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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.
Phototube Alpha Particle Track Phototube Phototube Electrode Phototube Experimental Setup
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.
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 . 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 , 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 τ1and τ2respectively. • 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 re-pumping 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.