# Presentation: 756396

## A System for Simultaneous Beta and Gamma Spectroscopy and its Application to Nuclear Non-Proliferation

- D.M. Hamby and A.T. Farsoni
- Nuclear Engineering and Radiation Health Physics
- College of Engineering
- Oregon State University
- Corvallis, Oregon

## Beta Spectroscopy

- Typically for beta dosimetry
- Open- and closed-window
- 		spectral stripping
- Solid-state, scintillators, thin plastics
- 20-yr history

## Gamma Spectroscopy

- Very common
- Simple to complex
- Solid-state, scintillators
- High/low resolution

## Scintillator Response

- The light output from most scintillators can be represented as a simple exponential decay.  The current pulse from the PM-tube anode is:

- where  is the scintillator light decay time and Q is the total collected charge

**NaI (Tl)**
**τ** = 230 nsec

**Plastic**
**τ** = 2.4 nsec

## Phoswich Detectors

_**Phoswich Detectors**_

-    The combination of dissimilar scintillators optically coupled to
-     a single PM tube (*phosphor sandwich*)

## Pulse processing algorithm is easy to edit

The algorithm is stable and reliable, no thermal noise or other fluctuations

No bulky analog electronics

Post-processing ability

More cost-effective

Effects, such as pile-up, can be corrected or eliminated at the processing level

Signal capture and processing can be based more easily on coincidence criteria between different detectors or different parts of the same detector

- The algorithm is stable and reliable, no thermal noise or other fluctuations
- No bulky analog electronics
- Post-processing ability
- More cost-effective
- Effects, such as pile-up, can be corrected or eliminated at the processing level
- Signal capture and processing can be based more easily on coincidence criteria between different detectors or different parts of the same detector

_**Digital Spectrometers**_*Advantages*

## Simultaneous Beta/Gamma Spectrometer

- A digital phoswich detection system capable of simultaneous spectroscopy with minimal cross-talk.

## Triple-Layer Phoswich Design

- 76.2 mm

- PMT

**Aluminized Mylar (6 ********m)**

**CaF****2****(Eu) (1.5 mm)**

**BC-400 (1.5 mm)**

**Quartz Optical Window (2.0 mm)**

**NaI(Tl) (25.4 mm)**

_**Triple-Layer Phoswich Design**_

## Electron & Photon Energy Deposition

_**Electron & Photon Energy Deposition**_

- Simulated detector response from different monoenergetic electrons in BC400.

- Simulated detector response from 1.0 MeV gamma-rays in the three scintillation layers.

*MCNP modeling for detector optimization.*

## Interaction Scenarios

- 1
- 2
- 3
- 4
- 5
- 6
- 7

**CaF****2**

**NaI:Tl**

**BC400**

- Scenario #

_**Interaction Scenarios**_

- Phoswich Detector

## Radiation Transport StudyBeta and Gamma Discrimination

- * Total probabilities are calculated for 1.0 MeV photon/electron events. Events with energies   less than 10 keV were excluded as electronic noise.

_**Radiation Transport Study**_*Beta and Gamma Discrimination*

## Data Acquisition System

## 12-bit, 100 MHz Digital Pulse Processor

- FPGA

- USB Port

- Preamplifier

- DAC

- Input signal
-  from anode

- Output signal
-  to oscilloscope

- ADC

## Data Acquisition SystemPulse Processing Algorithm

_**Data Acquisition System**_*Pulse Processing Algorithm*

- Four parameters, Baseline, P, M1 and M2 are used for analyzing the anode pulses.

- Sums A and B used for calculating the energy absorption in each layer of the phoswich detector.

- Fast-Ratio (FR) = (P – M1) / P
- Slow-Ratio (SR) = (M1 - M2) / M1

## Experimental ResultsMeasurements using the Phoswich/DPP - Energy Calibration

_**Experimental Results**_*Measurements using the Phoswich/DPP **- Energy Calibration*

- 14C

- 99Tc

**E****max ****= 292 keV**

**E****max ****=**** ****156 keV**

- 137Cs

- 137Cs + 60Co

- Photopeak at
- 662 keV

- Photopeak at
- 1172 keV

- Photopeak at
- 1332 keV

- Compton Edge at
- 477 keV

- Photopeak at
- 662 keV

- Beta particles (Emax) from 14C and 99Tc were used for **BC400** calibration.

- The Compton edge and photopeak of 137Cs were used for **CaF****2****(Eu)** calibration.

- The 662 keV and 1332 kev photopeaks, respectively from 137Cs and 60Co, were used for **NaI(Tl)** calibration.

## Experimental ResultsMeasurements using the Phoswich/DPP- Anode Signal Pulses

_**Experimental Results**_*Measurements using the Phoswich/DPP**- Anode Signal Pulses*

- Signal Pulses from 90Sr/90Y

- Signal Pulses from 137Cs

## Experimental ResultsMeasurements using the Phoswich/DPP- Real-Time and Simultaneous Beta/Gamma Spectroscopy

_**Experimental Results**_*Measurements using the Phoswich/DPP**- Real-Time and Simultaneous Beta/Gamma Spectroscopy*

- Pure Beta Source: 99Tc

## Experimental ResultsMeasurements using the Phoswich/DPP - Real-Time and Simultaneous Beta/Gamma Spectroscopy

_**Experimental Results**_*Measurements using the Phoswich/DPP **- Real-Time and Simultaneous Beta/Gamma Spectroscopy*

- (Beta-shielded) Gamma Source: 137Cs

## Experimental ResultsMeasurements using the Phoswich/DPP - Real-Time and Simultaneous Beta/Gamma Spectroscopy

_**Experimental Results**_*Measurements using the Phoswich/DPP **- Real-Time and Simultaneous Beta/Gamma Spectroscopy*

- Mixed Beta/Gamma Field: 99Tc + 137Cs

## Experimental ResultsMeasurements using the Phoswich/DPP - Response to Gamma Sources (beta-shielded)

_**Experimental Results**_*Measurements using the Phoswich/DPP **- Response to Gamma Sources (beta-shielded)*

- Percentage of events and simultaneous beta/gamma energy spectra from (a) 137Cs and (b) 60Co

- (a)

- (b)

- FWHM (662 keV)
- ~ 6.7%

- 60Co

- 137Cs

## Experimental ResultsMeasurements using the Phoswich/DPP - Response to Pure Beta Sources

- Percentage of events and simultaneous beta/gamma
-  energy spectra from (a)14C, (b) 99Tc and (c) 90Sr/ 90Y

- (a)

- (b)

- (c)

- 14C

- 99Tc

- 90Sr/ 90Y

_**Experimental Results**_*Measurements using the Phoswich/DPP **- Response to Pure Beta Sources*

## Slide 22

**Notes:**
321 stations, of which about a third are transmitting data to IMS

## Radioxenon Decay Productsenergy in units of keV

- 131m
- 133
- 133m
- 135

- max
- 346
- 901

- 
- 164
- 81
- 233
- 250

- c.e.
- 12945
- 199
- 214

- X ray
- 30
- 31
- 30
- 31

_**Radioxenon Decay Products**_*energy in units of keV*

**(********, ********) or (ce, X) Double Coincidence**

**(********, ce, X) Triple Coincidence**

## Dual Phoswich for Coincidence CountingUtilizing both light coincidence and signal coincidence

**Quartz**

**CaF****2**

**BC400**

**Sample**

**In**

**Out**

_**Dual Phoswich for Coincidence Counting**_*Utilizing both light coincidence and signal coincidence*

**HV**

**HV**

**Sig**

**Sig**

**PMT 1**

**PMT 2**

**NaI**

**Coupling**
**Sleve**

## 133mXe CoincidenceProbability: 0.115

- 199 keV c.e.

- 30 keV K X-ray

_**133m**__**Xe Coincidence**_*Probability: 0.115*

**Notes:**
92.1% of conversion electrons interact in BC400; 65.6% of x-rays interact in CaF2; this coincidence occurs in about 19% of decays; therefore total prob = 0.921 * 0.656* 0.19 = 0.115

## 133Xe Triple CoincidenceProbability: 0.017

- 346 keV max

- 45 keV c.e.

- 31 keV K X-ray

_**133**__**Xe Triple Coincidence**_*Probability: 0.017*

**Notes:**
64.7% of betas interact in BC400; 61.7% of conversion electrons interact in other 1⁄2 of BC400; and 65.6% of x-rays interact in CaF2; this triple coincidence occurs in about 13% of decays; therefore prob = 0.647*0.656*0.5*0.617*0.13 = 0.017

## Conclusions on Simultaneous Spectroscopy

_**Conclusions on Simultaneous Spectroscopy**_

- Prototype digital simultaneous beta/gamma spectrometer
- Customized digital pulse processor (12-bit, 100 MHz)
- Optimization through particle transport simulation (MCNP)
- Gamma resolution as low as 6.7% (@ 662 keV)
- < 2% false positives
- Better discrimination at lower energies
- Double/Triple coincidence detection
- High selectivity