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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)
- 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

- 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

- Gas Ionization
- Ionization in a Solid (Semiconductor detectors)
- Solid Scintillators
- Liquid Scintillators
- Nuclear Emulsions

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

Uses of Ion Chambers

- High radiation fields (reactors) measuring output currents.
- Need for exact measurement of ionization (health physics)
- Tracking devices

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.

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

Practical aspects

gas flow

typical gas: P10,

90% Ar,

10% methane

Sensitive to ,, X-rays, charged particles

Fast response, dead time ~ s

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

- “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

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”.

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.

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

Types of Si detectors

- Surface barrier, PIN diodes, Si(Li)
- Surface barrier construction

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

PIN diodes

- Cheap
- p-I-n sandwich
- strip detectors

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

Ge detectors

- Detectors of choice for detecting -rays
- Superior resolution

Scintillation detectors

- Energy depositlightsignal
- Mechanism (organic scintillators)

Note that absorption and re-emission have different spectra

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

- 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))

Emission of light by activator center

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

Distribution functions

Most general distribution describing radioactive decay

is called the Binomial Distribution

n=# trials, p is probability of success

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

Example of use of statistics

- Consider data of Table 18.2
- mean = 1898
- standard deviation, , = 44.2 where

For Poisson distribution

Interval distribution

Counts occur in “bunches”!!

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

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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