Transcript Laboratory Experiments in Radiation Detection and Measurement

Laboratory Experiments
in Radiation Detection
and Measurement
North Carolina Science Teacher’s Association Conference
Greensboro, NC
November 12, 2004
Gerald Wicks, CHP
North Carolina State University
Department of Nuclear Engineering
And the North Carolina Chapter of the Health Physics Society
Radiation Types







Alpha:
Beta:
Photons:
He nucleus, charge of +2
Electron, charge of -1 or +1
Electromagnetic radiation,
X-rays come from de-excitation of
electrons and are atomic in origin while
gamma rays come from de-excitation
of the nucleus and are nuclear in
origin, no charge, no mass
Neutron:
Neutral particle
Proton:
Particle with charge of +1, or H
nucleus
Fission fragments and recoil atoms are charged atoms
Mesons are charged particles with ~270 x mass of
electron
Radioactive Decay

Alpha Particle


Beta Minus Particle


Nucleus undergoes fission (heavy elements)
Delayed Neutron Emission


Atomic electron is captured by nucleus and combines with a proton
to form a neutron
Spontaneous Fission


Proton decays to neutron, beta plus particle (positively charged
electron) and neutrino, positron has energy up to a maximum value
Electron Capture


Neutron decays to proton, beta minus particle (electron) and antineutrino, beta particle has energy up to a maximum value
Beta Plus Particle


He nucleus tunnels out of nucleus, discrete energy
A few fission products undergo decay and emit neutrons
All of the above may produce gamma rays and x-rays
Chart of the Nuclides
Decay Scheme for Co-60
Charged Particle Interactions
Charged particle coulombic fields undergo
collision with bound atomic electrons
producing free electrons and charged
atoms, or ions, with kinetic energy
 Charged particles that pass near the
nucleus are accelerated and produce
x-rays of variable energy ranging up to the
kinetic energy of the incident particle, this
is also known as “Bremsstrahlung”

Photon Interactions



Photons, or x-rays and gamma rays, interact
with matter to produce electrons, scattered
photons, positrons and annihilation photons, and
charged recoiling atoms by various interactions
Majority of energy from photon interactions is
given to electrons, positrons, and photons
Annihilation photons result from combination of
positron and electron at rest and have an energy
of equal to the rest mass of an electron, 0.511
MeV
Neutron Interactions

A variety of absorption and capture interactions
occur in which radioactive atoms, charged
particles, photons, neutrons, recoil nuclei, and
fission fragments are produced
 (n,alpha)

(n,gamma) (n, fission) (n, proton) (n, triton)
Scattering interactions may produce scattered
neutrons of lower energy and a nucleus in an
excited state. Upon nuclear de-excitation,
photons are emitted
 (n,
n’) (n, n + gamma)
Neutron Activation
(absorption or capture reaction)
Neutron Activation








A = N [(1-exp(-ta)] exp(-td)
Where:
A = Activity (Bequerels)
N = number atoms of parent isotope
 = neutron flux (n/cm2/sec)
 = capture reaction cross section
 = decay constant
ta/td = sample activation and decay times
Radiation Terms and Units

Dose
 Energy deposited per unit
 RAD = 100 ergs / gram, 1

mass
Gy = 1 J / Kg = 100 RAD
Exposure
 Ionization
produced by photons in a mass of air,
Roentgen = 2.58 E-4 Coulombs / Kg

Dose-equivalent
 Tissue Dose X quality factor,
 REM = RAD X quality factor,
Q X Modifying Factor, N
N is typically 1
1 Sv = 100 REM
 Quality factor, Q, accounts for difference in the
biological damage caused by different radiation types
RADIATION DETECTORS
 Gas
filled detectors, e.g. Geiger-Mueller (GM)
counters, Proportional counters, and Ion
Chambers
 Solid detectors, e.g. scintillators and semiconductors
 Liquid detectors, e.g. scintillators and
chemical dosimeters
 Dosimeters, e.g. film, thermoluminescent
dosimeters
 Ratemeters and counters (scalers)
GM Detectors





A fill gas is contained in a sealed tube of various shapes,
sizes
Ionization occurs in the fill gas and in the detector wall
material (wall is at ground potential and serves as the
cathode)
Ions in the fill gas are accelerated and produce further
ionization giving a pulse of maximum amplitude that is
collected at the detector anode
GM detectors produce a full amplitude pulse (count) for
any ion that enters the detector, so distinction between
radiation types is not possible
GM detectors are very sensitive, inexpensive and rugged
and therefore commonly used
Ion Chambers




Fill gas is air that may or may not be contained
in a sealed chamber
Ionization occurs in the fill gas and in the
detector wall material (wall is at ground potential
and serves as the cathode)
Ions in the fill gas are collected at the detector
anode and produce a pulse height proportional
to the number of interactions, or ionization,
occurring in the detection volume
Ion chambers are very accurate and allow
distinction between radiation types based on
pulse height
Solid Detectors


Scintillation crystals respond to radiation
interactions by producing light. The light is then
detected and converted to an electrical signal
with a pulse height proportional to the radiation
energy deposition rate. Scintillators are
extremely sensitive to radiation and are often
used for obtaining energy spectra.
Semi-conductors respond to radiation by
producing electron-hole pairs. The pulse height
proportional to the radiation energy deposition
rate. Semi-conductors have superior resolution
compared to scintillators are therefore a better
choice for obtaining energy spectra.
Gamma Energy Spectrum
Liquid Detectors


Scintillation detectors respond to radiation
interactions by producing light which is then
detected and converted to an electrical signal.
The pulse height proportional to the radiation
energy deposition rate. The sample and
scintillator are mixed making alpha and beta
particle detection very efficient.
Chemicals respond to radiation by changing
oxidation state, which produces a change in
spectrophotometry in which the peak size is
proportional to the radiation energy deposition
rate
Radiation Dosimeters



Film responds to photons by changing its lattice structure
and upon development forms a latent image. The optical
density is related to the radiation energy deposited.
Films may also be used to detect particles, which
produce tracks upon being developed. The number and
length of tracks are used to determine the amount and
type of charged particles.
TLD are crystalline materials and respond to radiation by
forming electron –hole pairs. Some e-h pairs are
trapped at the time of formation in crystal imperfections.
Upon addition of heat the trapped e-h pairs become
mobile and release light which is then detected and
related to the radiation energy deposited.
Radiation Sources
Consumer products (lantern mantles,
welding rods, KI salt, smoke detectors)
 Radon and its decay products (decay
products are collected on furnace or
vacuum cleaner filters – radon can be
collected using charcoal canisters)
 Exempt quantities of radioactive materials
(under 10 CFR 30)

Radiation Sources

Naturally occurring sources;
C-14, K-40, Th-232, U-238 … radon
 Cosmogenic and terrestial sources
 H-3,

Technology enhanced sources;
 Mining
of U producing higher levels of
naturally occurring radioactive materials

Artificial sources;
 Fission
products, activation products,
accelerated produced radioactive materials
Laboratories using GM Detector
Radiation type identification
 Radiation shielding
 Radiation source detection
 Half-life determination
 Detection efficiency
 Detector resolving time
 Backscatter factor

Identification of Radiation Type
GM detector wall thickness affects
response. For the CDV-700 GM tube,
moderate energy beta particles and all
photons can be detected.
 Open window = beta + photon
 Closed window = photon
 Open – closed window = beta
NOTE:
YOUR CDV-700 IS NOT CALIBRATED!

Radiation Shielding

Count rate, or mR/h reading, from a
radiation source decreases as shielding is
added. Beta radiation is easily shielded
with paper, Al foil, or plastic. Photons are
shielded best with high atomic numbered
materials like Pb, W, or U although any
material will work.
Radiation Shielding


Beta shielding and range are related. Beta
particles have a defined range based on beta
particle energy. Range equations are given in
the attached file and on the next page.
Photon range has no endpoint in theory.
Shielding is based on photon energy.
R = R(0) exp (-uT)
where R is response, u is the linear attenuation
coefficient. u is a function of photon energy for a
given material.
Beta:
R = 0.412 E(1.265 - 0.0954 lnE)
where E < 2.5 MeV
R = 0.530 E - 0.106
where E > 2.5 MeV
E is maximum beta particle energy in MeV and R is g cm-2
R = dEn
Alpha, Proton, Beta:
where d and n are experimental parameters
and R is g cm-2
Material
Alph a
n
d
P roto n
n
d
Bet a
n
d
Water
Al
Pb
C
1.793
1.730
1.680
1.787
1.793
1.730
1.680
1.787
1.32
1.32
1.32
1.32
1.62E-4
3.15E-4
7.00E-4
1.86E-4
1.95E-3
3.47E-3
7.18E-3
2.22E-3
0.356
0.400
0.640
0.356
Radiation Shielding






Place source 2 inches away from open window GM counter
for beta particles
Maintain source-detector geometry and place materials of
known thickness between the source and detector
The exact thickness where the GM detector response equals
that of background occurs if the beta particles are completely
shielded. This material thickness is the range.
For photons, use a closed window. Determine the thickness
where the GM detector response, R, is half and also a tenth
of the unshielded response, R(0). These are the half value
layer (HVL) and tenth value layer (TVL), respectively.
Determine and compare the the linear attenuation
coefficients:
R/R(0) = exp (-uT) = 0.5 for HVL; u = -[ln 0.5/T]
R/R(0) = exp (-uT) = 0.1 for TVL ; u = -[ln 0.1/T]
Radiation Shielding



A curved line, or change in the shape of the line,
indicates that more than one radiation energy is
present, or multicomponent line.
For resolution, determine the shape of the high
energy line (high thickness readings) and
extrapolate back to lower energies (lower
thickness readings). This produces a third, low
energy line.
Extrapolate the low energy line to a zero reading
to determine the range of the lower energy
radiation
Radiation Source Detection
Perform instrument checks to verify
operation
 Determine background radiation level in
an area known to be free of radiation
sources
 Survey area and items with open window
 If elevated readings are found, survey with
open and closed window. Record data.

Half-life Determination







Obtain source with half-life of minutes or days range
Set up source next to open window detector
Record detector reading and time. Repeat this step to
obtain several data points.
Plot results on semi-log paper with detector readings on
y-axis (log scale) and time on x-axis (linear scale)
Determine slope, m, which is also the decay constant;
m = {ln[(y(1)] – ln[y(2)]}/[t(1) – t(2)]
Determine half-life using the slope (decay constant or λ);
ln 2 = λT1/2
Compare results to reported value for the source
Half-life Determination
Equations used in this lab:
A = N
A(t) = A(0) e-t
ln 2 = T1/2
Specific Activity = A/mass
Constants:
1 mole = 6.022 E23 atoms
3.7 E10 dps = 1 Ci
is decay constant
f is isotopic fraction
T1/2 is half- life
A is activity
t is decay time
N is number of atoms
Detector Efficiency




Used for detector calibration
Used in analysis of samples and sources
Used to determine detector sensitivity, or
detection limits
Efficiency, E = cpm / dpm or counts per decay
for a given radiation type and energy. E may
also be expressed as counts per beta particle,
counts per gamma photon, etc. by using the
radiation yield, Y;
E = cpm / dpm X 1 decay / Y
Detector Efficiency Curve for
Semi-Conductor Detector
42 % HPGE Efficiency @ C(s) on 7-29-02
Isotope Products Eu-152 3mm Diameter Point Source
y = 0.4224x-0.7096
Efficiency (CPS/DPS)
1.0E-02
8.0E-03
6.0E-03
4.0E-03
2.0E-03
0
200
400
600
800
1000
Gamma Energy (keV)
1200
1400
1600
Detector Efficiency
For GM detectors, beta efficiency is
affected by distance and beta particle
energy
 Typical efficiencies for CDV-700 for
moderate to high energy beta particles
range between 0.01 to 0.05 c/d

Detector Resolving Time





Resolving time is used to correct for count rate,
cpm, losses. These losses occur because the
counting rate exceeds the ability of the detector
to process the pulses.
Obtain two sources, one of low activity and a
second of higher activity
Place the first source near the open window and
record the cpm reading, or R(1)
Place the second source near the open window
and record the cpm reading, or R(2)
Place the both sources near the open window
and record the cpm reading, or R(1+2)
Detector Resolving Time





Calculate detector resolving time as follows:
t = [R(1) + R(2) – R(1+2)] / [2 R(1)R(2)]
Typical GM detector resolving times, t, are
hundreds of microseconds, us
(e.g. 100 us = 1.67 E-6 minutes)
R = R(0) / [1-R(0) t], if R(0) is cpm, use t in
minutes
R(0) is observed count rate, in cpm
R is the true count rate, in cpm
Backscatter Factor





Backscattering is the deflection of radiation by scattering
processes through angles greater than 90 degrees with
respect to the original direction
Alpha radiation travels in a straight line motion while beta
particles will alter their path direction significantly upon
interacting with matter. As a result, backscattering may
be significant for beta radiation sources counted by a
GM detector depending on the beta particle energy and
source backing material.
As a result of beta particle backscattering, the detector
count rate increases
Backscatter factors may be as high as 40%
Corrections for backscattering may be necessary for
analytical measurements
Backscatter Factor








Obtain a high energy beta emitting radioactive source in liquid form
Uniformly disperse and evaporate on an extremely thin layer of
material (saran wrap, scotch tape, mylar). Also cover the source
with a thin layer of material to prevent the spread of contamination.
Place the GM open window tube near the source and record the
cpm reading, C(0)
Place additional backing material of known thickness underneath
the source (minimize air gaps by using adhesive tape or epoxy) and
record the cpm reading, C
Continue the previous step until a maximum cpm reading is
observed. This is the saturation point, which is approximately 0.3
times the beta particle range.
The linear thickness of a particular material to produce
backscattering is approximately proportional to the reciprocal of the
density of the material
If interested, repeat using other materials and observe the change
on backscattering. Their should be a dependence on atomic
number.
% Backscatter = [C – C(0)/C(0)] x 100
C is cpm with backing material, C(0) is cpm with minimal backing