This presentation discusses calorimetry dosimetry, which measures energy imparted to matter through temperature change. It can provide the most precise absolute dose measurements. Thermocouples and thermistors are commonly used temperature sensors. Thermistors are preferable due to their greater sensitivity. Different materials have different thermal capacities that determine the temperature increase per unit of absorbed dose. Energy fluence calorimeters contain a stopping material core to measure radiation beam energy, and calculate fluence based on temperature increase and material properties. Calorimetry provides advantages at high dose rates where other dosimeters saturate.

1. Presentation on Dosimetry with calorimeter
By
Emmanuel W. Fiagbedzi
Master of medical physics student
ICTP-University of trieste
Italy
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2. INTRODUCTION
Calorimetry dosimetry is the most precise of all
absolute dosimetry techique.
It is a way of measuring energy imparted to matter by
radiation through temperature change
This means that the dose absorbed in the sensitive
volume is proportional to change in temperature.
Only relatively small corrections for thermal leakage
and for chemical reactions are necessary
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3. In principle any kind of thermometer can be applied
in a calorimeter if the temperature change is large
enough to measure with sufficient accuracy and
precision
In practice only thermocouples and thermistors are
sufficiently sensitive and small; thermistors are
usually preferable because of their greater sensitivity
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4. The temperature increase per unit of absorbed
dose to the material in the calorimeter’s sensitive
volume depends on its thermal capacity, which is
usually expressed in cal/g °C or J/kg °C
The exact value of the calorie (i.e, the energy
required to raise 1 g of water 1 °C) depends upon
the temperature of the water to which it refers
Usually thermal-capacity (or specific-heat) tables
assume the value of the calorie for water at 15 °C;
hence 1 cal = 4.185 J, and 1 cal/g °C = 4185 J/kg °C
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5. For a sensitive volume containing a material of
thermal capacity h (J/kg °C), mass m (kg), and
thermal defect δ, and that absorbs E joules of
energy, the temperature increase is given by
where D̅ is the average absorbed dose (Gy) in the
sensitive volume
(1 ) (1 )
( C)
E D
T
hm h
δ δ− −
∆ = = o
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6. Thus a measurement of D̅ does not require explicit
knowledge of m if h is known
The thermal defect δ is the fraction of E that does not
appear as heat, due to competing chemical reactions,
if any
δ is negative for exothermic reactions
A few typical values of h are given in the following
table
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7. Thermal capacity of several calorimetric materials
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8. For example, in Al a dose of 1 Gy causes a temperature
increase of 1.12 × 10-3
°C
To measure this temperature rise with 1 % precision
would require a thermometer capable of detecting
temperature changes of the order of 10 µ°C
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9. Thermocouples typically have temperature
coefficients of 40 – 70 µV/°C
A temperature change of 10 µ°C would then give a
potential change of (4 – 7) × 10-10
V
This is too small to detect with available
instruments, such as a nanovoltmeter
Increasing the dose to 100 Gy would cause a
temperature rise of 0.112 °C, requiring detection of
(4 – 7) × 10-8
V for 1 % precision, which can be
accomplished with a nanovoltmeter
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10. Thermocouples are generally found to be most
useful in calorimeters where large doses (> 10 Gy)
are given, usually in a short enough time period
for thermal leakage to be negligible (i.e., under
adiabatic conditions)
Thermocouple sensitivity can be multiplied by
constructing a thermopile, consisting of a number
of thermocouples in series, but this is usually not
practical for calorimetric dosimetry because of the
increase in perturbation of the medium and the
number of thermal leakage paths
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11. Thermistors can be obtained in sized comparable
to thermocouples
They are semiconductors made of metallic oxides
and other constituents that are usually not
specified by the manufacturer
They exhibit negative temperature coefficients of
the order of several percent per °C at room
temperatures, increasing in negative coefficient
with decreasing temperature.
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12. The resistance of a thermistor at room
temperature is typically 103
– 105
Ω, which can be
conveniently measured with great precision and
accuracy by a Wheatstone bridge as shown in the
following diagram
The bridge null detector must be sensitive enough
so that the power dissipated in the thermistor is
negligible compared to the radiation heating
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13. A Wheatstone bridge circuit for measuring the resistance of a thermistor in the
sensitive volume (or “core”) of a calorimetric dosimeter. When Rx is set to
produce a null current reading,
Rc/Rx = R1/RJ, from which Rc can be determined.
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14. There are three general types of radiometric
calorimeter designs, depending on whether absorbed
dose in a reference medium, energy fluence in a
radiation beam, or power output of a radioactive
source is to be measured.
They are 1.Energy fluence calorimeter 2.Absorbed
dose calorimeter 3. calorimeters for measuring the
power ouput of a radioactive source.
We shall discuss only one which is the Energy
fluence calorimeter.
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15. An energy-fluence calorimeter contains a
core,usually consisting of a cylindrical piece of
dense material such as lead or gold,large enough
to stop an incident beam of radiation.
The core is suspended by nylon strings in an
insulated vacuum chamber,sometimes adjacent to
a twin core that serves as a control to determine
thermal leakage.
More than one thermistors may be necessary to
sample the temperature adequately and also the
heater should be designed to distribute the heat
uniformly.
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17. The energy fluence of the radiation beam passing
through the aperture of the area A is calculated by
h in the equation can be determined through
electrical calibration
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At high dose rate where other dosimeters shows
saturation effects,calorimeters are at their best

19. Attix, Introduction to radiological physics and
Radiation Dosimetry. pages 426-436
Padovani, Dosimetry 6 lecture notes slides
GRAZIE!!!
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