Alpha Particle Scintillation Alpha Particle Scintillation Analysis in High Pressure Analysis in High Pressure

ArgonArgonDaniel Saenz, Rice UniversityDaniel Saenz, Rice University

Advisor: Dr. James White, Texas A&M Advisor: Dr. James White, Texas A&M UniversityUniversity

Experimental Dark MatterExperimental Dark Matter

Dr. White is involved in experimental dark Dr. White is involved in experimental dark matter science.matter science.

WIMP DetectionWIMP Detection

Subatomic interactions in gaseous nobles.Subatomic interactions in gaseous nobles.

Involved in numerous projects and Involved in numerous projects and collaborations.collaborations.

SIGN, Zeplin II, etc.SIGN, Zeplin II, etc.

CoatingsCoatings

Phototubes are often used in Dr. White’s Phototubes are often used in Dr. White’s work. Charge will build up on them unless work. Charge will build up on them unless conductive coatings are added.conductive coatings are added.

We needed a coating material and We needed a coating material and thickness that would transmit the most thickness that would transmit the most light and conduct. Tried gold, aluminum, light and conduct. Tried gold, aluminum, platinum, etc.platinum, etc.

Coating ChamberCoating Chamber

Pump with roughing Pump with roughing and a turbo.and a turbo.

Up to four hours to Up to four hours to pump to ideal pump to ideal pressure of <10pressure of <10-5 -5 torr.torr.

Material on a Material on a conducting boat conducting boat which current passes which current passes through to evaporate.through to evaporate.

MCF Coating MachineMCF Coating Machine

This metal evaporator This metal evaporator can reach pressures can reach pressures <10<10-6 -6 in under half an in under half an hour.hour.

GarfieldGarfield

Garfield is software made by CERN for the Garfield is software made by CERN for the purpose of simulating drifting and particle purpose of simulating drifting and particle tracking in electromagnetic fields in two tracking in electromagnetic fields in two dimensions.dimensions.

ProjectProject

Normal matter interacts in characteristic ways Normal matter interacts in characteristic ways with Argon, Xenon, etc.with Argon, Xenon, etc.

Gamma rays, alpha particles, neutrons, etc. can Gamma rays, alpha particles, neutrons, etc. can all scintillate by ionizing molecules.all scintillate by ionizing molecules.

These particles excite an atom and bumps These particles excite an atom and bumps electron up a level. It then decays and releases electron up a level. It then decays and releases a photon.a photon.

We can detect these photons using phototubes.We can detect these photons using phototubes.

PhototubesPhototubes

A phototube is a A phototube is a device that can detect device that can detect single photons. It does single photons. It does this when a photon this when a photon hits it and emits an hits it and emits an electron via the electron via the photoelectric effect.photoelectric effect.

The current is then The current is then amplified to get a amplified to get a sizeable reading.sizeable reading.

Phototubes

Types of ScintillationTypes of Scintillation

Typically, primary and secondary Typically, primary and secondary scintillation is observed.scintillation is observed.Primary:Primary: caused by an excited atom (as previously caused by an excited atom (as previously

discussed).discussed).

Secondary:Secondary: An electrode in the center of the chamber An electrode in the center of the chamber

creates a field. The ionized atom drifts to the creates a field. The ionized atom drifts to the electrode, and the accelerated charge electrode, and the accelerated charge radiates energy.radiates energy.

RecombinationRecombination

An electric field has an effect on An electric field has an effect on scintillation, particularly secondary. Just scintillation, particularly secondary. Just how much this effects alpha particle how much this effects alpha particle scintillation events is part of my scintillation events is part of my experiment.experiment.

Recombination nevertheless occurs when Recombination nevertheless occurs when due to a low field, photons are reabsorbed due to a low field, photons are reabsorbed before making the way to a phototube.before making the way to a phototube.

Project: Alpha Particle ScintillationProject: Alpha Particle Scintillation

Went out to the Went out to the Nuclear Science Nuclear Science Center to the Center to the accelerator.accelerator.

Proton SourceProton Source

Duoplasmatron Ion Duoplasmatron Ion SourceSource

Using Using 77(Li(p,n)(Li(p,n)77Be Be reactions, we achieve reactions, we achieve up to monochromatic up to monochromatic 2 MeV neutrons.2 MeV neutrons.

Tandem van de Tandem van de Graaff accelerator Graaff accelerator used to speed up used to speed up particles.particles.

Experimental SetupExperimental Setup

Alpha Particle Track

Electrode

Phototube

Phototube

Phototube

Phototube

Acquiring the DataAcquiring the Data

Scintillation pulse is read out using a 500 Scintillation pulse is read out using a 500 MHz waveform digitizer which is fed into a MHz waveform digitizer which is fed into a PC.PC.On the computer, we used a data analysis On the computer, we used a data analysis program called PAW to make graphs and program called PAW to make graphs and view data.view data.To view individual events with much To view individual events with much flexibility, however, a lot of C++ code has flexibility, however, a lot of C++ code has to be written and maintained.to be written and maintained.

Neutron EventsNeutron Events

Though we detected some neutron events, Though we detected some neutron events, we could not calibrate the beam enough to we could not calibrate the beam enough to get a sizeable number of neutron events.get a sizeable number of neutron events.

More useful to study alpha particle events.More useful to study alpha particle events. Alpha particle events have an effect more on Alpha particle events have an effect more on

the primary scintillation rather than the the primary scintillation rather than the secondary scintillation.secondary scintillation.

Primary ScintillationPrimary Scintillation

Here is a sample event Here is a sample event showing primary showing primary scintillation only (using scintillation only (using zero field).zero field).

Fast componentFast component Slow componentSlow component

Each event shows an Each event shows an alpha particle (alpha particle (238238U to U to 234234Th + Th + αα) exciting many ) exciting many atoms, which are either in atoms, which are either in a singlet or triplet state.a singlet or triplet state.

Complete Event w/ FieldComplete Event w/ Field

Area under the curveArea under the curve

The ratio of the area The ratio of the area under the fast part of under the fast part of the curve and the the curve and the area under the slow area under the slow part of the curve is part of the curve is plotted for two plotted for two potentials (red high potentials (red high and blue zero):and blue zero):

Varying the PotentialVarying the Potential

Taking data of the Taking data of the light shape over a light shape over a wide range of wide range of voltages (0, 500, voltages (0, 500, 1000, 1500, and 2000 1000, 1500, and 2000 volts) yielded only a volts) yielded only a very small effect of very small effect of field on the field on the scintillation pulse scintillation pulse shape.shape.

Hump ShapeHump Shape

The hump shape in the The hump shape in the event is not characteristic event is not characteristic of pure argon [1]. of pure argon [1]. According to the According to the literature, it is explained literature, it is explained that this may possibly be that this may possibly be due to the presence of due to the presence of xenon.xenon.This was an opportunity This was an opportunity to do another smaller to do another smaller experiment – to use the experiment – to use the shape of primary shape of primary scintillation to determine scintillation to determine the composition of gas.the composition of gas.

FitFit

Using an equation of fit used by [1], we Using an equation of fit used by [1], we attempted to fit a curve for light as a attempted to fit a curve for light as a function of time.function of time.

l(t) = Al(t) = A11ee--t/t/ττ1 1 + A+ A22ee--t/t/ττ

2 2 –A–A3 3 ee--t/t/ττdd

The AThe A11 and A and A2 2 terms are the fast and slow terms are the fast and slow decay terms with decay constants decay terms with decay constants ττ11 and and ττ22 respectively.respectively.

The AThe A33 term subtracts from the function, term subtracts from the function, allowing for a hump.allowing for a hump.

FitFit

Using LabFit, we were Using LabFit, we were able to fit an excellent able to fit an excellent curve with the following curve with the following parameters: parameters:

AA11 = 913.8 = 913.8

AA22 = 1920 = 1920

AA33 = 1929 = 1929

ττ11 = 13.11 ns = 13.11 ns

ττ22 = 417.5 ns = 417.5 ns

ττdd = 177.6 ns = 177.6 ns

Flush the ChamberFlush the Chamber

Took greater Took greater precaution to ensure precaution to ensure that argon was truly that argon was truly argon.argon.

Re-pumped the Re-pumped the chamber and took chamber and took more data.more data.

Indeed, the data lost Indeed, the data lost the hump shape.the hump shape.

Fast-to-Slow RatiosFast-to-Slow Ratios

Here are the Here are the resulting ratios resulting ratios after re-after re-pumping the pumping the argon argon chamber.chamber.

Next StepNext Step

Much research is taking place in dark Much research is taking place in dark matter science looking for dark matter matter science looking for dark matter events in liquid argon (rather than gaseous events in liquid argon (rather than gaseous argon).argon).

We can compare our decay constants to We can compare our decay constants to those of liquid argon.those of liquid argon.

Fit to CurveFit to Curve

Equation w/o Equation w/o hump hump structure:structure:

l(t) = Al(t) = A11ee-t/ -t/ ττ1 1 + +

AA22ee-t/ -t/ ττ22

AA11 = 1.737 = 1.737

AA22 = 0.6144 = 0.6144

ττ11 = 36.94 ns = 36.94 ns

ττ22 = 1217 ns = 1217 ns

Fit to Curve (Cont.)Fit to Curve (Cont.)

We found the decay constant to be significantly We found the decay constant to be significantly higher than the 2-8 ns range for liquid argon. higher than the 2-8 ns range for liquid argon. This makes sense because Ar atoms are much This makes sense because Ar atoms are much closer to each other in the liquid phase.closer to each other in the liquid phase.

Part of what makes argon event detection Part of what makes argon event detection difficult is the natural decay of difficult is the natural decay of 4040K to Argon. K to Argon. There is about one decay per kilogram of liquid There is about one decay per kilogram of liquid argon. One has to differentiate between these argon. One has to differentiate between these normal decays and new ones.normal decays and new ones.

ConclusionsConclusions

Alpha particle shape was recorded at a Alpha particle shape was recorded at a specific pressure. With good accuracy, we specific pressure. With good accuracy, we found an equation that fits our data at this found an equation that fits our data at this pressure.pressure.

The results revealed that scintillation due The results revealed that scintillation due to alpha events in high pressure argon to alpha events in high pressure argon atoms are very sensitive to xenon atoms are very sensitive to xenon imperfections.imperfections.

Conclusions (Cont.)Conclusions (Cont.)

Furthermore, we verified that at this Furthermore, we verified that at this pressure, the field has only a small effect pressure, the field has only a small effect on the yield of the light.on the yield of the light.

This is a verification that can hopefully This is a verification that can hopefully make dark matter or WIMP events stand make dark matter or WIMP events stand out someday.out someday.

ReferencesReferences

1. 1. 1. S. Kubota et al., NIM A 327, (1993) 71-74.