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Lesson 17. Detectors. Introduction. When radiation interacts with matter, result is the production of energetic electrons. (Neutrons lead to secondary processes that involve charged species)

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Lesson 17 l.jpg

Lesson 17

Detectors


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Introduction

  • When radiation interacts with matter, result is the production of energetic electrons. (Neutrons lead to secondary processes that involve charged species)

  • Want to collect these electrons to determine the occurrence of radiation striking the detector, the energy of the radiation, and the time of arrival of the radiation.


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Detector characteristics

  • Sensitivity of the detector

  • Energy Resolution of the detector

  • Time resolution of the detector or itgs pulse resolving time

  • Detector efficiency


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Summary of detector types

  • Gas Ionization

  • Ionization in a Solid (Semiconductor detectors)

  • Solid Scintillators

  • Liquid Scintillators

  • Nuclear Emulsions


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Detectors based on gas ionization

  • Ion chambers

35 eV/ion pair>105 ion pairs created.

Collect this charge using a capacitor, V=Q/C

NO AMPLIFICATION OF THE PRIMARY IONIZATION


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Uses of Ion Chambers

  • High radiation fields (reactors) measuring output currents.

  • Need for exact measurement of ionization (health physics)

  • Tracking devices


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Gas amplification

  • If the electric fields are strong enough, the ions can be accelerated and when they strike the gas molecules, they can cause further ionization.



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Proportional counters

  • Gas amplification creates output pulse whose magnitude is linearly proportional to energy deposit in the gas.

  • Gas amplification factors are 103-104.

  • Will distinguish between alpha and beta radiation


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Practical aspects

gas flow

typical gas: P10,

90% Ar,

10% methane

Sensitive to ,, X-rays, charged particles

Fast response, dead time ~ s


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Geiger- Müller Counters

  • When the gas amplification factor reaches 108, the size of the output pulse is a constant, independent of the initial energy deposit.

  • In this region, the Geiger- Müller region, the detector behaves like a spark plug with a single large discharge.

  • Large dead times, 100-300µs, result

  • No information about the energy of the radiation is obtained or its time characteristics.

  • Need for quencher in counter gas, finite lifetime of detectors which are sealed tubes.

  • Simple cheap electronics


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Semiconductor Radiation Detectors

  • “Solid state ionization chambers”

  • Most common semiconductor used is Si. One also uses Ge for detection of photons.

  • Need very pure materials--use tricks to achieve this



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p-n junction

Create a region around the p-n junction

where there is no excess of either n or p

carriers. This region is called the “depletion

region”.


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Advantages of Si detectors

  • Compact, ranges of charged particles are µ

  • Energy needed to create +- pair is 3.6 eV instead of 35eV. Superior resolution.

  • Pulse timing ~ 100ns.


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Ge detectors

  • Ge is used in place of Si for detecting gamma rays.

  • Energy to create +- pair = 2.9 eV instead of 3.6 eV

  • Z=32 vs Z=14

  • Downside, forbidden gap is 0.66eV, thermal excitation is possible, solve by cooling detector to LN2 temperatures.

  • Historical oddity: Ge(Li) vs Ge


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Types of Si detectors

  • Surface barrier, PIN diodes, Si(Li)

  • Surface barrier construction


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Details of SB detectors

  • Superior resolution

  • Can be made “ruggedized” or for low backgrounds

  • Used in particle telescopes, dE/dx, E stacks

  • Delicate and expensive


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PIN diodes

  • Cheap

  • p-I-n sandwich

  • strip detectors


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Si(Li) detectors

  • Ultra-pure region created by chemical compensation, i.e., drifting a Li layer into p type material.

  • Advantage= large depleted region (mm)

  • Used for -detection.

  • Advantages, compact, large stopping power (solid), superior resolution (1-2 keV)

  • Expensive

  • Cooled to reduce noise


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Ge detectors

  • Detectors of choice for detecting -rays

  • Superior resolution


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Scintillation detectors

  • Energy depositlightsignal

  • Mechanism (organic scintillators)

Note that absorption and re-emission have different spectra


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Organic scintillators

  • Types: solid, liquid (organic scintillator in organic liquid), solid solution(organic scintillator in plastic)

  • fast response (~ ns)

  • sensitive (used for) heavy charged particles and electrons.

  • made into various shapes and sizes


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Liquid Scintillators

  • Dissolve radioactive material in the scintillator

  • Have primary fluor (PPO) and wave length shifter (POPOP)>

  • Used to count low energy 

  • Quenching


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Inorganic scintillators (NaI (Tl))

Emission of light by activator center


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NaI(Tl)

  • Workhorse gamma ray detector

  • Usual size 3” x 3”

  • 230 ns decay time for light output

  • Other common inorganic scintillators are BaF2, BGO





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Distribution functions

Most general distribution describing radioactive decay

is called the Binomial Distribution

n=# trials, p is probability of success


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Poisson distribution

  • If p small ( p <<1), approximate binomial distribution by Poisson distribution

    P(x) = (xm)x exp(-xm)/x!

    where

    xm = pn

  • Note that the Poisson distribution is asymmetric


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Example of use of statistics

  • Consider data of Table 18.2

  • mean = 1898

  • standard deviation, , = 44.2 where

For Poisson distribution



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Interval distribution

Counts occur in “bunches”!!


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Table 18-3. Uncertainties for some common operationsOperation Answer UncertaintyAddition A+B (σA2+σB2)1/2Subtraction A-B (σA2+σB2)1/2Multiplication A*B A*B((σA/A)2+(σB/B)2)1/2Division A/B A/B((σA/A)2+(σB/B)2)1/2



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Uncertainties for some common operationsOperation Answer UncertaintyAddition A+B (σA2+σB2)1/2Subtraction A-B (σA2+σB2)1/2Multiplication A*B A*B((σA/A)2+(σB/B)2)1/2Division A/B A/B((σA/A)2+(σB/B)2)1/2


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