Alpha Particle Scintillation
Analysis in High Pressure
Argon
Daniel Saenz, Rice University
Advisor: Dr. James White, Texas A&M
University
Experimental Dark Matter
Dr. White is involved in experimental dark
matter science.
WIMP Detection
Subatomic interactions in gaseous nobles.
Involved in numerous projects and
collaborations.
SIGN, Zeplin II, etc.
Coatings
Phototubes are often used in Dr. White’s
work. Charge will build up on them unless
conductive coatings are added.
We needed a coating material and
thickness that would transmit the most
light and conduct. Tried gold, aluminum,
platinum, etc.
Coating Chamber
Pump with roughing
and a turbo.
Up to four hours to
pump to ideal
pressure of <10-5 torr.
Material on a
conducting boat
which current passes
through to evaporate.
MCF Coating Machine
This metal evaporator
can reach pressures
<10-6 in under half an
hour.
Garfield
Garfield is software made by CERN for the
purpose of simulating drifting and particle
tracking in electromagnetic fields in two
dimensions.
Project
Normal matter interacts in characteristic ways
with Argon, Xenon, etc.
Gamma rays, alpha particles, neutrons, etc. can
all scintillate by ionizing molecules.
These particles excite an atom and bumps
electron up a level. It then decays and releases
a photon.
We can detect these photons using phototubes.
Phototubes
Phototubes
A phototube is a
device that can detect
single photons. It
does this when a
photon hits it and
emits an electron via
the photoelectric
effect.
The current is then
amplified to get a
sizeable reading.
Types of Scintillation
Typically, primary and secondary
scintillation is observed.
Primary:

caused by an excited atom (as previously
discussed).
Secondary:

An electrode in the center of the chamber
creates a field. The ionized atom drifts to the
electrode, and the accelerated charge
radiates energy.
Recombination
An electric field has an effect on
scintillation, particularly secondary. Just
how much this effects alpha particle
scintillation events is part of my
experiment.
Recombination nevertheless occurs when
due to a low field, photons are reabsorbed
before making the way to a phototube.
Project: Alpha Particle Scintillation
Went out to the
Nuclear Science
Center to the
accelerator.
Proton Source
Duoplasmatron Ion
Source
Using 7(Li(p,n)7Be
reactions, we achieve
up to monochromatic
2 MeV neutrons.
Tandem van de
Graaff accelerator
used to speed up
particles.
Experimental Setup
Phototube
Alpha Particle
Track
Phototube
Phototube
Electrode
Phototube
Acquiring the Data
Scintillation pulse is read out using a 500
MHz waveform digitizer which is fed into a
PC.
On the computer, we used a data analysis
program called PAW to make graphs and
view data.
To view individual events with much
flexibility, however, a lot of C++ code has
to be written and maintained.
Neutron Events
Though we detected some neutron events,
we could not calibrate the beam enough to
get a sizeable number of neutron events.
More useful to study alpha particle events.

Alpha particle events have an effect more on
the primary scintillation rather than the
secondary scintillation.
Primary Scintillation
Here is a sample event
showing primary
scintillation only (using
zero field).


Fast component
Slow component
Each event shows an
alpha particle (238U to
234Th + α) exciting many
atoms, which are either in
a singlet or triplet state.
Complete Event w/ Field
Area under the curve
The ratio of the area
under the fast part of
the curve and the
area under the slow
part of the curve is
plotted for two
potentials (red high
and blue zero):
Varying the Potential
Taking data of the
light shape over a
wide range of
voltages (0, 500,
1000, 1500, and 2000
volts) yielded only a
very small effect of
field on the
scintillation pulse
shape.
Hump Shape
The hump shape in the
event is not characteristic
of pure argon [1].
According to the
literature, it is explained
that this may possibly be
due to the presence of
xenon.
This was an opportunity
to do another smaller
experiment – to use the
shape of primary
scintillation to determine
the composition of gas.
Fit
Using an equation of fit used by [1], we
attempted to fit a curve for light as a
function of time.
l(t) = A1e-t/τ1 + A2e-t/τ2 –A3 e-t/τd
The A1 and A2 terms are the fast and slow
decay terms with decay constants τ1 and
τ2 respectively.
The A3 term subtracts from the function,
allowing for a hump.
Fit
Using LabFit, we were
able to fit an excellent
curve with the following
parameters:
A1 = 913.8
A2 = 1920
A3 = 1929
τ1 = 13.11 ns
τ2 = 417.5 ns
τd = 177.6 ns
Flush the Chamber
Took greater
precaution to ensure
that argon was truly
argon.
Re-pumped the
chamber and took
more data.
Indeed, the data lost
the hump shape.
Fast-to-Slow Ratios
Here are the
resulting ratios
after repumping the
argon
chamber.
Next Step
Much research is taking place in dark
matter science looking for dark matter
events in liquid argon (rather than gaseous
argon).
We can compare our decay constants to
those of liquid argon.
Fit to Curve
Equation w/o
hump
structure:
l(t) = A1e-t/ τ1 +
A2e-t/ τ2
A1 = 1.737
A2 = 0.6144
τ1 = 36.94 ns
τ2 = 1217 ns
Fit to Curve (Cont.)
We found the decay constant to be significantly
higher than the 2-8 ns range for liquid argon.
This makes sense because Ar atoms are much
closer to each other in the liquid phase.
Part of what makes argon event detection
difficult is the natural decay of 40K to Argon.
There is about one decay per kilogram of liquid
argon. One has to differentiate between these
normal decays and new ones.
Conclusions
Alpha particle shape was recorded at a
specific pressure. With good accuracy, we
found an equation that fits our data at this
pressure.
The results revealed that scintillation due
to alpha events in high pressure argon
atoms are very sensitive to xenon
imperfections.
Conclusions (Cont.)
Furthermore, we verified that at this
pressure, the field has only a small effect
on the yield of the light.
This is a verification that can hopefully
make dark matter or WIMP events stand
out someday.
References
1. 1. S. Kubota et al., NIM A 327, (1993)
71-74.