MEASUREMENT OF THE 21Na(p,γγ)22Mg REACTION
WITH THE DRAGON FACILITY AT TRIUMF-ISAC
A.A. Chen a, S. Bishop b, L. Buchmann c, M.L. Chatterjee c,k, J.M. D’Auria b, S.
Engel d, D. Gigliotti e, U. Greife f, D. Hunter b, A. Hussein e, D.A. Hutcheon c, C.
Jewett f, J. King g, S. Kubono h, A. Laird c, M. Lamey b, R. Lewis i, W. Liu b, S.
Michimasa h, A. Olin c,l, D. Ottewell c, P.D. Parker i, J. Rogers c, F. Strieder d, M.
Wiescher j, C. Wrede b
a
Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1, Canada
b
Department of Physics, Simon Fraser University, Burnaby, BC V5A 186, Canada
c
TRIUMF, Vancouver, BC V6T 2A3, Canada
d
Institut für Experimentalphysik III, Ruhr-Universität, Bochum, Germany
e
Department of Physics, University of Northern British Columbia, Prince George, BC V2N 4Z9, Canada
f
Department of Physics, Colorado School of Mines, Golden, CO 80401, USA
g
Department of Physics, University of Toronto, Toronto, ON M5S 1A7, Canada
h
Center for Nuclear Study, University of Tokyo, RIKEN Campus, Hirosawa 2-1, Wako, Saitama 31-0198, Japan
i
A.W. Wright Nuclear Structure Laboratory, Yale University, New Haven, CT 06511, USA
j
Department of Physics, University of Notre Dame, South Bend, IN 46556, USA
k
Saha Institute of Nuclear Physics, Calcutta 700 064, India
l
Department of Physics, University of Victoria, Victoria, BC V8W 3P6, Canada
Abstract. The DRAGON recoil separator facility, designed to measure the rates of radiative proton and
alpha capture reactions important for nuclear astrophysics, is now operational at the TRIUMF-ISAC
radioactive beam facility in Vancouver, Canada. We report on first measurements of the 21Na(p,γ)22Mg
reaction rate with radioactive beams of 21Na.
21
INTRODUCTION
Explosive hydrogen burning plays a key role in the
nucleosynthesis and energy generation of novae,
where the outburst happens as a result of accretion
onto the surface of a white dwarf star from a
companion star. Radiative proton capture reactions on
pre-existing seed nuclei result in the formation of
cycles, such as the HotCNO cycle or the Neon-Sodium
(NeNa) cycle, with the latter potentially dominating
the nuclear burning in novae whose ejecta are rich in
neon, sodium, magnesium, aluminum, and sulphur
[1,2]. The timescale of the NeNa cycle at nova
temperatures is limited by the beta-decays of the
longest-lived isotopes, the most important of which is
Na with a half-life of 22 seconds. However, the rate
of the 21Na(p,γ)22Mg reaction will determine the
effective lifetime of 21Na in the outburst environment,
thereby affecting the rate of energy generation and
subsequent nucleosynthesis. In addition, since further
proton capture on 22Mg is hindered due to the high
photodisintegration rate of 23Al, most of the 22Mg
produced in the explosion will decay to 22Na. The rate
of the 21Na(p,γ)22Mg reaction therefore determines
how much 22Na is produced in the outburst. This
particular isotope is important since its daughter, 22Ne,
has been found in pre-solar meteoritic grains [3], and
also because it will serve as a target for gamma-ray
telescopes. The production of 22Na is a therefore a
constraint on models of explosive nucleosynthesis in
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
237
the Galaxy, both now and when the solar system was
formed.
In nova environments, the 21Na(p,γ)22Mg reaction
rate is dominated by narrow resonances in 22Mg with
energies up to 600 keV in the center of mass system.
Recent studies have helped clarify significantly the
structure of 22Mg [4], although full spectroscopic
information on the important resonances is not yet
available, and consequently the 21Na(p,γ)22Mg reaction
rate is poorly known [5]. With the advent of
radioactive beam facilities, the rates of reactions like
21
Na(p,γ)22Mg can be determined through direct
measurements of the important resonance strengths
[6]. In the following, we report on first measurements
of the 21Na(p,γ)22Mg reaction using the DRAGON
recoil separator at the TRIUMF-ISAC radioactive
beam facility.
dispersive elements of the electromagnetic separator
are two magnetic dipoles (MD) and two electric
dipoles (ED), configured into two consecutive stages
of mass separation. The first section selects one
charge state of the recoil nuclei and achieves a large
part of the beam rejection, while the second section
serves to remove most of the remaining beam ions.
Additional quadrupole and sextupole elements, and
steering devices are used for efficient transport of the
recoils through the separator. The recoils and “leaky”
beam ions are then detected in a double-sided silicon
strip detector of 5 × 5 cm2, which measures their
energies, and horizontal and vertical positions. A
more detailed discussion of the DRAGON facility and
commissioning will be published elsewhere [7].
THE DRAGON FACILITY AT ISAC
The ISAC facility has been designed to produce
beams of unstable isotopes for use in a wide program
of basic and applied science. For nuclear astrophysics
studies, ISAC delivers isotopically pure radioactive
beams of energies between 0.15 and 1.5 MeV/u to
either a recoil separator for radiative capture studies,
or a general-purpose scattering chamber for charged
particle reaction studies. The former, called DRAGON
(Detector of Recoils And Gammas Of Nuclear
reactions), has been specifically designed for (p,γ) and
(α,γ) reactions in inverse kinematics.
The DRAGON facility consists of a differentially
pumped, windowless gas target surrounded by an array
of gamma-ray detectors, followed by an
electromagnetic recoil separator and a recoil detection
station (Figure 1). The gas pressure in the target cell is
regulated to be typically 4.5 Torr, and the gas density
is uniform over the 11cm length of the cell. The cell
contains a solid state detector used for beam
normalization through the detection of elastically
scattered target ions. The pressure downstream of the
target is reduced to about 10-6 Torr by a second set of
differentially pumped tubes. Gamma-rays from the
de-excitation of the recoil nucleus is detected in an
array of 30 BGO (Bismuth Germanate) crystals, which
are placed in close geometry around the target. The
typical energy resolution for 6.13 MeV gammas is
about 7% (FWHM).
The recoil particles, along with the beam ions,
emerge from the target within a cone of ±20 mrad or
less, within the acceptance of the separator. The main
238
FIGURE 1. A schematic representation of the DRAGON
recoil mass spectrometer, with simulated ion trajectories.
EXPERIMENT
The 21Na beam was produced at the ISAC facility
by bombarding a thick target of SiC pellets with a 500
MeV proton beam from the TRIUMF cyclotron.
Following extraction and ionization, the 21Na ions
were accelerated through an RFQ-DTL system to the
required energies. The final pulsed beam had
intensities of 108 particles per second on target.
In order to test the DRAGON performance with
radioactive beams, initial measurements were
performed by measuring the 21Na(p,γ)22Mg reaction
through a resonance at Ecm = 822 keV. This state had
been observed as a strong broad state in a recent
measurement of 21Na+p elastic scattering [8], and
therefore provided a good source of (p,γ) events for
calibrating the DRAGON facility. An excitation
function for this broad resonance has been measured
with beam energies between 800 - 900 keV/u, and the
data analysis is presently in progress. At these
energies, when recoils are detected in singles mode in
the double-sided silicon strip detector (DSSSD), a
clean separation in energy between the recoils and
beam ions is observed.
Following
these test
measurements, the
22
Na(p,γ) Mg reaction was measured at the energies
of interest to nova nucleosynthesis. Previous
calculations of the reaction rate indicate that the
dominant 22Mg resonance at nova temperatures is
located at Ecm = 212 keV, and therefore our first
measurements targeted this resonance. Measurements
were performed with the 21Na beam at energies
between 210 – 230 keV/u. Since the resonance is
much narrower than the energy width of the target, the
plateau of the curve corresponds to the thick-target
yield of the reaction through this resonance. Figure 2
illustrates the DRAGON performance with data taken
under thick-target yield conditions. The upper left
panel shows a DSSSD singles energy spectrum, where
the peak corresponding to beam ions is dominantly
visible. Comparing this spectrum to the one in the
lower left panel, where the events displayed are the
ones that satisfy a gamma-recoil coincidence
requirement, the recoils’ peak is now evident. To
confirm that this peak is well separated from random
coincidences between recoils and gammas from
positron decay of the beam in the target, the spectrum
also shows events taken with an off-time window 10
times longer than that of the true events.
the elastic scattering monitor located in the gas target.
In addition, a beta monitor located near the slits after
the first electric dipole, where most of the beam is
dumped, provides an alternative method for beam
normalization. For both the Ecm = 822 keV and 212
keV resonances, the charge state distribution of the
emerging recoils was measured.
21
The panels on the right side display DSSSD spectra
of horizontal position, as measured by the strip
segmentation of the detector (each strip is 3mm wide).
The upper right panel corresponds to the singles data,
while the lower one shows the coincidence data.
These spectra illustrates that all the recoils of interest
are well contained within the horizontal range of the
detector. The same is true of the vertical range.
In order to determine the absolute cross section, the
beam current was normalized to measurements of the
beam on target in a Faraday cup upstream of the target,
before and after each run. Fluctuations in the beam
intensity were measured as changes in the count rate of
239
FIGURE 2. Double-sided silicon strip detector spectra from
the measurements of the Ecm = 212 keV resonance at a beam
energy of 220 keV/u: (a) Singles energy spectrum; (b)
Singles horizontal position spectrum; (c) Energy spectrum
for recoil-gamma coincidence events; (d) Coincidence
horizontal position spectrum. In (c) and (d), the crosshatched events correspond to random coincidences in an offtime window.
RESULTS
As already mentioned, the yield curve for the Ecm =
822 keV data is presently under analysis. We expect
to be able to extract a value for the resonance strength
for this state.
A preliminary thick-target yield curve for the Ecm =
212 keV resonance is shown in Figure 3. The yield is
plotted as the number of observed 22Mg recoils per
1012 beam ions, and includes corrections for gamma
detection efficiency and the selected charge state
fraction of the recoil. Two different measurements
were taken at the same energy as a consistency check.
Additionally, a measurement was taken with a
different gas target pressure, also to verify consistency.
new rate on nova nucleosynthesis and energy
generation will then be investigated.
CONCLUSIONS
First measurements with radioactive beams using
the DRAGON facility at ISAC have been performed.
The 21Na(p,γ)22Mg reaction, an important link in the
NeNa cycles in novae, has been measured through the
resonances at Ecm = 212 keV and 822 keV. Yield
curves for both resonances were determined. Data
taken on other resonances are presently being
analyzed. Future studies at DRAGON will cover other
reactions in explosive hydrogen burning. In particular,
the next planned experiments are direct studies of the
19
Ne(p,γ)20Na and the 13N(p,γ)14O reactions, important
in the breakout from the Hot-CNO cycles and CNO
cycle, respectively.
FIGURE 3. Yield curve for the Ecm = 212 keV resonance
(preliminary). The hatched band shows one standard
deviation about the mean of the measured thick target yield.
The extreme points show our limits for off-resonance
background or direct capture yields.
A typical data point in the plateau region consists
of the yield summed over several runs, with each run
requiring about 9 hours of beam on target. The
vertical error bars for each point are determined by
combining the 1σ confidence intervals for the runs that
comprise the data point. The mean yield was
calculated from a maximum likelihood analysis of the
data points between Ecm = 204 – 210 keV, assuming
Poisson statistics for the observed yield in each run
comprising a given data point. The error on the mean
yield was determined by combining the 1σ error of the
likelihood analysis with the statistical error in the
beam current. The analysis is still in progress.
The mean thick target yield was used to calculate
the resonance strength for this state, and a preliminary
value of ωγ = (1.08 ± 0.14) meV has been determined.
From the low energy rise of the yield curve, the
resonance energy has been determined to be Ecm ≈
204.5 keV, i.e., lower than the currently accepted
value [4] by over 7 keV. Efforts toward confirming
these results are in progress. Further measurements on
other resonances of interest at higher energies (Ecm =
336, 460 and 541 keV) have been performed or are
presently also in progress. In particular, a significant
yield was observed for beam energies around 560
keV/u, which could correspond to the known state at
Ecm = 541 keV. Once the measurements and analysis
are completed, the 21Na(p,γ)22Mg reaction rate at nova
temperatures will be calculated. The impact of the
240
ACKNOWLEDGMENTS
Thanks are due to the TRIUMF and ISAC staff for
producing and delivering the beams, and to all
colleagues who helped in the design and construction
of the DRAGON facility. This work was supported by
NSERC (Canada).
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