Chapter 17

Radiation Detection and

Measurement

Presented by Mingxiong Huang, Ph.D.

mxhuang@ucsd.edu

Basic Concepts

Charged Particle Interacts with Detector:

Excitation, De-Excitation

Excitation

• Imparted E < B.E.

• e- at higher energy state

• 70% of all particulate

interactions are non-

ionizing

De-excitation with radiation

• Photon or Auger electron

• Imparted E > B.E.

• Ion pair results

• Secondary ionization

Charged Particle

Interacts with Detector:

Ionization

Photons Interact with Detector

Visible /

UV

lights

Detecting Radiation

Photon

or

particle Interaction

with detector

(small E)

Amplification

Electrical

signal

Interaction with detector:

Ionization

Excitation

Two modes of operation:

Pulse: Each interaction detected separately

Current: Net current due to summed interactions

Detector Properties

Dead time effect in Pulse Mode

•Current mode avoids dead time:

•e.g., CR, Image intensifier, CT, dose calibrator

•But info about individual interactions is lost

Spectroscopy

Pulse Height Spectrum

•Pulse mode: Amplitude is proportional to energy deposited

•Provides spectrum of energy deposited in detector

•E Not necessarily the same as the incident energy spectrum

Detector Efficiency (Sensitivity)

Efficiency = (Geometric Efficiency) x (Intrinsic Efficiency)

Efficiency = (# detected) / (# emitted)

Geometric Efficiency

Intrinsic Efficiency: Absorption Efficiency x Conversion Efficiency

Intrinsic Efficiency (Quantum Detection Efficiency)

of the Detection Material

Intrinsic Efficiency = 1 – e-μx = 1 –e-(μ/ρ)ρx

•μ is the linear attenuation coefficient of the material

•μ/ρ is the mass attenuation coefficient

•x is the thickness of the detector

•ρ is the density of the material

•Mass attenuation coefficient increases with the atomic

number of the material, but decreases with the photon

energy, with the exception of the absorption edge.

Gas-Filled Detectors

(Ionization chamber, GM counter, proportional counter)

Gas-Filled Detector for Charged Particles

(Ionization chamber, GM counter, proportional counter)

(thin wall)

incident

photon is

blocked

(thick wall)

Gas-Filled Detector for Charges Particles:

Applied voltage determines type of operation

No current

flows

Gas

multiplication

“Avalanche”

Portable air-filled ionization chamber

survey meter, maily in current mode

Measures

exposure rate

<mR/hr to R/hr

Also has

integrate mode

for exposure

(mR)

Port

able

air

-fill

ed io

niz

atio

n c

ha

mbe

r

su

rve

y m

ete

r, m

aily

in

cu

rre

nt

mo

de

Dose Calibrator, current mode, no dead-

time issue

Portable Geiger-Muller Survey Meter,

Pulse Mode, with Dead-time Problem

Thin window

“pancake” probe

Used to search

for contamination

or spills

Cannot really

measure

exposure (R), just

count rate (a few

hundred cps)

Paralyzable!

Scintillation Detectors, High Efficiency

Detectors for X-ray and Gamma-ray Photons

•Some common materials:

•NaI(Tl): Nuclear Medicine

•CsI: Image intensifiers, CT, DR

•Bi4Ge3O12: PET

•Gd2O2S(Tb): Radiographic screens

•ZnCdS(Ag): Image intensifier output phosphor

•CdWO4: CT

Scintillation Detector X-ray and Gamma-

ray Photons

Photons

Excitation and

Light emission

Light detection

by PMT, film,

photodiode

Electrical

signal

•Luminescence: fluorescence and phosphorescence

(afterglow)

•Desirable properties of a scintillation crystal (e.g., NaI(Tl)):

•high detection (absorption) efficiency

•high energy conversion efficiency

•short decay time of excited state (low afterglow)

•transparency to its own emissions

•emitted light matched to sensitivity of light detector

Photomultiplier Tube (PMT)

1. Convert visible / UV photons to electrical signal

2. Signal amplification (millions to billions)

scintillator

optical

coupler

reflector

evacuated

Photomultiplier Tube (PMT)

Scintillators with electron trapping

• Thermoluminescent Dosimeters (TLDs) – Heating releases trapped electrons

– Subsequent light emission proportional to absorbed energy

– LiF

– Personal monitoring, therapy dosimetry

• Photostimulable phosphors (PSPs) – Computed Radiography

– Laser releases trapped electrons, emitting light

– BaFBr

Semiconductor Crystal Detectors, mainly

for X-ray

Energy Bands

Difference is the magnitude of energy gap

Excitation to conduction

band by light, ionizing

radiation, heat

e-

(hole)

P-type and N-type impurities in a

semiconductor

mobile e- in

conduction band

mobile holes in

valence band

Semiconductor Diode

Equivalent to a solid

state ion chamber:

photodiode, diode

detector, good energy

resolution, need

cooling

Gas multiplication occurs in _____.

a. Geiger-Mueller counters

b. Scintillation detectors

c. Semiconductor detectors

d. Ionization chambers

e. Thermoluminescent dosimeters

Gas multiplication occurs in _____.

a. Geiger-Mueller counters

b. Scintillation detectors

c. Semiconductor detectors

d. Ionization chambers

e. Thermoluminescent dosimeters

Of the following, assuming each detector has the

same active volume, the most efficient detector

for x- and gamma-rays is a _____.

a. Geiger Mueller counter

b. NaI(Tl) detector

c. Single channel analyzer

d. Ionization chamber

e. Dose calibrator

Of the following, assuming each detector has the

same active volume, the most efficient detector

for x- and gamma-rays is a _____.

a. Geiger Mueller counter

b. NaI(Tl) detector

c. Single channel analyzer

d. Ionization chamber

e. Dose calibrator

Which of the following instruments does NOT suffer

from dead-time effects?

a. Geiger-Mueller counter survey meter

b. NaI(Tl) well counter

c. PET camera

d. Scintillation camera

e. Dose calibrator with ion chamber detector

Which of the following instruments does NOT suffer

from dead-time effects?

a. Geiger-Mueller counter survey meter

b. NaI(Tl) well counter

c. PET camera

d. Scintillation camera

e. Dose calibrator with ion chamber detector

Pulse Height Spectroscopy

Single Channel Analyzer (SCA) System

Cable short as possible

Energy Discrimination

A logic pulse is

transmitted when

conditions of UL/LL

are met.

Calibration

(peaking) involves

matching V to E.

Multi-Channel Analyzer (MCA)

Multi-Channel Analyzer System

Calibration

similar to SCA

Sample assays are better with semiconductor detectors due to superior

energy resolution. High detection efficiency of NaI is preferred for imaging.

Acquisition of a Spectrum

X-ray and Gamma-Ray Spectroscopy with

Sodium Iodide Detectors

Photoelectric Effect (Absorption from Chap 3)

• The products of interaction are:

– 1. Photoelectron (ejected electron)

– 2. Positive ion (remaining atom)

– 3. Characteristic radiation (discrete x-rays emitted when electron drops to fill vacant shells) or Auger electrons

• Original photon disappears (absorption).

• X-ray energy is unique to element (characteristic)

• Most interactions in NaI detector are with Iodide

53I

• Occurs for loosely bound

electrons with negligible B.E.

• Source of undesirable scatter

radiation that reduces S/N

• Input: photon

Output: photon + electron

• h·inc = h·scat + K.E. e-

• Scattered photon: 0° 180°

• Scattered electron: 0° φ 90°

Compton Scattering

(increase the

background, Chap 3)

φ

Interactions of x-rays and gamma rays

with a detector

A - Photoelectric (PE)

B - Compton + PE

C - Compton + escape

of scattered photon

D - PE + loss of

characteristic x-ray

E - Compton scatter

from the shield

F – PE + Characteristic

x-ray from the shield

Shield needed to reduce background radiation.

Spectrum of Cesium-137 A – 662 keV PE, total

absorption

B - Compton + escape

C - Compton edge

D – Backscatter from the

shield

E – 32 keV PE, Barium K-

shell x-rays after internal

conversion electron

F – PE with lead shield, K-

shell x-rays from shield 72-88

keV

90% gamma-ray

10% internal conversion

Spectrum of Technetium-99m

A - PE, total absorption

B - PE with Iodine K-shell

x-ray escape (28-33 keV)

C – Lead K-shell x-rays from

the shield

Low Compton plateau since

PE dominates in Iodine at 140

keV

89% isomeric decay

11% internal conversion

Spectrum of Iodine-125—Sum Peak

Sum peak

%100Peak ofCenter at Height Pulse

FWHMResolutionEnergy

Energy Resolution

NaI Energy Resolution of Cs-137 (662 keV) = 7-8%

Pulse Pileup

Non-Imaging Detectors

Thyroid Probe System

Thyroid probe system

• For measuring the uptake of I-123 or I-131 by

the thyroid glands of patients.

• Connects to an SCA or MCA

• Two-capsule method (one capsule placed in a

neck phantom (“Standard”), one capsule

swallowed by the patient.

• One-capsule method

• Initial counts, then measures at 4-6 hours after

the administration, and 24 hours after.

• Measurements at the patient’s thyroid and

distal thigh for non-thyroidal activity.

Thyroid Uptake Measures

Two-capsule method (accurate, more measures, higher cost):

phantomin capsulepatient ofcount Initial

phantomin standard ofcount Initial

count Backgroundphantomin standard ofCount

countThigh count ThyroidUptake

One-capsule method (fewer measures, lower cost, subject to instability,

technologist error, and dead-time effects):

2/1/693.0

count Backgroundphantomin capsule ofCount (Initial)

countThigh count ThyroidUptake

Tte

Th

yro

id P

robe

Syste

m

Sodium Iodide Well Counter

• Used for Schilling test (a test of

vitamin B12 absorption), Plasma or

blood cell volume determination,

Radioimmunoassays.

• High geometric efficiency, High

sensitivity,1 nCi activity.

• It usually connects with an SCA or

MCA.

• Suffer from dead-time, sum peak,

Not used for activity exceeding

5,000 cps (~1.35 x 10-7 Ci = 135

nCi).

• Quality Assurance, see Page 658.

Automatic Gamma Well Counter

Automatic Gamma Well Counter

Dose Calibrator

Nuclear Regulatory Commission

(NRC) Quality Assurance:

•Accuracy on installation and

annually thereafter

•Linearity on installation and

quarterly thereafter

•Constancy before its first use

daily

•Test for Geometry Dependence

on installation

• Gas-filled, ionization

chamber

• Operated in current mode,

not subject to dead-time

effects.

• It can accurately assay

activities as large as 2 Ci

Dose Calibrator

Dose Calibrator: testing for linearity

Molybdenum-99 Concentration Testing

•Mo-99 / Tc-99m beta-minus decay,

•Radiation dose: emitting high energy beta particles with 66-hour

half-life

•High-energy gamma rays will degrade resultant images

•NRC requirement: M0-99 < 0.15 μCi

• The wall of lead container are sufficiently thick to stop all

gamma rays from Tc-99m (140 keV), but thin enough to be

penetrated by many high-energy gamma rays from Mo-99 (740

and 778 keV).

•First, empty lead container is first assayed in the dose calibrator

•Second, the vial of Tc-99m alone is assayed.

K is a correction factor

Example, Complicated Beta-minus Decay

Self-reading: Sr-82 and Sr-85

concentration test

Counting Statistics

Binomial Probability Distribution

Gaussian Probability Distribution

Counting Statistics

Basic rules:

1. For N counts:

(only true when binominal approaches Gaussian)

2. Fractional error =

3. Confidence Intervals: Interval about measurements Probability that mean is within interval (%)

±0.674σ 50.0

±1σ 68.3

±1.64σ 90.0

±1.96σ 95.0

±2.58σ 99.0

±3σ 99.7

)( NorN

NN

N

N

1

Fractional Errors

Propagation of Error Equations

Operation Standard deviation

cN cσ

N/c σ/c

N1+N2

N1-N2

12 2

2

12 2

2

Example:

Sample counts: 1600 over 5 min

Background counts: 900 over 5 min

What is the mean and fractional error (uncertainty) of the

actural sample count rate (in cts/min)?

N1 = 1600cts for 5min, N2 = 900cts for 5min,

σ1 = 40cts for 5min, σ2 = 30cts for 5min,

N = N1-N2 = 700cts for 5min

= 50cts for 5min

Answer: 140 +/- 10 cts/min, 7.1% fractional error

12 2

2