dosimetry with calorimeter
TRANSCRIPT
Presentation on Dosimetry with calorimeter
By
Emmanuel W. Fiagbedzi
Master of medical physics student
ICTP-University of trieste
Italy
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INTRODUCTIONCalorimetry 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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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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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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For a sensitive volume containing a material of thermal capacity h (J/kg C), mass m (kg), and thermal defect , and that absorbs E joules of energy, the temperature increase is given by
where ̅D is the average absorbed dose (Gy)
in the sensitive volume
(1 ) (1 ) ( C)
E DT
hm h
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Thus a measurement of ̅D does not require explicit knowledge of m if h is known
The thermal defect is the fraction of E that does not appear as heat, due to competing chemical reactions, if any
is negative for exothermic reactionsA few typical values of h are given in the
following table
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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 C
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Thermocouples typically have temperature coefficients of 40 – 70 V/C
A temperature change of 10 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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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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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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The resistance of a thermistor at room temperature is typically 103 – 105 , 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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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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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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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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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