Transcript Lesson 17 - Oregon State University

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.
The Result
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
Semiconductor physics
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
QuickTime™ and a
decompressor
are needed to see this picture.
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 depositlightsignal
• 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
NaI detector operation
Nuclear electronics
Nuclear statistics
Table 18-2 Typical Sequence of
Counts of a long-Lived Sample
(170Tm)*
Measurement
cp0.1m
xi-xm
Number
1
1880
-18
2
1887
-11
3
1915
17
4
1851
-47
5
1874
-24
6
1853
-45
7
1931
33
8
1886
-32
9
1980
82
10
1893
-5
11
1976
78
12
1876
-22
13
1901
3
14
1979
81
15
1836
-62
16
1832
-66
17
1930
32
18
1917
19
19
1899
1
20
1890
-8
*We are indebted to Prof. R.A. Schmitt for providing these data.
(xi-xm)2
324
121
289
2209
576
2025
10899
1024
6724
25
6084
484
9
6561
3844
4536
1024
361
1
64
Distribution functions
Most general distribution describing radioactive decay
is called the Binomial Distribution
x
n-x
P(x)=(n!/((n-x)!x!)p (1-p)
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
N
2 
x  x 
2
i
i 1
N 1
For Poisson distribution

m
Gaussian (normal) distribution
P(x) 

2 

x  xm 
1


exp



2x
2xm
m


Interval distribution
1
I(t)  exp( t / tm )dt
tm
Counts occur in “bunches”!!

Operation
Answer
Uncertainty
Addition
A+B
(A2+B2)1/2
Subtraction
A-B
(A2+B2)1/2
Multiplication A*B
A*B((A/A)2+(B/B)2)1/2
Division
A/B
A/B((A/A)2+(B/B)2)1/2
Simple statistics
Uncertainties for some common operations
Operation
Addition
Subtraction
Multiplication
Division
Answer
A+B
A-B
A*B
A/B
Uncertainty
(σA2+σB2)1/2
(σA2+σB2)1/2
A*B((σA/A)2+(σB/B)2)1/2
A/B((σA/A)2+(σB/B)2)1/2