Developments in Radiation Detection Systems

ByAlex Rak

03/17/2011

Presentation Outline

• About Me• Introduction to Scintillation Detectors• Scintillation Detector System

– Mechanical Design– Test Procedure– Final Results

• Introduction to Gamma Cameras• Gamma Camera System

– Theory– Test Procedure– Results

• Summary

About Me

Bardeen Engineering Fellowship CandidateAge: 23Hometown: Burlington, ILEducation: • B.S. in Mechanical Engineering from Bradley

University• M.S. in Mechanical Engineering from Bradley

University Expected in May

About Me Continued

Work Experience:• Graduate Researcher, Bradley University (May-Dec.

2010)• Graduate Assistant, Bradley University (Aug.-Dec. 2009)• Drafting Intern, Flexible Steel Lacing Co. (Summer 2007)

Other Experience:• President, Sigma Phi Epsilon Fraternity (2006-2007)• Vice-President, Inter-Fraternity Council (2008-2009)• Senator, Student Senate (2006)

Scintillation Detectors

2 Key Components:• Scintillation Crystal

– Can be Crystal, Plastic, Liquid, or Glass• Photomultiplier Tube

– Contains Photocathode, Several Dynodes, and an Anode

How Scintillation Detectors Work

• Scintillation Light is Released when Struck by Ionizing Radiation – Converts Gamma Rays to Visible Blue Light

• The Light is Converted into an Electric Pulse by the Photocathode

• The Signal is then Amplified in the Photomultiplier Tube by Passing Through a Series of Dynodes Until it Reaches the Anode

Senior Project

• Funded by Los Alamos National Laboratory (LANL)– Est. 1943 in Los Alamos, NM 87545

• LANL wanted an Automated Omni-Directional Sensor to Locate Radioactive Material

• Contacts at LANL– Dr. Gregory Dale– Dr. Michele DeCroix– Mr. Robert Wheat

System Requirements

• Use Standard 3 in. Right Circular Cylinder NaI Detector

• Detect and Locate Source Within 10 Minutes• Transportable Pieces Weighing Less than 60 lbs.• Locate the Source Within 1 ft. of Actual Location

at a Distance of 5 ft. from the Detector• Scans Continuously Without User Feedback• Scan an Area 90⁰ Left and Right of Center and 60⁰

Down

System Design

• Motorized Linear Slide was Used to Move Scintillation Detector in and out of a Collimator– 1.7 in. Thick Lead Cylinder Used for

the Collimator• Second and Third Motor Used to

Tilt and Rotate the Entire System– 100:1 Gearbox Used with Both

Motors• Ball Bearings were Used to Smooth

the Movement of the Device

Motor Mount Analysis

• Made from A36 Hot Rolled Steel Plate–Yield Strength of 400 MPa

• Max Stress – 180 kPa• Max Deformation - 3.3 x 10-4 mm

Scintillation Detector Arm Analysis

• Built from ¾ in. OD Mild Steel Tubing– Yield Strength of 500 MPa

• Max Stress – 44.8 MPa• Max Deformation – 0.5 mm

Base Analysis

• Constructed out of 6061 Aluminum– Yield Strength of 275 MPa

• Max Stress – 53.9 MPa• Max Deformation – 0.8 mm

A-Frame Analysis

Forces: Buckling:∑Fx = 0 σact = F1-2/A∑Fy = 0 σallow = Pcrit/APcrit = π2EI/(KL)2 n =σact /σallow

• Modeled with a Truss Analyzer• Built from ¾ in. OD Mild Steel Tubing

– Yield Strength of 500 MPa• Max Stress – 1.2 MPa• Buckling Factor of Safety – 2031

Search Algorithm

• A C++ Compiler was Used to Determine the Next Step in the Search Algorithm– Based on Concepts of Windowing and Interpolation

• There were 4 Stages to the Algorithm– Every Scan was Taken for 30 sec.

• The Locations of the Sections, the Positions of the Motors, and the Depth of the Scintillation Detector were Calculated and Set in the C++ Code

• Poisson Statistics were Used to Determine if the Scintillation Detector saw a Source of Radiation in a Particular Section

Statistics

• Scanning with the Scintillation Detector Yields a Value with Units of Counts/Second

• The Background Radiation, or B, with no Source Present was Recorded for the Testing Room

• Each Scan Yields a Total Counts, T• The Source Counts/Second, or S, is Found Using the Following

EquationS=T-B

• The Standard Deviation of the Source Counts of the Initial Scan was Determined Following the Rules of Poisson Statisticsσinitial=√(Ti+B)

Statistics Continued

• A Threshold of 3 was Chosen to Determine if the Source was Within a Window

• This is Represented in the Equation BelowS/σinitial≥3

• If the Threshold Condition is met by the Counts in a Particular Window, the Source is Determined to be in that Window– The Scan then Moves to the Next Stage and

Continues from the Window with the Source

Stages 1 and 2

• Stage 1 is the Initial Search and Scans the Entire Search Area• By Increasing the Detector’s Depth in the Collimator, its

Circular Viewing Area was Decreased• Positioning was Adjusted so the Circular Viewing Area

Encompassed the Entire Rectangular Window

Stage 2 - WindowingDetector Depth = 1.29”

Stages 3 and 4

Stage 3 - WindowingDetector Depth = 3.59”

Stage 4 - InterpolationDetector Depth = 5.26”

Stage 4 Continued

• The Interpolation Stage Records the Values in All 4 Quarters and Plugs them into the this Equationy = -4E-08x6 + 1E-05x5 - 0.0013x4 + 0.0674x3 - 1.4691x2 + 4.5456x + 205.58

R2 = 0.9996

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80 90 100

• In this Equation, x is the Angle Off the Center Axis of the Scintillation Detector and y is the Counts/Second

• The Angle Off the Center Axis is Used as the Radius of a Circle that is Centered in the Middle of Each Quarter

• It is Known that the Quarter with the Highest Recorded Counts Contains the Source

• This Quarter will have the Smallest Value for the Angle Off the Center Axis and Therefore the Smallest Circle

Final Source Location

• The Intersections of the Circles Provide a More Accurate Location

• The Black Dots are the Intersections of the Circles and the Red Dot is the Experimental Source Location

Test Setup

• A 10 μCi Source of Cesium-137 was Used• 30 Scans were Performed with the Source at

Random Locations• 4 of the Sources were Placed Outside of the

Required Search Area to Test for False Positives• The Sources were Placed at Different Distances from

the Scintillation Detector Ranging from 1.1 to 8 ft.– These Distances Corresponded to the Limits of the

Detector

Results

• The 4 Sources Placed Outside the Search Area were not Found as Expected

• All 26 Sources Placed Within the Area were Found Within the Tolerance Required

• The Average Distance from the Experimental Source to the Actual Source was 4.5 in.

• The Entire System was Designed to Break Apart into 3 Pieces Each Weighing 54 lbs.

• One Full Scan Took About 9.5 Minutes to Run

Scintillation Detector System

Gamma Cameras

Components:• Large, Thin Sheet of Scintillation Crystal• Can have a Collimator in Front of the Crystal• Array of Small Photomultiplier Tubes• Complex Electronics to Interpret Signal and Send to

Computer

Gamma Cameras Continued

• The Size of the Crystal can Vary Depending on the Application

• The More Photomultiplier Tubes in the Array, the Better the Spatial Resolution

• Position of the Incident Gamma Ray is Calculated Using the Signal Measured in the Surrounding Photomultiplier Tubes

Gamma Camera System Goals

• Design a System that can be Easily Integrated with a Standard Gamma Camera

• Develop a more Accurate Method for Determining the Location of a Source of Radiation

• Design a System that can be Used for Different Radioactive Sources

• Eliminate the Need for Moving Parts to Reduce the Chances of Failure

System Theory

• By Suspending a Lead Bar in Front of the Gamma Camera in a Known Location, a Portion of the Incident Radiation is Blocked

• When the Data Recorded by the Camera is Converted into an Image the Portion that was Partially Blocked will Appear as a Shadow

• The Direction of the Origin of the Radiation can be Found from this Image by Using some Transforms and Geometry

Lead Bar Design

• Lead Bar was Sized at 0.4 x 2 x 10 in.– 0.4 in. was the Thickness Required to Stop 99% of the

Radiation from the 10 μCi Cesium-137 Source Determined Using Lambert’s LawI(x)=I0e-αx where α = linear attenuation coefficient

x = length of materialand I0 = initial intensity

• The Length and Height were Chosen Because they Matched Standard Sizes from the Manufacturer, NucLead

Lead Bar Apparatus

• Standard Plumbing Pipes and Fittings were Used to Keep the Costs Low

• Thin L-Brackets were Used to Hold the Bar in Place to Minimize the Interference with the Final Image

• The Bar was Suspended 2 in. Away from the Camera Screen, with its Longest Side Horizontal, and with its Thinnest Face Oriented Up and Down– This Configuration Provided the Best Image

Apparatus Image

• The Metal Fittings in the Apparatus Blocked a Negligible Amount of the Radiation

Gamma Camera Procedure

• Measure the Background Radiation in the Test Room with no Source Present with a 600 sec. Scan

• Place the Lead Bar in the Apparatus and the Cesium-137 Source in a known Location

• Initiate a 600 sec. Scan to Record the Incident Radiation from the Source

• Import the Background and Source Scans into the Wolfram Mathematica Code

• Import the Mathematica Output File into the MATLAB Code as an Image and Determine Experimental Source Location

Scanning Software

Syngo:• Software Installed on Siemens

Workstation that is Used to Setup the Type and Length of a Scan

• Outputs a List of Energies and Positions of all Gamma Events

Siemens e.soft:• Directly Connected to the Gamma

Camera• Transmits the Information Recorded by

the Camera to the Syngo Workstation

Scanning Hardware

Siemens e.cam Gamma Camera:• Non-Production Test Model• 3/8 in. Thick NaI(Tl) Crystal• 16 x 22 in. Viewing Area• Array of 59 Photomultiplier

Tubes– 53 with a 3 in. Diameter– 6 with a 2 in. Diameter

Mathematica Code

• Originally Developed by Dr. John Engdahl in 2009• Creates a List of Gamma Events

– Subtracts Background Radiation– Removes Markers Placed in the List by e.soft– Eliminates Values Beyond 1 Standard Deviation of the

Mean• Outputs a Comma Separated Value File that Can

be Opened as a Matrix or Image

MATLAB Code

• Imports the CSV Fileas an Image

• Performs a Mean andMedian Filter to Image− 3 x 3 Filter Kernel

MATLAB Code Continued

• Finds Edges Using Built-in Canny Edge Finder

• Performs a Hough Transform to Find the Location of the Edges

Hough Transform

• Take all the Points in an Image and Transforms them into Parameter Space

• Each Point is Assigned a Line Equation Containing a Sine and Cosine Termxicosθ + yisinθ = ρ

• These Lines are Plotted Over a Specified Range of the Parameters ρ and θ

• The Intersections of the Sine and Cosine Lines Correspond to Line in Real Space– The Endpoints are Saved as Variables to be Used Later

Other Operations

• Saves Values for the Known Position of the Lead Bar• Adjusts the Values of the Endpoints of the Lines so

they are Relative to the Center of the Gamma Camera• Runs through Multiple If, Elseif, and Else Loops to

Determine which Points Correspond to which Value and what Side of the Screen the Shadow is on– The Hough Transform Assigns the Values by their Order in

Parameter Space and not Real Space– This Allows the same Naming System to be Used no

Matter where the Shadow is Projected

Source Trajectory

• Using the Endpoints Found through the Hough Transform, the Center of the Shadow is Determined

• This Point is Connected with a Point at the Center of Mass of the Bar to Create a Line

• The Experimental Source Location is at some Distance Along the Line– This Line can be Extended as far

as Needed

Sample Result

• The Images on Previous Slides came from a Scan with the Source Located at (12”, -4”, 60”) Relative to the Center of the Gamma Camera

• After the Analysis was Finished the Experimental Source Trajectory Passed the Actual Location by 11.6”

• Need to Take more Scans to see how the Source Location Affects the Result– Gamma Camera was Moved so it Needs to be Powered

on and Calibrated

Ideal Test Results

• Sketches were Created in Pro-E so Tests could be Performed Using Images with Ideal Edges

• 12 Mock Source Locations were Used and Images were Created by Projecting Lines from the Source

• The Same Code was Used to Analyze the Ideal Images– The Average Distance Away from the

Actual Source Location was 2.4 in.

Example Usage

• Shipping Container is Suspected of Containing Radioactive Material

• Someone could Search the Entire Container with a Geiger Counter by Hand– Could Take a Long Time and Puts People in Danger

• A Van Containing the Gamma Camera System could Park Next to the Container and Scan it– This would Narrow the Search Area Within the Container and

Determine the Radiation Level of the Source

Improvements

• Modify the Image Processing to get Better Edges from the Real Scan Images

• Find an Experimental 3D Point Source Location Instead of a Trajectory through Space

• Add Computer Codes to Completely Automate the Process

• Output the Average Energy Level of Gamma Rays so the Type of Radioactive Material can be Determined

References

• Gamma Cameras, David S. Graff Ph.D.• Attenuation of Radiation in Matter, Peter B. Siegel

Ph.D.• The Gamma Camera, UBC Physics & Astronomy• Interaction-Detectors, University of Waterloo• Detectors of Light, UC Davis University of California• Digital Image Processing, Rafael C. Gonzalez,

Richard E. Woods

Acknowledgments

• Senior Project Team– Keegan Roach– Derek Blunier– Eugene Kim

• Dr. John Engdahl• Bradley University• Fermilab

– Elaine G. McCluskey– Shelley A. Krivich– Bardeen Fellowship Committee

Summary

• About Me• Introduction to Scintillation Detectors• Scintillation Detector System

– Mechanical Design– Test Procedure– Final Results

• Introduction to Gamma Cameras• Gamma Camera System

– Theory– Test Procedure– Results

Questions?