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Resonance States in 22Mg, 26Si for Reaction Rates in the RP-Process Y. Shimizu
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
Figure 1. Detailed configuration of new Faraday cups installed inside, near the exit of
the first dipole magnet of the Grand Raiden spectrometer.
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
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.
Excitation energy in "Mg
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
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
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.
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
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~.
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.
Excitation energy in ''Mg
Figure 4. Excitation energy of 22Mg spectrum at several scattered angles. The dashed
line shows a-threshold (Ex= 8.14 MeV).
Table 1. Energy levels of 22Mg7in units of keV
6041 f 11
8396 % 15
6255 f 10
8 5 4 7 f 18
10090 f 29
6606 f 11
8613 f 20
10297 f 25
6767 f 20
10429 f 26
6889 f 10
10570 f 25
9 0 6 6 f 18
7169 f 11
7674 f 11
10078 f 24
( 9172 f 23 )
( 92485 20 )
7402 5 13
10660 f 31
10844 f 38
7784 f 18
9 5 2 6 f 10
9533 f 24
7964 f 16
9712 f 21
8062 f 16
9827 f 44
8203 f 23
9924 rir 28
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
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
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).
Energy levels of 26Si, in units of keV
4211 f 16
7019 f 10
7900 5 22
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
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.
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.
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THE 21Na(p,y)22MgREACTION FROM E C M = 200 TO
850 KEV IN EXPLOSIVE STELLAR EVENTS
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
Ruhr- Uninersitat, Bochum, Germany
D. GIGLIOTTI AND A. HUSSEIN
Uniuersity of Northern British Columbia, Prince Georye, B.ritish Cohmbia,
U. GREIFE AND C. C. J E W E T T
Colorado School of Mines, Golden, Colomdo, USA
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
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.
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 =
Figure 1 shows our present understanding of the level scheme
*Present address: Heavy Ion Nuclear Physics Laboratory, RIKEN, Wako, Saitama,
+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