Alpha Particle Scintillation Analysis in High Pressure Argon Using Photomultiplier Tubes

1
Problem Scintillation is a flash of light that
occurs as a result of a subatomic particle
interaction. The pulse shape of the scintillation
in argon gas depends upon the particle type. This
project focuses on alpha particle events in high
pressure argon. This will later be compared to
pulses from gamma rays and nuclear recoils. Our
motivation is to develop innovative ways to
differentiate nuclear recoils for neutron and
dark matter detection.
Our fit indicates that xenon is present in the
chamber at a level less than 6 ppm. Since the
cell had been previously filled with xenon, we
concluded that some xenon must have been trapped
in teflon insulators used in the chamber. Further
data was then taken with this gas at different
potentials on the center electrode to investigate
the size of recombination effects. The figure
below shows the pulse shape for summed events
with varying potentials. No appreciable
difference is seen except possibly at long decay
times.
Alpha Particle Scintillation Analysis in High
Pressure Argon Using Photomultiplier Tubes Daniel
Saenz, Rice University Advisor Dr. James White,
Texas AM University
Experimental Setup
Alpha decay
PMT
The shape of the pulse includes a fast and a slow
component. The fast component is caused by the
decay of the singlet state 1Su while the slower
component is due to the triplet state 3Su of the
Ar2 molecule.
Pulse shapes with potentials of 0, 500, 1000,
1500, and 2000 V.
A cylindrical gas cell is surrounded by four
PMTs. Scintillation pulse is read out using a
500 MHz waveform digitizer and analyzed with a
PC. The chamber was pumped to a pressure less
than 10-7 torr, then filled with argon to a
pressure of 50 atm. Then, we triggered our data
to only show alpha particles.
However, the hump shape is not a trait of pure
argon. Instead, it indicates the presence of a
contaminant such as Xenon 1. To analyze this
contamination, a large number of events were
summed and fit to a curve given by 1.
To check our contamination hypothesis, the
chamber was flushed and refilled with argon. The
resulting pulse shape is shown in the figure
below. No hump structure is observed.
Data/Analysis
Pulse shape from argon with xenon impurities
removed.
Conclusion Our pulse shape analysis indicates a
strong sensitivity to xenon impurities in argon
as well as a weak interaction with electric
fields.
Blue is the summed events and the red curve is
the fit from the given equation.
This is a sample event of scintillation from
alpha decay.
1. S. Kubota et al., NIM A 327, (1993) 71-74.