Astrophysics with DRAGON: The 26gAl (p,?) 27Si Reaction

1
Astrophysics with DRAGON The 26gAl
(p,?) 27Si Reaction Heather Crawforda,1 for the
DRAGON Collaborationb aSimon Fraser University,
Burnaby, B.C., Canada bTRIUMF, Vancouver, B.C.,
Canada
Abstract The 26gAl(p,?)27Si reaction is
important for nuclear astrophysics, as 26Al is
directly observable in supernovae explosions due
to its decay with a characteristic gamma. This
allows comparison of observational data with
models, the accuracy of which depends on how well
known the reaction rates for the processes
involved are. As the only direct destruction
pathway for 26Al aside from its beta decay, the
26gAl(p,?)27Si reaction is an integral part of
the 26Al system, and an accurate measure of its
rate, determined mainly by the strength of
available resonance reactions, is critical. The
strength of the 188 keV resonance is currently
being directly studied for the first time in
inverse kinematics, using the DRAGON facility at
TRIUMF. A 26Al radioactive beam incident on a
windowless H2 gas target gives rise to 27Si
recoils, which are detected through the
coincidence of a prompt gamma, and a heavy ion
signal at the end detectors. Data is being
analyzed to separate true events from background
and determine the thick target yield. Also
important is an analysis of beam intensity and
composition, using data from DRAGON detectors and
faraday cups. Results from these latter aspects
of the study will be reported on.

• Normalization of Beam
• Determination of the number of beam particles on
  target over the course of a given run involves a
  number of steps.
• A measure of absolute beam intensity was
  determined with the use of a Faraday cup 2
  metres upstream of the gas target
• Since beam intensity varies over the course of
  a two-hour run, a reliable monitor of relative
  beam intensity was chosen
• A relationship between the absolute beam
  intensity and the relative value was established
  (normalization factor)
• Monitor response was integrated over run, and
  normalization factor was used to determine the
  total beam on target
• Two different relative beam monitors within the
  DRAGON system were used for purposes of
  normalization for different groups of the 250
  runs taken.

(I) Current deposited on the left mass slit
after the first mass separation is proportional
to the number of beam particles and produces a
good relative beam intensity profile (figure 4).
The normalization constant was a simple ratio,
defined as below. This ratio was calculated
for each run and then averaged. The charge on
the mass slit directly integrated, and used to
determine the number of beam particles on target
by multiplication by the normalization factor.
The average number of beam particles on target
over a 2-hour run determined using this method
was on the order of 1012 particles.
(3)
Figure 4
Motivation 
(II) A surface barrier detector within the
gas target, located at 30 to the beam axis,
detects Rutherford elastically scattered protons,
the number of which is proportional to the number
of beam particles. Figure 5 shows a relative beam
intensity profile. The number of protons was
determined from the pulse height spectrum, which
had virtually no background in the region where
the protons appeared (figure 6). Given the
well-known dependence of Rutherford scattering on
the target pressure and beam energy, a
normalization factor independent of these
quantities was established (shown below).
The 26gAl(p,?)27Si reaction, believed to
occur in novae and supernovae exposions, involves
radiative capture on a radioactive species. This
reaction has a significant and direct impact3 on
the abundance of 26gAl, a relatively long-lived
radionuclide which is produced as a part of the
Mg-Al cycle, shown in figure 2.
26gAl undergoes positron decay to the first
excited state of 26Mg which then rapidly decays
with a characteristic 1.809 MeV gamma ray,
meaning 26gAl can be directly observed by
orbiting gamma telescopes. Direct observation of
abundances allows comparison with calculated
values from network calculations and models
attempting to describe novae and supernovae
explosions. The relevant reaction rates are
important parameters within these models, and
these rates, including that of the 26gAl(p,?)27Si
reaction, must be determined experimentally.
(4)
Figure 5
Normalization values were calculated for runs in
which the first 300s of the trigger rate spectrum
showed a relatively constant beam intensity, as
in figure 7. These values are shown in figure 8.
Average normalization factors were then
determined, and used with the total number of
scattered protons determine the beam particles on
target. On average, for a two hour run, the
number of beam particles on target determined
from this method was on the order of 1012.
Figure 2
Measurement of Resonance Strength Thick Target
Yield Stellar nuclear reaction rates are usually
dominated by a few narrow resonance reactions4.
These narrow resonances are characterized by
resonance strengths (??). Resonance strength is
experimentally determined by measuring the thick
target yield, given by the following
equation  Experimental determination of the
thick target yield requires accurate knowledge of
the number of recoils detected, the number of
beam particles incident on the target, the
efficiency of the BGO array used for detection of
gamma rays and the fraction of the recoils in the
charge state used (CSF), shown by the following
equation Recoils are detected at the DRAGON
end detectors using ?-heavy ion coincidence
detection. Preliminary data analysis suggests
that 11 recoils, or true events, were detected
during the experiment. This work focused on
determining the number of 26gAl beam particles on
target over the course of the experiment.
Figure 8
Figure 6
Figure 7
Beam Contamination
(1)
The number of beam particles
includes both the number of 26gAl nuclei as well
as the number of contaminant particles, including
26Na and 26mAl. For calculation of the
resonance strength, the number of 26gAl particles
incident on the target is required, so the number
of contaminant particles was determined and
subtracted from the number of beam particles.
26mAl was detected using a pair of
NaI detectors located on either side of a horn
situated above the mass slits where this species
was expected to be deposited. Since 26mAl decays
through positron emission, some positrons make it
into the horn and annihilate, emitting a pair of
511 keV gamma rays, which are detected in
coincidence, by the pair of NaI detectors. After
? de Broglie wavelength of projectile
e stopping power of the target material
m and M masses of the target and projectile
nuclei respectively
?? resonance strength (? is the statistical
spin factor and ? is a level width dependent term)
compensating for the charge state fraction of the
contaminant species, and detection efficiency,
the
Figure 9
(2)
contamination levels of 26mAl were determined for
each run these are plotted in figure 9.
26Na was similarly quantified using a
HPGe detector pointed at the left mass slit where
this species was expected to be deposited. Since
26Na decays with a characteristic 1.809 MeV gamma
ray, the integral of the relevant gamma peak in
the spectrum was used to determine the
contamination levels. Again, after compensating
for the charge state fraction and detection
efficiency, the contamination levels of 26Na
were determined run-by-run these values are
plotted in figure 10.
Figure 10

• The Experiment
• Intense radioactive 26Al beam is incident on 6
  Torr H2 target with 202 keV/u of energy
• Particles pass through target reaching resonance
  energy (188 keV in center of mass frame) near
  middle of target -- some react to produce 27Si
  recoils, most pass straight through
• Particles that react emit a number of gamma rays
  (gamma ray cascade) which are detected by the BGO
  array surrounding the target (gamma signal)
• Recoils emerge with same momentum as beam, with
  a small angular spread (? 15 mrad), and move
  through the separator to isolate 4 charge state
  27Si recoils, which are detected at the end
  detector (heavy-ion signal)

Summary The 26gAl(p,?)27Si reaction is still
ongoing at DRAGON, with a continuation of the
experiment scheduled for this October to achieve
improved statistics. However, the work completed
this past summer has contributed to establishing
a more standard method of beam normalization,
which can be used for the upcoming and ideally
other future experiments. In fact, this work
produced two normalization methods which
validated one another, producing values in
agreement within 8 for over 150 experimental
runs. Methods for monitoring contamination
levels have also been considered and incorporated
into normalization to determine the desired value
for the calculation of resonance strength the
number of 26gAl particles on target.
Acknowledgements I would like to thank the entire
DRAGON group, particularly Dr. Chris Ruiz, Anuj
Parikh, and Dr. Dave Hutcheon for their help and
guidance in this work. I would also like to
thank Dr. John DAuria for allowing me the
opportunity to work with the DRAGON group this
past summer and gain invaluable experience.
Figure 3
1Authors Email hcrawfor_at_sfu.ca 2D.
Hutcheon et al., Nuclear Instruments and Methods
in Physics Research A 498, 190 (2003).
3Ruiz, C, E989 Astrophysical studies using 26Al
ground-state and isomer beams, TRIUMF research
proposal (internal), (2004). 4C. Rolfs and W.
Rodney, Cauldrons in the Cosmos (University of
Chicago Press, 1988).