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Measurement of Ionizing Radiation

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Introduction to Health Physics
Chapter 9
Health Physics Instrumentation
RADIATION DETECTORS
• Instruments used in the practice of health
physics serve a wide variety of purposes
• one finds instruments designed specifically for
the measurement of a certain type of
radiation, such as low-energy X-rays, highenergy gamma rays. fast neutrons, and so on
RADIATION DETECTORS
• The basic requirement of any such instrument
is that its detector interact with the radiation
in such a manner that the magnitude of the
instrument's response is proportional to the
radiation effect or radiation property being
measured
Radiation Measurement Principles
Signal дїЎи™џ Amplification
Physical 物理
Chemical еЊ–е­ё
Biological 生物
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Reader
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Calibration
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RADIATION DETECTORS
PARTICLE-COUNTING INSTRUMENTS
• Gas-Filled Particle Counters
– variable voltage source V, high-valued resistor R
– a gas-filled counting chamber D, which has two
coaxial electrodes that are very well insulated
from each other
– All the capacitance associated with the circuit is
indicated by the capacitor C
PARTICLE-COUNTING INSTRUMENTS
• Gas-Filled Particle Counters
– If the time constant RC of the detector circuit is
much greater than the time required for the
collection of all the ions resulting from the
passage of a single particle through the detector,
then a voltage pulse of magnitude
PARTICLE-COUNTING INSTRUMENTS
• Gas-Filled Particle Counters
– A broad-output pulse would make it difficult to
separate successive pulses.
– if the time constant of the detector circuit is made
much smaller than the time required to collect all
the ions. This pulse, allows individual pulses to be
separated and counted
Gas-Filled Particle Counters
• Ionization Chamber Counter
– the range of voltage great enough to collect the
ions before a significant fraction of them can
recombine yet not great enough to accelerate the
ions sufficiently to produce secondary ionization
by collision
– The exact value of this voltage is a function of the
type of gas, the gas pressure, and the size and
geometric arrangement of the electrodes
Gas-Filled Particle Counters
• Ionization Chamber Counter
– the number of electrons collected by the anode
will be equal to the number produced by the
primary ionizing particle
– the gas amplification factor is equal to one
– The pulse size from a counter depends on the
number of ions produced in the chamber makes it
possible to use this instrument to distinguish
between radiations of different specific ionization
Gas-Filled Particle Counters
• Ionization Chamber Counter
– disadvantages : the relatively feeble output pulse
Gas-Filled Particle Counters
• Proportional Counter
– As the voltage across the counter is increased
beyond the ionization chamber region, a point is
reached where secondary electrons are produced
by collision. This multiplication of ions in the gas,
which is called an avalanche
– The output voltage pulse is proportional to the
high voltage across the detector
– The pulse size dependence on ionization for the
purpose of distinguishing between radiations
Gas-Filled Particle Counters
• Proportional Counter
– The gas amplification factor is greater than one
– to use a very stable high-voltage power supply
– the gas amplification depends on
• the diameter of the collecting electrode
• the gas pressure
Gas-Filled Particle Counters
• Geiger Counter
– increase the high voltage beyond the proportional
region will eventually cause the avalanche to
extend along the entire length of the anode
– the size of all pulses - regardless of the nature of
the primary ionizing particle- is the same
– When operated in the Geiger region, therefore, a
counter cannot distinguish among the several
types of radiations
Geiger-Muller Counter
Gas-Filled Particle Counters
• Geiger Counter
– avalanche
ionization
Gas-Filled Particle Counters
• Quenching a Geiger Counter
– After the primary Geiger discharge is terminated, the
positive ions slowly drift away from the anode wire and
ultimately arrive at the cathode or outer wall of the counter.
Here they are neutralized by combining with an electron
from the cathode surface. In this process, an amount of
energy equal to the ionization energy of the gas minus the
energy required to extract the electron from the cathode
surface (the work function) is liberated. If this liberated
energy also exceeds the cathode work function, it is
energetically possible for another free electron to emerge
from the cathode surface---and thereby produce a spurious
count
Gas-Filled Particle Counters
• Quenching a Geiger Counter
– Prevention of such spurious counts is called
quenching
– External quenching
• electronically, by lowering the anode voltage
after a pulse until all the positive ions have
been collected
– Internal quenching
• chemically, by using a self-quenching gas
Resolving Time
• The negative ions, being electrons, move very rapidly
and are soon collected, while the massive positive
ions are relatively slow-moving and therefore travel
for a relatively long period of time before being
collected
• These slow-moving positive ions form a sheath
around the positively charged anode, thereby greatly
decreasing the electric field intensity around the
anode and making it impossible to initiate an
avalanche by another ionizing particle. As the positive
ion sheath moves toward the cathode, the electric
field intensity increases, until a point is reached when
another avalanche could be started
Resolving Time
• dead time
– The time required to attain this electric field
intensity
• recovery time
– the time interval between the dead time and the
time of full recovery
• resolving time
– The sum of the dead time and the recovery time
Resolving Time
• dead time, recovery time, resolving time
Resolving Time
• Measurement of Resolving Time
• the "true“ counting rate
• the observed counting rate of a sample is R0
Scintillation Counters
• A scintillation detector is a transducer that
changes the kinetic energy of an ionizing
particle into a flash of light
Scintillation Counters
• Whereas the inherent detection efficiency of
gas-filled counters is close to 100% for those
alphas or betas that enter the counter, their
detection efficiency for gamma rays is very
low-usually less than 1%
• Solid scintillating crystals have high detection
efficiencies for gamma rays
Scintillation Counters
Scintillation Counters
scintillator
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Photomultiplier
Tube й›»еўћз®Ў
PM Tube
Semiconductor Detector
• A semiconductor detector acts as a solid-state
ionization chamber
• The operation of a semiconductor radiation
detector depends on its having either an
excess of electrons or an excess of holes.
• A semiconductor with an excess of electrons
is called an n-type semiconductor, while one
with an excess of holes is called a p-type
semiconductor
Semiconductor Detectors
DOSE-MEASURING INSTRUMENTS
• Radiation flux VS radiation dose rate
– Example 9.2
• Consider two radiation fields of equal energy density. In
one case, we have 0.1-MeV photon flux of 2000 photons
per cm^2/s. In the second case, the photon energy is 2MeV and the flux is 100 photons per cm^2/s. The energy
absorption coefficient for muscle for 0.1-MeV gamma
radiation is 0.0252 cm^2/g; for 2-MeV gamma the energy
absorption coefficient is 0.0257 cm^2/g. The dose rates
for the two radiation fields are given by:
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з« ж‰‹жЏђеЃµжЄўе™Ё
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Ionization Chamber Dosimeter
Personal Pen Dosimeter
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Diagnostic IC
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Therapeutic IC
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Survey meter
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OD пЂЅ log
L0
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OD
Film Dosimeters
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( L0 пЂЅ 10 Lt )гЂЂOD пЂЅ 1
гЂЂгЂЂ
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( L0 пЂЅ 100 Lt )гЂЂOD пЂЅ 2
Dose (log)
Thermoluminescent Dosimeters
Glow curve
(з™је…‰ж›Із·љ)
Energy Dependence
Film
TLD
DOSE-MEASURING INSTRUMENTS
• Electronic Dosimeters
– employ solid-state
semiconductors, silicon
diodes, to detect beta and
gamma radiation over a
very wide range of dose
rates and doses
– measure and display
instantaneous dose rate
and integrate over time
NEUTRON MEASUREMENTS
• Detection Reactions
– Neutrons, like gamma rays, are not directly
ionizing; they must react with another medium
to produce a primary ionizing particle
– Because of the strong dependence of neutron
reaction rate on the cross section for that
particular reaction,
• use different detection media, depending on the
energy of the neutrons that we are trying to measure,
• modify the neutron energy distribution so that it will be
compatible with the detector
NEUTRON MEASUREMENTS
• Detection Reactions
– 10B(n,a)7L
• either as BF3 gas or as a thin film on the
inside surfaces of the detector tube
• The ionization due to the alpha particle and
the 7Li recoil nucleus is counted
– Elastic scattering of high-energy neutrons by
hydrogen atoms. (scattered proton)
– Nuclear fission: fissile material (n,f) fission
fragments
– Neutron activation: threshold detectors
NEUTRON MEASUREMENTS
• Neutron Dosimetry
– The dose equivalent (DE) from neutrons
depends strongly on the energy of the neutrons,
We therefore cannot simply convert neutron flux
into dose equivalent unless we know the energy
spectral distribution of the neutrons
– Commercially available neutron dose-equivalent
meters, utilize a thermal neutron detector
surrounded by a spherical or semispherical
moderator
NEUTRON MEASUREMENTS
• Neutron Dosimetry
– Commercially available neutron dose-equivalent
meters
NEUTRON MEASUREMENTS
• Neutron Dosimetry
– Bubble Dosimeter
• completely unresponsive to
gamma radiation
• allowing calibration and
readout directly in
microsieverts or in millirems of
neutron dose
• The number of bubbles is
directly proportional to the
neutron-equivalent dose.
• 相互認可協議 (Mutual Recognition Arrangement, MRA)
C IPM 關 鍵 比 對 (key com parisons)與 區 域 性 關 鍵 比 對 (regional key
com parisons)гЂЃ й›™ й‚Љ й—њ йЌµ жЇ” е°Ќ (bilateral com parisons)зљ„ зµђ еђ€ ж–№ ејЏ
пјљ еЏѓ еЉ C IPM й—њ йЌµ жЇ” е°Ќ зљ„ ењ‹ 家 еЇ¦ й©— 室
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пјљ еЏѓ еЉ C IPM 與 еЌЂ еџџ жЂ§ жЇ” е°Ќ зљ„ ењ‹ 家 еЇ¦ й©— 室
:其他國家實驗室
пјљй›™й‚Љй—њйЌµжЇ”е°Ќ
• 量測追溯體系Traceability of Measurement
ењ‹йљ›еє¦й‡ЏиЎЎе±Ђ(BIPM)
International Bureau of Weights & Measures
各國家標準實驗室
大陸
NIM
и‹±ењ‹
NPL
еѕ·ењ‹
PTB
確保輻射量的量測一致性
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游離輻射國家標準
實驗室 NRSL
зѕЋењ‹
NIST
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ARPANSA
жЄў ж Ў еЇ¦ й©— 室
最終使用者
其他行業
иЈЅйЂ е·Ґе» ж ёиѓЅй›»е» й†«з”ЁиЁєз™‚й™ў
ж—Ґжњ¬
ETL
COUNTING STATISTICS
• Because of this fluctuating rate, it is not correct to
speak of a true rate of transformation (which implies
no statistical error in the measurement) but rather of
a true average rate of transformation.
• When we make a measurement, we estimate the
true average rate from the observed count rate
• The error of a determination is defined as the
difference between the true average rate and the
measured rate
APPLICATIONS OF
STATISTICAL MODELS
Application B: Estimation of the
Precision of a Single Measurement
COUNTING STATISTICS
• The Binomial Distribution
– n is the number of trials
– for which each trial has a success probability p,
then
– the predicted probability of counting exactly x
successes
COUNTING STATISTICS
• The Binomial Distribution
– Exp. dice throw :
• 3 ones in 3 consecutive throws. n=3, p=1/6, x=3
P(3)=[3!/3!]*(1/6)^3=1/216
• 2 ones in 3 consecutive throws. n=3, p=1/6, x=2
P(2)=[3!/(1!*2!)]*(1/6)^2*(5/6)=5/72
• 1 ones in 3 consecutive throws. n=3, p=1/6, x=1
P(1)=[3!/2!]*(1/6)*(5/6)^2=25/72
COUNTING STATISTICS
• The normal distributions
– As n increases, the
distribution curve
becomes increasingly
symmetrical around
the center line
– For the case where n
is infinite, we have
the familiar bellshaped normal curve
COUNTING STATISTICS
• The normal distributions
– 34% of the area lies
between the mean and
1s above or below the
mean.
– about 14% of the area is
between 1s and 2s
– only about 2% of the
total area lies beyond
either + or - 2s from the
mean
COUNTING STATISTICS
• The normal distributions
– Since the curve is
symmetrical about the
mean, 68% of the area
lies between п‚±1s
– 96% of the area is
included between п‚±2s
COUNTING STATISTICS
• The Poisson distributions
– Many categories of binary processes can be characterized
by a constant and small probability of success for each
individual trial. Included are most nuclear counting
experiments in which the number of nuclei in the sample is
large and the observation time is short compared with the
half-life of the radioactive species
– Under these conditions, p << 1, the binomial distribution
approaches the Poisson distribution
COUNTING STATISTICS
• The Poisson distributions
– standard deviation:
– coefficient of variation CV
– Variance: s 2
COUNTING STATISTICS
• The Poisson distributions
– sum or difference:
COUNTING STATISTICS
• The Poisson distributions
– Exp. A) 10000 counts in 10-min, s = 100 per 10 min
1000 п‚± 10 cpm, %CV = (10/1000)*100%=1%
– Exp. B) 1000 counts in 1-min, s = 32 per 1-min
1000 п‚± 32 cpm, %CV = (32/1000)*100%=3.2%
– When two quantities, each of which has its own variance
COUNTING STATISTICS
• The Poisson distributions
– Example 9.6
– A 5-min sample count gave 510 counts, while a 1-h
background measurement yielded 2400 counts. What is
the net sample counting rate and the standard deviation ,
of the net counting rate?
COUNTING STATISTICS
• The Poisson distributions
– product or a quotient
COUNTING STATISTICS
• The Poisson distributions
– Example 9.7
COUNTING STATISTICS
• The Poisson distributions
– Example 9.9
иЁ€ж•ёзµ±иЁ€ ( Counting statistic )
иЁ€ж•ёзµ±иЁ€ ( Counting statistic )
PROBLEMS
• 9.1, 9.2, 9.4, 9.8, 9.18, 9.19, 9.20, 9.23,
9.24, 9.27, 9.33
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