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Enter The DRAGON
Investigating the
13C(p,γ)14N
Aaron M. Bebington
reaction
a, 1
(and the DRAGON Collaboration b)
aUniversity
Of Surrey, Guildford, Surrey, England
bTRI-University
Meson Facility (TRIUMF), Vancouver, BC, Canada
Abstract
The 13N(p,γ)14O reaction is very important for our understanding of explosive astrophysical sites, such as novae and supernovae. This reaction determines the
conditions under which the CNO cycle changes to the Hot CNO cycle. If temperatures are hot enough, 13N will capture a proton before it has chance to beta
decay, forming 14O which initiates the HCNO cycle. The beta decay of 14O (t1/2 = 70.6secs) is much quicker than the beta decay of 13N (t1/2 = 9.97mins), which
means that the HCNO cycle produces energy much faster. The DRAGON collaboration at TRIUMF plans to measure the cross-section of the 13N(p,γ)14O
reaction at energies around the Gamow window, relevant to novae temperatures. This region of energy is lower than the resonance peak energy, which has
been measured previously. As 13N is radioactive, and is very close in mass to 13C (a difference of 0.002383 amu), a pure 13N beam is difficult to produce,
because 13C will contaminate the beam. Initially we studied the 13C(p,γ)14N reaction so that its contribution could be compensated for when studying the
13N(p,γ)14O reaction. The 13C(p,γ)14N reaction was used to probe the DRAGON not only because it has similar properties, but because 13C(p,γ)14N
measurements have been made before by King et al2.
Data Analysis from the 13C(p,γ)14N reaction
DRAGON
The DRAGON (Detector of Recoils And Gammas Of Nuclear reactions) is situated
at TRIUMF, Canada’s National Laboratory for Particle and Nuclear Physics, which
houses the world’s largest cyclotron. DRAGON was designed to measure radiative
capture reactions in inverse kinematics using a hydrogen or helium gas target.
The DRAGON system is basically a 21m recoil mass spectrometer which can
create elements via proton or alpha capture reactions, and then separates them
based on mass, in two stages.
Beam enters a windowless gas target box, which is surrounded by a closelypacked array of 30 gamma detectors made of BGO (Bismuth Germanium Oxide)
scintillation crystals. A series of pumps are found either side of the entrance and
exit to the target, and are used to keep the entrance and exit in vacuum (~10-7
Torr), allowing the beam to pass cleanly through the target.
On leaving the target, the products (recoils) of the nuclear reaction (together with original beam, known as leaky beam) enter
the first stage of the mass spectrometer. The mass spectrometer is made up of magnetic dipoles (M), magnetic quadrupoles
(Q), magnetic sextupoles (S), and electrostatic dipoles (E), arranged in a two stage mass separation:
(QQMSQQQSE)(QQSMQSEQQ). The magnetic dipoles are used in such a way as to separate out the charge state of
interest, and the quadrupoles and sextupoles focus the beam through the spectrometer. The electrostatic dipoles are used in
such a way as to separate the recoils from the leaky beam using their different momentums.
At the end of DRAGON is a choice of two end detectors, a double-sided-silicon-strip detector (DSSSD) and an ionization
chamber (IC). The DSSSD measures energy, position, and time-of-flight. The IC measures energy and change in energy
(ΔE). The IC was used for the 13C(p,γ)14N experiment, and will be used for the future 13N(p,γ)14O experiment, because it can
be used to separate out all the contaminate elements, using ΔE measurements.
Figure 1 shows a typical coincidence gamma energy spectrum from a DRAGON run of the
13C(p,γ)14N reaction. A coincidence gamma is one that is associated with a recoil heavy ion
of 14N as detected in the end detector of DRAGON. The cγ0 means that the data put into this
spectrum is from the most energetic coincidence gamma ray detected by a single BGO per
event by the BGO gamma array. The main peak will correspond to the energy of the gamma
rays cascading from the 8MeV excited state to the ground state. The various other peaks are
from either: the cascade gammas to other excited states, or from the main 8MeV gammas
that did not deposit all of their energy into a single BGO.
The data analyzer used by DRAGON is a MIDAS program which looks at runs online and
offline. Analyzing a run offline means that we can pass the run through the analyzer many
times, and by having made changes to the online data base (ODB), we can eliminate more
and more unwanted background events. These changes in the ODB mean that we can also
look at spectrums not set up in the online ODB. For example, instead of looking at the most
energetic coincidence gamma per event, we can look at the sum of all gammas that trigger a
BGO per event (see figure 2). By summing the gammas we have eliminated the lower
energy peaks which were triggered by 8MeV gammas depositing their energy over more
than one BGO.
On analysis of the 14N recoils, we see what appears to be “clipping” of lower energy recoils,
in the peak (figure 3). The 13C(p,γ)14N reaction has a large cone angle of approximately
19mrad, which is beyond the design limits of DRAGON (approximately 16mrad). Therefore,
some recoils will not make it through the beam tubes out of the gas target box, but will be
“clipped”, staying in the gas target box. To find out what percentage of recoils weren’t making
it to the end detector, we needed to create and run GEANT simulations of DRAGON and this
reaction.
GEANT
GEANT is a Detector Description and Simulation Tool. It is a program that simulates the way in which elementary particles pass
through matter. It was originally designed for High Energy Physics but is also today used in medical and biological sciences, and
astronautics. The main applications of GEANT for High Energy Physics are the tracking of particles through an experimental
setup for the simulation of detector response, and the graphical illustration of the setup and of the particle trajectories.
We used GEANT to create a replica of the DRAGON separator, for simulations of different astrophysical reactions, such as the
13C(p,γ)14N reaction.
Figure 1
BGO simulations
Due to the many excited states of 14N, and hence the large amount of
gamma cascades, I would start the analysis of 13C(p,γ)14N by
concentrating solely on the 8MeV ground state gamma, which could be
compare with King et al2. But how could I separate out the ground state
gammas from the cascades? The GEANT simulation of DRAGON’s BGO
array3 was used to calculate the percentage of 8MeV gammas that
deposited all of their energy in a single BGO. However, the BGO gamma
array only covers 92% of the solid angle of the gas target, and hence not
all gammas are registered. Of the gammas that did register, 85.3%
deposited their energy in a single BGO, and 13.9% deposited their energy
in a BGO and its neighbour. From the diagram of the simulated BGO
array to the right, you can see that it is very complex, so defining a
neighbouring BGO is difficult. To simplify, the GEANT simulation was
updated to use a cuboid technique, where by if a BGO fires and another
fires a certain distance away which is within the cube, then it is said to be
a neighbouring BGO.
13C(p,γ)14N
Creating an ionization chamber in the DRAGON simulation
Using schematic diagrams of DRAGON’s actual ionization chamber, I was able to simulate a simple ionization
chamber with ‘cuboids’ and ‘cylinders’. A lot of FORTRAN code was needed for the simulation and their were a lot of
problems with the designing. After months of work, the simulated ionization chamber was operational, and the
DRAGON simulation was able to track recoil particles through it.
Figure 2
Figure 3
simulations
By creating an input file for the 13C(p,γ)14N reaction, I was able to simulate this reaction through DRAGON
to compare with the actual data. Figure 3 shows a recoil spectrum with a peak energy value of around
5MeV for an actual DRAGON run. Simulating the same conditions with the DRAGON GEANT simulation
gave a peak energy of 6.55MeV (figure 4).
This 1.5MeV energy difference was believed to happen as the recoils pass through the entrance window
(a mylar foil) of the ionization chamber.
To test this theory, I started working on creating an ionization
chamber within GEANT for our DRAGON simulation. Other
motivations for simulating the ionization chamber were to:
a) get a proper estimate of energy straggling,
b) find out what anode the recoil ion stops in,
c) get a proper energy spectrum,
d) compare with the real data and estimate the acceptance loss,
e) simulate the correct geometry features of the energy loss,
f) test recoils in different pressures within the ionization
chamber.
Figure 4
Summary
Acknowledgements
I would like to thank Dr Chris Ruiz, Dr Alison Laird, Dr Sabine Engel, Dario Gigliotti, and Mike Lamey, for their close help and
support, throughout this project, and their friendship during my year at TRIUMF. Also, I like to thank Professor John D’Auria for
giving me this excellent opportunity to come to this facility, and experience nuclear astrophysics outside of the classroom.
1
Author’s Email Address: [email protected],
2
J. King et al., Nuclear Physics A 567 (1994) 354-376,
3
Initially, the 13C(p,γ)14N reaction experiment was used as
an acceptance test for DRAGON. We were pushing the
limits of angular acceptance of the DRAGON, due to the
large 19.4mrad cone angle for this reaction. Also, the very
small cross-section meant a simulation was necessary to
investigate the acceptance loss specific to this reaction.
My simulations of the 13C(p,γ)14N reaction in DRAGON,
with the new ionization chamber, are still continuing, with
histogram updates, and running with different mistunes of
distance, angle and percentage, of the beam in the gas
target.
Once complete, my analysis of the 13C(p,γ)14N data
compared with analysis from my 13C(p,γ)14N simulations,
will provide DRAGON with sufficient enough results to be
able to compensate for this reaction occurring with the
13N(p,γ)14O reaction. My creation of the ionization
chamber in the DRAGON simulation will not only aid the
DRAGONeers in distinguishing the different elements in
their future 13N(p,γ)14O data, but also help in the analysis
of future reaction studies when the ionization chamber is
used.
D.Gigliotti, Master’s thesis, University of Northern British Columbia (in preparation) 2003