Radiation Detectors - Overv

7/6/2010 
Introduction to Radiation Detectors

Two Categories of Detector Systems

1. Passive Integrating Detector Systems

- do not require power during deployment- do not record individual interactions of radiat- might or might not need to be processed

- cannot provide real time readout (usually)

cannot provide real time readout (usually)

- most passive detectors are dosimeters- e.g., TLD, film dosimeter

2. Active Detector Systems

- require power- real time readout- usually register individual interactions of rad

- e.g., GM detector
Passive Integrating Detec

Systems

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General

• Radiation interactions produce some type of chan

“detector.” 

• The accumulated change due to multiple interacti

radiation is determined at the end of the deploym

Passive Integrating Detector System• The accumulated change is related to the quantity 

interest.

• Primarily used as dosimeters to measure dose.

• Might be used to assess the average level of radi

during deployment, e.g., radon concentration.
Examples 

Detector Effect Related to 

Exposure

Principle TypeRadiation Ass

Thermoluminescent dosimeters (TLD)

Intensity of emitted light when heated

Gamma rays, neutrons

Optically Stimulated Luminescence (OSL) 

dosimeter

Intensity of emitted light when exposed to light

Gamma rays, 

Passive Integrating Detector System

dosimeterFilm dosimeters Darkening (OD) of film Gamma rays, Pocket dosimeters (electroscope)

Displacement of fiber due to decrease in charge

Gamma rays, neutrons

Bubble dosimeters Number of bubbles 

produced

Neutrons

Electrets Decrease in 

voltage/charge

Radon, gamm

Chemical dosimeters Change in color Gamma rays, DIS dosimeters Change in current Gamma rays, 
Active Detector Systems

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System Components

Systems operating in the pulse mode consists of a:

1. Detector2. Pulse processing device3 Pulse analysis device

Active Detector Systems

. Pulse analysis device

Simplified block diagram:

Detector Pulse Processor 

A
1. Detector

• An electronic pulse is produced each time a "part

radiation (alpha, beta, gamma, neutron, etc.) intethe detector. The greater the intensity of the radia

more pulses per unit time.

• With many detectors (e g proportional counter NActive Detector Systems

With many detectors (e.g., proportional counter, Ndetector) the greater the energy deposited in the the larger the pulse size. 

• With some detectors (e.g., GMs), the size of the p

related to the energy deposited.

• If the pulses are very small and difficult to distingu

electronic noise, the system might measure the cthe detector rather than analyze the individual pu
1. Detector

Three categories of active detectors:

• Gas detectors (three types)• Scintillation detectors (many types)• Semiconductor detectors (three types)

Active Detector Systems

Semiconductor detectors (three types)
1. Detector

Types of Gas Detectors 

Detector Principle Types of Radiation As

Ion Chamber Gamma rays, x-raysProportional 

C t

Alphas, betas, neutrons

Active Detector Systems

Counter

Geiger-Mueller (GM)Detector

Gamma rays, x-rays, betas
1. Detector 

Examples of Scintillation Detectors 

Detector Principle Types of Radiation 

Sodium Iodide (NaI) Gamma rays, x-raysLanthanum Bromide or 

Chloride (LaBr3, LaCl3)

Gamma rays, x-rays

Active Detector Systems

Chloride (LaBr3, LaCl3)Cesium Iodide (CsI) Gamma rays, x-raysPlastic Gamma rays, x-rays, betasZinc Sulfide (ZnS) AlphasLiquid Scintillation (LSC) Betas, alphasLutetium Oxyorthosilicate (LSO)

Annihilation photons

Bismuth Germanate (BGO) Annihilation photonsGlass (lithium, cerium) Neutrons
1. Detector 

Types of Semiconductor Detectors 

Detector Principle Types 

Radiation Asses

Diode alphas, betas

Active Detector Systems

Lithium Drifted:

Silicon (SiLi)Germanium (GeLi)

x-raysgamma rays

High Purity Germanium (HPGe) gamma rays, x-rays
2. Pulse Processor 

• Usually a preamplifier and linear amplifier. • Less commonly, a biased amplifier might be emp• Some detectors (e.g., GM detectors) might not re

amplifiers

Active Detector Systems

. 

• Linear Amplifiers:

- increase the size of all the pulse by the same fa

gain. 

- change the shape of the pulses. In doing so, th

filter out noise.
2. Pulse Processor 

• Biased amplifiers:

- apply a linear gain to those pulses coming out o

linear amplifier above a specific amplitude (the level). 

Active Detector Systems

- this eliminates pulses from the linear amplifier b

certain size. 

- the remaining pulses are spread over the entire 

spectrum. 

- most common application is in alpha spectrosc
3. Pulse Analysis

• Threshold

- Pulses smaller than the threshold setting are as

be electronic noise and are not analyzed. 

The appropriate threshold setting depends on t

Active Detector Systems

- The appropriate threshold setting depends on t

magnitude of the noise and the size of the radiagenerated pulses. This depends on the detector 

- Since GM detector pulses are large, a high thre

setting can be used, e.g., 30 - 300 mV. 

- Some pulses produced by scintillators and prop

counters are small. As such, these detectors emmuch lower threshold, e.g., 5 mV. 
3. Pulse Analysis

• Threshold cont.

- If different types of detector might be connecte

same rate meter or scaler, a compromise thresbe used, e.g., 30 mV. 

Active Detector Systems

- The threshold is often set at the factory and no

changed. However, with some meters the threbe displayed and adjusted without having to opinstrument. 

- With so-called "smart" systems, the meter can 

the probe to which it is connected and adjust ththreshold accordingly. 
3. Pulse Analysis

• Window

- In some situations, it can be advantageous to l

analysis to radiation-generated pulses in a certrange.

Active Detector Systems

- Some meters have two “windows” that allow th

select the size of the pulses to be counted (e.gpulses, large pulses, or small and large pulses

- Other systems employ upper and lower level 

discriminators that screen out pulses above anuser specified size. The threshold can be conszero adjust for the lower level discriminator.
3. Pulse Analysis

Pulse analysis can involve:

a. Counting the pulsesb. Pulse height analysis

c Pulse shape (duration) analysis

Active Detector Systems

. Pulse shape (duration) analysis
3. Pulse Analysis

a. Counting the Pulses

The count or count rate can be related to the:

• exposure rate (e.g., mR/hr) 

• dose rate (e g mrad/hr mGy/hr)

Active Detector Systems

dose rate (e.g., mrad/hr, mGy/hr)

• dose equivalent rate (e.g., mrem/hr, mSv/• activity of a source (e.g., dpm, Bq)

The device used to count the pulses is either a:

• rate meter, or• scaler
3. Pulse Analysis

a. Counting the Pulses – Count Rate Meter

• For our purposes, a count rate meter reads ou

(or cps) or has been calibrated to convert the into a quantity such as exposure rate (mR/hr) 

equivalent rate (mrem/hr mSv/hr)

Active Detector Systems

equivalent rate (mrem/hr, mSv/hr). 

• Count rate meters employ an analog display (

bouncing needle) or a digital display wherein tdisplayed count rate is "updated“ every secon

• When used to search for radiation/radioactivity

rate meter is usually set to a "fast response.” response is preferred when accurate measuredesired. 
3. Pulse Analysis

a. Counting the Pulses - Scaler

• Scalers perform counts over a selected time (e

• Primary applications: laboratory analyses, me

alpha and beta surface contamination. 

Active Detector Systems

• Scalers permit the count to be read from an un

digital display. Nevertheless, their main advanability to measure smaller increases above bathan can be detected with count rate meters.

• Some combination instruments can function e

ratemeter or as a scaler.
3. Pulse Analysis

b. Pulse Height Analysis

• If the detector pulse height is related to the am

energy deposited in the detector by the radiatiheight analysis (PHA) can be used to determin

energy of the radiation

Active Detector Systems

energy of the radiation. 

• Since the energy of radiation is characteristic 

radionuclide that produced it, pulse height anabe used to identify the radionuclide(s). 

• The process by which we measure the energy 

radiation is called spectroscopy. 
3. Pulse Analysis

b. Pulse Height Analysis

• The number of pulses displayed as a function 

height (or energy) is a spectrum.

Gamma Spectrum Alpha Spe

Active Detector Systems

Number of Pulses

Pulse Height

Number of Pulses

Pulse Heig
3. Pulse Analysis

b. Pulse Height Analysis

• A very simple form of pulse height analysis mi

used to identify the type of radiation responsib

pulse. 

Active Detector Systems

• For example, alpha pulses produced by propo

counters are usually larger than pulses producbeta particles. By dividing the pulses into two categories, it is possible to obtain simultaneouseparate, measurements of alpha and beta ac
3. Pulse Analysis

c. Pulse Shape Analysis

• Pulses produced by different types of radiation 

alphas and betas) can differ in shape. As sucshape analysis might be used to distinguish th

di ti t

Active Detector Systems

radiation types.

• The pulse shapes might differ because the pu

time reflects the distribution within the detectoof the deposited radiation energy and this disvaries for different types of radiation. 
3. Pulse Analysis

c. Pulse Shape Analysis

• Pulse shape analysis can also be utilized to p

overlapping pulses from being misinterpreted large pulse. This process is referred to as pile

j ti

Active Detector Systems

rejection.

• Individual "legitimate" single pulses coming ou

amplifier have a near-gaussian shape unlike opulses which can appear distorted.