Transcript Q Radiation detectors

Half-life
• It's impossible to predict when a specific atom is
going to decay, but you can predict the number of
atoms that will decay in a certain time period.
• The halflife is the amount of time it takes for half
of the atoms in a sample to decay. The halflife for
a given isotope is always the same ; it doesn't
depend on how many atoms you have or on how
long they've been sitting around.
Carbon-dating
• Carbon has 3 isotopes, 2 of which are stable
(carbon-12 and carbon-13) and one which is
radioactive (carbon-14). Of these isotopes, the
most common in nature is carbon-12.
• Production of Carbon-14
• Carbon-14 is produced in the atmosphere by the
interaction of neutrons produced by cosmic rays
with the stable isotope of nitrogen, nitrogen-14:
1
0
n N  C  H
14
7
14
6
1
1
• The carbon-14 atoms produced are then
incorporated into carbon dioxide molecules to
produce 14CO2 molecules which mix with the most
common 12CO2 molecules in the atmosphere.
• The 14CO2 enters plant tissue as a result of
photosynthesis or absorption through the roots.
• 14C enters animal tissue when animals eat plants
containing 14C.
• The amount of 14C produced in the atmosphere is
balanced by the continual decay of 14C to produce
14N and a beta-particle:
C N  e
14
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14
7
0
1
• When a plant or animal dies it stops taking
in carbon-14 and radioactive decay begins
to decrease the amount of carbon-14 in the
tissues.
• The age of the plant or animal specimen
containing carbon, such as wood, bones,
plant remains, is determined by measuring
the ratio of carbon-12 to carbon-14.
• The half-life of carbon-14 is 5730 years, because
of its relatively short half-life, carbon-14 can only
be used to date specimens up to about 45,000
years old. After this the amount of carbon-14
present in the sample is too small to be measured
precisely.
• Carbon-14 can not be used to measure the age of
very young specimens as the change in the ratio
between the amount of carbon-12 and carbon-14
will not be sufficient to be detected.
• The tube is filled with
Geiger Counter
Argon gas, and a voltage
is applied between the
wire and the case.
• When a particle enters
the tube, it ionizes an
Argon atom. The
electron is attracted to
the central wire, and as it
rushes towards the wire,
the electron will knock
other electrons from
Argon atoms, causing an
“avalanche” (gas
amplification)
•Thus one single incoming particle will
•
• cause many electrons to arrive at the wire,
•creating a pulse which can be amplified and
• counted. This gives us a very sensitive detector.
Proportional Counter
• Detector and amplifier (gas amplification)
• Radiation causes primary then secondary
ionization - but just in small region of tube.
• For G-M -ionization of whole tube
• The smaller amount of ionization in the
proportional counter allows for smaller
dead-times and for us to have proportional
counting. But higher LOD than G-M
Proportional Counting
• Pulses are fed into a counter (pulse height
discriminator) which requires that the pulses
be of a certain height before it will count
them.
• The height of the pulse depends on the
energy lost by the particle.
• Hence can distinguish β’s from different
sources
Semiconductor detector
• Based on ionization of semiconductor
•  causes formation of e--hole pairs in
depletion zone of reversed-biased p-n
junction
• Pulse size (current produced) is proportional
to energy lost by  (~3.6 eV/ion pr in Si)
• Good energy resolution
• Can only be made fairly small
NaI Scintillation Counter
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Single NaI crystal is grown from molten salt.
Can be up to a 25 cm NaI crystal (big = $$$)
O.5 mole % TlI is added as activator
Ionizing radiation produces a flash of light
330-500 nm
• Detect with PMT
• Size of pulse is proportional to energy lost
NaI Scintillation Counter
• Good for stopping high-energy  rays.
• I has larger Z than Ge - more efficient at
stopping  rays.
• Energy resolution is not as good as with a
semiconductor detector but the counting
efficiency is higher
Liquid Scintillation counting
• Good for counting low energy ’s (tritium
and C-14) and ’s.
• Scintillation cocktail - eg toluene and pterphenyl and 2,5-diphenyl oxazole
• Energy from  excites solvent. Energy is
then transferred to primary fluor then
secondary fluor then get flash of light
• Coincidence counting with two PMT’s to
overcome interference by dark current
LiquidScintillation
Counting
Liquid Scintillation counting
• Pulse height is proportional to the energy
adsorbed from the radiation
• Can set the pulse height discrimination in
several channels so you can count several
’s at once
• eg 14-C and 3-H
Cerenkov counting
• Bluish white light 300 - 700 nm
• It is emitted when an e- passes through the
medium at a speed greater than the speed of
light in that medium
• In water,the lowest energy particles that will
cause this are 0.26 MeV
• ie need a hard beta
• P-32, K-42, Sr-90
Cerenkov counting
• Use polyethylene vials because glass would
absorb
• Advantage over scintillation counting is it
can be done in water and is cheaper and less
complicated
• Disadvantage - no good for soft betas from
C-14 and tritium.