instrum. h4eshodos Phys. Res. A 422, 484 (1999).

6. T. Wakasa, K. Hatanaka: Y. Fujita, G.P.A. Berg, H. Fujimura, H. Fujita,

&I. Itoh, J. Kamiya, T. Kawabata, K. Nagayania, T. Noro, H. Sakaguchi,

Y. Shimbaxa, H. Takeda, K. Tamura, H. Ueno, M. Uchida, M. Uraki, and

pvl. Yosoi, Nucl. Instrum. Methods Phys. Res. A 482;79 (2002).

7. Y. Fujita, K. Hatanaka, G.P.A. Berg, K. Hosono, N. hfatsuoka, S. Morinobu,

T. Noro, M. Sato, K. Tamura, and H. Ueno, Nucl. Instrum. Methods Phys.

Res. B 126: 274 (1997).

8. P.M. Endt, Nucl. Phys.; A521, (1990) 1.

9. A.A. Chen, R. Lewis, K.B. Swartz, D.W. Visser, and P.D. Parker, Phys. Rev.

C 63, 065807 (2001).

10. D.W. Bardayan, J.C. Blackmon, A.E. Champagne, A.K. Dummer, T. Davinson, U. Greife, D. Hill, C.Iliadis, B.A. Johnson, R.L. Kozub, C.S. Lee,

M.S. Smith, and P.J. Woods, Phys. Rev. C 6 5 : 032801 (2002).



THE 21Na(p,y)22MgREACTION FROM E C M = 200 TO

850 KEV IN EXPLOSIVE STELLAR EVENTS



s. BISHOP:



J. M. D’AURIA, A. C H E N ~D. HUNTER? M. LAMEY, w . LIU

AND c. W R E D E ~

Simon Fraser Uni.ue,rsity, Burnaby, B.ritish Columbia, Canada



R. E. AZUMA AND J. D. KING

University of Toronto, Toronto, Canada

L. BUCHMANN, D. HUTCHEON, A. M. L A I R D ~ A .OLIN, D. OTTEWELL

AND J. ROGERS

TRIUMF, Vancouver, B&ish Columbia, Canada



M. L. CHATTEWEE

Saha Institute of Nuclear Physics, Calcutta, India

S. ENGEL

Ruhr- Uninersitat, Bochum, Germany

D. GIGLIOTTI AND A. HUSSEIN

Uniuersity of Northern British Columbia, Prince Georye, B.ritish Cohmbia,

Canada



U. GREIFE AND C. C. J E W E T T

Colorado School of Mines, Golden, Colomdo, USA

3. JOSE

Institut d’Estudis Espacials de Catalunya, CSIC/UPC, Barcelona, Spain



S. KUBONO AND S. MICHIMASA

University of Tokyo, Tokyo, Japan

R. LEWIS AND P. PARKER

Yale University, New Haven, CT, USA



375



376

The long-lived radioactive nuclide 22Na

= 2.6 y) is, in principle, an astrcnomical observable for understanding the physics processes of oxygen-neon novae.

Production and abundance yields of 22Na in these events are dependent t o the

hitherto unknown rate of the 21Na(p,y)22Mgreaction. Using a high intensity radioactive 21Na beam at the TRIUMF-ISAC facility, direct measurements of the

strengths of six potentially astrophysicdy important resonances have been made

at center of mass energies in the range: Ecm = 200 to 850 keV. Reported herein are

preliminary results obtained for these strengths and their respective contributions

to the 21Na(p,y)22Mgstellar reaction rate.



1. Introduction

Explosive steller events, such as classical novae and X-ray bursts, are two

astrophysical sites wherein the “burning” of heavy elements can proceed

by way of successive proton capture on radioactive nuclei. The high tenlperatures and densities within the burning zones of these events are such

that radiative proton capture rates can exceed the competing beta decay

rates of the reactant radioactive nuclei. Novae are presently understood to

be the result of a thermonuclear runaway (TNR) across the surface of a

white dwarf star within a binary star system. The densities and temperatures within the TNR allow proton capture on the seed nuclei comprising

the white dwarf surface, resulting in the production of intermediate mass

nuclides that are ejected into the interstellar medium (ISM). The mechanism for an X-ray burst event is considered to be essentially that of a novae

event, with the important distinction that the underlying progenitor of the

explosion is a neutron star. However, material is not ejected into the ISM

due to the high escape velocity of neutron stars.

Nova temperatures and densities are such that, given Coulonib barrier

constraints, the proton capture reaction flow predominantly occurs along

the periphery of the proton-rich side of the valley of stability. With a

neutron star as the underlying progenitor for X-ray bursts, burning zone

temperatures and densities can be at least an order of magnitude greater

than in novae, resulting in a reaction flow occurs further removed from

the valley of stability, even merging with the proton drip-line beyond A =

38

Figure 1 shows our present understanding of the level scheme

li2l3.



*Present address: Heavy Ion Nuclear Physics Laboratory, RIKEN, Wako, Saitama,

Japan, bishop@rarfaxp.riken.jp

+Present address: McMaster University, Hamilton, Ontario, Canada

Present address: Langara College, Vancouver, British Columbia, Canada

§present address: Yale University, New Haven, CT. USA

TPresent address: University of York, York, England



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