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RP-SVS Health Physics Services, TA-36-1

MCNP Modeling and Benchmarking of a Xenon Gas Proportional Counter

J. Heard, T. McLean, J. Bland, &

A. Justus September 2014 HPIC Meeting

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Outline

Introduction Materials Methods Results Conclusion References Acknowledgments

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Introduction

There is currently no reliable method of rapidly determining the cause of alarms on personnel contamination monitors (PCMs). The problem is distinguishing transuranic activity from naturally occurring radon progeny.

The results on the PCM are displayed in two channels, one for gross beta and one for gross alpha. This means the PCM is unable to identify the radionuclides present, i.e. man made from naturally occurring activity.

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Introduction cont.

Radon is a naturally occurring gas and a member of the U-238 decay series with a 3 ½ day half-life.

In order to determine if the radionuclide present is in fact radon progeny we must employ other techniques to be discussed momentarily.

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Materials Quaesta Module • Manufacturer: Quaesta Instruments,LLC • User Interface: Tera Term Pro • Capabilities

i. Regions of Interest capabilities (ROI) ii. High Voltage Supply iii. Multi-channel Analyzer

iv. Timing capabilities

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Materials cont. Xenon Gas Proportional Probe • Manufacturer: LND, INC. • Part Number: 49727 200 cm2 Beta-Gamma Proportional Counter • Gas Filling: Xenon Pressure: 800 torr • Physical Dimensions: 279.5mm/11.01inch x 103.2 mm/4.06inch • Window: Titanium 0.00047in. (0.0011938 cm) - 0.00050in.

(0.00127cm)

Proportional Probe

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Materials cont. Monte Carlo N-Particle Transport Code • Los Alamos National Lab • Software package capable of simulating nuclear processes. • Simulate pulse height spectrum in the Xenon Gas Proportional

probe. • Used to manipulate probe optimization

i. Gas Pressure ii. Window Thickness iii. Chamber Depth

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Methods Monte Carlo Simulation with MCNP6 Modeled current probe design • Ran pulse height tallies for alpha/beta sources and mono-

energetic alphas/beta. • To obtain energy calibration of the probe an f8 tally was ran for

alpha energy deposition. • Compared with experimental data to obtain an effective resolution. Adjusted probe design • Varied window thickness to view effect on beta and alpha spectra

and detection efficiencies. • Adjusted chamber depth and tested various gas pressures for

optimum detection.

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Methods cont.

Experimental Data with Xenon Gas Proportional Probe Evaluated operating parameters

• Ideal amplifier gain. • Optimum High Voltage and regions of interest determined.

(alpha low, etc) Tested detection efficiency at probe’s center

• Alpha: 239Pu, 241Am, and 230Th • Beta: 14C, 99Tc, 36Cl, 90Sr/Y

Facial Uniformity • Area detection tests on the horizontal and vertical axes. • Analyzed spectral dependence on irradiated portions of the

detector’s surface.

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Methods cont.

Methods of Distinguishing Radon/Thoron Progeny from Transuranics

• Decay in alpha count rate with time. • Pseudo-Coincidence although observed with low efficiency can

be used to tag Rn progeny. • Beta/Alpha Ratio • Change in ratio of high alpha energy events to low energy

events. • A combination of all or some of the above techniques.

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Results As of now we can discern from the data that unlike the MCNP model which is configured to display symmetry, the actual probe does not have a constant facial uniformity .

Measured Horizontal Facial Uniformity

Measured Vertical Facial Uniformity

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Results cont.

Unlike the assumptions in the MCNP model the standard titanium window thickness must be reduced in order to maximize alpha efficiency and energy resolution.

MCNP Titanium Widow Thickness vs Alpha Energy Efficiency

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Results cont.

Shown in the chart below is the MCNP data demonstrating that the gas proportional probe will give a higher pulse height to the Polonium (alpha emitter) proving that it can deposit more energy in the chamber than the transuranic series (Americium).

MCNP Polonium 218 and Americium 241 Spectra

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Conclusion

This is an on-going study and has only just made it past stage one. The next stage incorporates the MCNP modeled data and its ideal development for the probe. We will then begin to manipulate the physical probe in order to achieve optimum detection of alpha and beta activity. Ultimately an instrument that is user friendly and can make a quick determination using one or more of the techniques discussed earlier to distinguish radon/thoron progeny from transuranic activity is the goal.

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References • Klett, Alfred; Pseudocoincidence Techniques, 2011

• Porstendorfer, J; Properties and Behavior of Radon and Thoron

and Their Decay Products in the Air,1993

• Canberra;Peronnel Contaimination and Objcect Monitors,2014

• World Nuclear Association; Radioactive Decay in Thorium and Uranium Series, 2014

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Acknowledgments

We would like to express the deepest appreciation to Ches Simpson (NEN-2) for allowing Jadtrl Heard the opportunity to work along side the RP-SVS as an intern this summer. We would also like to thank all of the workers located at TA-36 Radiation Instrumentation Repair and Calibration facility at LANL, TA-36.

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Questions