Jun Chen

1
Study of Astrophysically important states in 26Si
with 27Si(p, d)26Si and p(25Al, p)25Al
Jun Chen Department of Physics and Astronomy,
McMaster University, Canada For the McMaster-NSCL
and McMaster-CNS collaborations
MOTIVATION
EXPERIMENT TWO 25Alp elastic scattering with
CRIB
Set-up

• 25Al beam made by the in-flight method using
  24Mg(d,n)25Al reation
• 5105 pps 25Al _at_ 3.5 MeV/u on target
• Thick CH2 targets (6.58 mg/cm2)?
• Scanned 3.4 MeV in centre-of-mass energy
  (energy level up to 8.9 MeV)
• Maximum proton energy 13 MeV
• 10.48 mg/cm2 Carbon target used for background
  check

• Galactic 26Al important probe for interstellar
  medium, characterized by the emission of 1.8 MeV
  gamma rays from the decay of 26gAl (Fig.1)?

Fig.1 1.8 MeV Gamma-ray emission from the ground
state of 26Al

• 25Al(p,?)26Si bypasses the production of 26gAl
  in nova explosions (red region in Fig.3) since
  26Si decays through 26mAl without the emission of
  1.8 MeV gamma rays (Fig.1 Fig.2)?

Fig.7 Schematic of the CRIB spectrometer at CNS
Fig.2 Reaction paths for 26Al production in nova
explosion

• Forward scattering favored in inverse kinematics
• 3 ?E-E telescopes for recoil PID and tracking
  (Fig.9 Fig.10)
• 2 PPACs for beam tracking
• NaI detectors used above the target for
  detecting inelastic events

• At even higher temperatures (grey region in
  Fig.3) in supernova explosions, 26mAl will be
  excited to the higher levels by thermal
  excitation and then quickly gamma-decay to the
  ground state
• States in 26Si in these Gamow windows need to be
  well understood in order to reduce the remaining
  uncertainty in the reaction rate (two
  experiments)

Fig.8 Configuration of the silicon detectors and
PPACs in F3 chamber
Fig.3 Astrophysically important energy levels
in 26Si and its mirror nucleus. In the Gamow
window at nova temperature (red region), states
are uncertain in resonant energies and
spin-parity assignments.
EXPERIMENT ONE 27Si(p,d)26Si _at_ NSCL
Set-up

• Primary beam 150 MeV/u 36Ar
• Secondary beam 89 MeV/u 27Si
• Intensity of 1 107 pps with purity of about 36
  
• Target 250 mg/cm2 CH2 foil
• Detectors segmented germanium detector array
  (SeGA) detecting the gamma decay from 26Si in
  coincidence with the recoil at the focal plane
  (Fig.4)

Fig.10 PID for low energy protons stopped in the
?E detector
Fig.9 PID for high energy protons penetrating
the ?E detector
Results
89 MeV/nucleon radioactive 27Si beam
Fig.4 Schematic of the spectrometer NSCL
Fig.12 A sample preliminary R-Matrix fit for
resonance at Er 2.186 MeV, Jp 3-
Fig.11 Excitation function in the centre-of-mass
frame

• 2 Ge detector arrays at 37o and 90o around the
  target position (Fig.5)
• 32 segments for each detector providing accurate
  3D position for Doppler broadening correction of
  the measured gamma rays

The final excitation function was obtained by
subtracting the carbon background in the CH2
target as well as correcting for the energy loss
of protons in the target which was simulated by
using SRIM calculations. The R-Matrix fit is for
single channel only and does not include the
inelastic scattering.
Ongoing Analysis

• Extract energies, widths, and spin-parities from
  R-Matrix fits for all found resonances
• Calculate the new reaction rate

Fig.5 Front view of the SeGA germanium detector
arrays at NSCL
Results
1796 keV
993 keV
SUMMARY
1796 keV
993 keV
1537 keV
1400 keV
Two different experiments have been performed for
the measurements of the astrophysical important
states in 26Si with the aim to reduce the
remaining uncertainty in the 25Al(p,?)26Si
reaction rate at nova temperatures. The final
analysis for extracting the physical parameters
is in progress.
2647 keV
2647 keV
849 keV
2355 keV
1987 keV
Fig.6 Doppler corrected gamma-ray spectrum
Fig.7 A sample ?-? coincidence spectrum for 1399
keV gamma ray
Ongoing Analysis
References

• Extract cascades from the coincidence analysis
• Determine the energies and widths for found
  levels
• Make spin-parity assignments
• Calculate the new reaction rate

6 A. H. Hernandez et al., NMB 143 (1998)
569-574 7 S. Kubono et al., Eur. Physi. J. A
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Stopping and Ranges of Ions in Matter, vols. 3
and 5, Pergamon Press, Oxford, 1980. 9 A. M.
Lane, and R. G. Thomas, Rev. Mod. Phys. 30 (1958)
257
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