1
Calorimetry:
•
This is the only way to directly measure the dose – all ways it is deposited. The
temperature rise in an isolated medium.
•
As we saw in the first lecture, the energy deposited from radiation is very small, so
this task is difficult.
•
We must know the following: 1. The transport of heat throughout the calorimeter,
and 2. The amount of endothermic and exothermic chemical reactions.
•
The average dose to the medium assuming no heat loss is the following:
D=
h ⋅ ∆T
1−δ
Where,
-- h is the specific heat capacity (or just “specific heat”) of the medium.
Units: [h] = J/(kg oC).
-- δ is the thermal defect. This is the small fraction of energy that does not
appear eventually as thermal energy – because of chemical reactions. δ is
negative for exothermic reactions.
•
Standards laboratories have traditionally used carbon in the form of graphite as
the sensitive volume of the calorimeter, because of its similar atomic number
compared to the effective atomic number of water. It also has a δ = 0. The value
of h for graphite is 7.1x102 J/(kg oC). therefore a 1oC temperature rise will require
710 Gy !!
•
Obviously we need a very sensitive measurement device. Thermistors are small and
can measure temperature on the order of a few µoC because resistance changes of
about 5000 Ω/oC are possible.
Lecture 29 MP 501 Kissick 2016
2
•
The figure from Domen (The Dosimetry of Ionizing Radiation, Vol. II, p. 273) below
shows a graphite calorimeter used at NBS (now it is NIST):
•
Here we make sure ∆T=0, so that q~ ∆T =0 in both the core and the jacket. It is
hard.
•
No matter how hard we try, heat will transport. Two ways to deal with it:
-- use insulation to prevent conductive losses and reflective coatings to prevent
radiative losses.
-- use intelligent feedback to keep the jacket and the absorber at the same
temperature. With no temperature gradient, there is no heat flow! This is
called an adiabatic calorimeter.
Lecture 29 MP 501 Kissick 2016
3
•
Really to do a good job, we need both approaches. After all we can do with
hardware, we still need to be clever in the data interpretation. We can extrapolate
the temperature rise linearly which assumes no heat loss. Then, at the end of the
beam on time, we can use the deviation from the actual temperature rise to achieve
a correction. A heated wire is often used to do this, then this correction factor
can be applied to the radiation case.
Temperature rise
Purely linear extrapolation:
no heat losses
Extrapolate to get
this correction for
heat loss.
Beam ‘on’
time
•
One can make a water calorimeter by sandwiching a thermister between two thin
plastic sheets. It is not run in adiabatic conditions, but instead relies heavily on the
extrapolation technique described above. The water calorimeter has the advantage
of getting the dose to water, and because it’s a calorimeter, there is no reference
to another beam required! Below is a figure of a water calorimeter from Domen,
page 294:
Lecture 29 MP 501 Kissick 2016
4
•
In fact, we are building a calorimeter in our department’s calibration lab. Student
Martha Malin has sent me these pictures with descriptions of the device she has
been working on:
A CAD rendering (artificially colored)
of the absorber (where the radiation
is absorbed to induce a temperature
change), the detector housing (the
absorber sits within a cavity of the
detector housing. It's used to create
a stable thermal environment), and
the source delivery assembly (the
source is attached to this and is
repeatedly raised and lowered into
the absorber).
A picture of the absorber used for a
measurement of the energy emitted from the
source
A picture of the partially assembled
calorimeter. You're looking at the top of the
detector housing. Also shown are the LHe
fill and vent tubes, the source transfer
tubes (the small Kapton tubes in the center
of the image. They are used to guide the
source delivery assembly into the absorber),
and the wiring/vacuum pass through
Lecture 29 MP 501 Kissick 2016
5
Integrating Dosimeters:
•
Integrating dosimeters “store” their energy until it is “read” later. Sometimes, this
process only happens once (the dosimeter cannot be used again, like films).
Sometimes, it can be re-set like thermoluminescent dosimeters described below.
•
The following integrating dosimeters will be discussed:
-- thermoluminescent dosimeters (TLDs)
-- radiographic films (going out of use !)
-- radiochromic films (emerging as useful, finally, perhaps)
-- other chemical dosimeters, specifically Fricke solutions
•
Thermoluminescence (TL) Dosimetry:
•
John Cameron, who founded this department, and brought it early fame with this,
and the AAPM was inspired by a geology lecture that used TL technology. He
realized that what he saw could be used for radiation dosimetry! Therefore, he
invented this general process for our applications.
•
Scintillation materials, when exposed to ionizing radiation, emit light in the form of
prompt fluorescence due to the excitation of the material. Scintillators are used in
pulse dosimetry.
•
Phosphorescent materials are crystalline materials in which metastable energy
states store some of the excitation energy in the form of electron-hole pairs in the
band structure of the crystals. There are traps that can hold the electrons or the
holes and prevent them from recombining for long periods of time. The traps are
created with impurities in the crystal’s structure, by heating in such a way as to
cause dislocations, and by the radiation itself.
•
Good scintillators have few or no traps like these.
•
TL dosimeters (TLDs) on the other hand are designed to have traps to prevent as
much prompt recombination as possible. The base crystal is often LiF.
Lecture 29 MP 501 Kissick 2016
6
•
The recombination of electrons and holes occurs at luminescence centers in the
crystal. The luminescence centers may be at a hole trap or at a different location.
hν
Conduction band
Electron
trap
Creation of an
electron-hole pair:
Hole
trap
Valence band
hν
hν
Ways to recombine
electron-hole pairs:
TL photons released
•
The recombination process is fundamentally stochastic. The probability (‘time for’)
recombination, P, is given by the Randall-Wilkins theory:
P = τ −1 = α ⋅ e − E /( kT )
Where,
-- τ is the mean lifetime in the trap.
-- E is the energy depth of the trap.
-- k is the Boltzman constant = 8.62x10-5 eV/K
-- T is the temperature, units=K
-- α is the ‘frequency factor,’ a constant of proportionality.
•
Each trap type has a characteristic α .
•
Larger energy depths require higher temperatures to dislodge electrons or holes
from the trap.
Lecture 29 MP 501 Kissick 2016
7
•
For a given trap, the probability for release increases with increasing temperature.
•
One reads-out the TLD by heating it, and measuring the emitted light, usually with
photomultiplier tubes.
•
As the TLD is heated, lower energy traps release first, followed by deeper traps.
•
The shape of this TL intensity of light emitted is called the “glow curve.”
•
Each material has its own characteristic glow curve.
•
Each peak in the glow curve corresponds to a different trap’s energy depth.
•
The peak’s peak intensity happens at the temperature of this maximum as follows:
Tm = (489 K / eV ) E (eV )
Where,
-- E is the trap’s energy magnitude in eV.
•
The shape of the peak is described pictorially as follows:
Lecture 29 MP 501 Kissick 2016
8
•
Examples of glow curves are shown here:
•
Ideally, the traps will not release until heated, but at room temperature, because
of the stochastic nature of the process, spontaneous release is possible. This is
reduced to a huge degree by refrigeration until read-out time.
•
The low temperature peak at 100oC has only a few days half-life, so its use should
be avoided.
•
Before reuse, the TLD needs to be fully cleared. This high temperature “annealing”
is required then after the glow curve is read and before reuse.
•
The annealing process may also induce or destroy traps. These mobile traps can
then destroy the crystal’s functioning.
•
John Cameron invented a recipe to avoid damage: 400oC for 1 hr. followed by quick
cooling (“quenching”), then reheat to 80oC for 24hours.
Lecture 29 MP 501 Kissick 2016
9
•
Be careful with peaks above 250oC because mechanical defects get released and
leads to bursts of spurious light that will interfere with the glow curve
measurements. This is called triboluminescence.
•
You can see triboluminescence by crunching Wintergreen Lifesavers between your
teeth in a dark room!
•
Also at high temperatures dirt and organics can lead to blackbody radiation
contamination. It is best to use tweezers or gloves – don’t touch the crystals!
•
Optical filtration and nitrogen flowing over the crystal also help reduce blackbody
radiation effects.
•
Because of all these many issues, care and frequent system calibration is required.
•
To give reproducible results, the TL system should have constant sensitivity. This
done by batching or binning the material into closely matched sensitivities.
•
The dosimetry cycle one uses is shown below:
•
The TLD reader also needs to be maintained. PM tube drifts, dirty filters,
fluctuating power supplies, etc… can lead to drifts in the sensitivity. This can be
checked with LED tests.
Lecture 29 MP 501 Kissick 2016
10
•
The typical TLD reader:
•
The light output of most phosphors increases slightly at high temperatures. This is
called “supralinearity,” and is most likely due to radiation-induced trap formation.
At even higher doses, radiation can destroy traps so the sensitivity decreases.
TL intensity
Supralinearity
At higher doses
Significant
radiation
trap destruction
Absorbed dose, D
Lecture 29 MP 501 Kissick 2016
11
•
Characteristic of some typical TL phosphers:
Lecture 29 MP 501 Kissick 2016
12
•
TLDs show a lot of LET dependence: can be greater than or less than for Co-60.
•
TLDs are extremely versatile! Either as a powder, or as chips with or without
Teflon encapsulation, suspended as powder in water or gel, single large crystals or
formed into teeth: note that tooth enamel is a TL material !! We all have TLDs in
our teeth.
•
Famously, TLD materials can be chosen to be used for neutron dosimetry. Li-6 has
a very large cross-section for 36 Li (n, α )13H . The 2.07 MeV alpha and the 2.74 MeV
triton are then absorbed by the phosphor, resulting in a high TL signal.
•
Trade-names from Harshaw Chemicals are TLD-700, TLD-100, TLD-600, and they
have isotopic compositions of Li-6 as follows:
-- TLD-700: 6 Li / Li ≈ 0
best for gamma sensitivity.
-- TLD-100: Li / Li ≈ 7%
6
-- TLD-600: 6 Li / Li ≈ 96% best for neutron sensitivity.
Lecture 29 MP 501 Kissick 2016
13
The responses of TLD-600 and TLD-700:
Lecture 29 MP 501 Kissick 2016