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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


Introduction l.jpg

  • 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.

Detector characteristics l.jpg
Detector characteristics

  • Sensitivity of the detector

  • Energy Resolution of the detector

  • Time resolution of the detector or itgs pulse resolving time

  • Detector efficiency

Summary of detector types l.jpg
Summary of detector types

  • Gas Ionization

  • Ionization in a Solid (Semiconductor detectors)

  • Solid Scintillators

  • Liquid Scintillators

  • Nuclear Emulsions

Detectors based on gas ionization l.jpg
Detectors based on gas ionization

  • Ion chambers

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

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


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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

Semiconductor radiation detectors l.jpg
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

P n junction l.jpg
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


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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

Liquid scintillators l.jpg
Liquid Scintillators

  • Dissolve radioactive material in the scintillator

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

  • Used to count low energy 

  • Quenching

Inorganic scintillators nai tl l.jpg
Inorganic scintillators (NaI (Tl))

Emission of light by activator center

Nai tl l.jpg

  • Workhorse gamma ray detector

  • Usual size 3” x 3”

  • 230 ns decay time for light output

  • Other common inorganic scintillators are BaF2, BGO

Distribution functions l.jpg
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!


    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

Interval distribution l.jpg
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

Slide41 l.jpg

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