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13 Cards in this Set
- Front
- Back
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What is the wavelength and frequency of a 125 Kev photon beam?
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needs answer
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Define "Roengten"
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(R)
Unit of Exposure A measure of ionization produced in air by photons. The quotient of dQ by dm where dQ is the absolute value of the total charge of teh ions of one sign produced in air when all the electrons liberated by photons in air of mas dm are completely stopped in air. 1 R = 2.58 X 10^-4 C/kg air |
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Define "Electronic equilibrium"
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When the ionization lost is compensated by the ionization gained. Under this condition, the definition of roentgen is effectively satisfied.
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Describe the Free-air Ionization Chamber and it's uses.
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The fee-air ionization chamber is an absolute ion chamber for measuring exposure.
Their main function is in the standardizing laboratories where they can be used to calibrated field instruments. The upper energy limit occurs at approximately at 3 MeV. |
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Describe the Thimble Chamber and it's uses.
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The thimble chamber is a secondary instrument for measuring exposure.
The air volume required for electronic equilibrium can be substituted by a small aircavity with solid air-equivalent wall sothat the electronic equilibrium is main-tained in the air cavity. This is the principle of a thimble chamber. Ionization produced in the air cavity is by electrons crossing the cavity, not by photon interactions with the cavity air. |
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Describe the Condenser chambers and it's uses.
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A condenser chamber is a thimble ionization chamber connected to a condenser.
May be used for exposure measurements up to cobalt-60 Suitable for measuring EXPOSURE RATE in air for relatively lower-energy beams (<_2 MeV) |
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Describe the Farmer Chamber and it's uses.
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Farmer-type chambers are the most commonly used ion chambers for the calibration of all beam energies used in radiation therapy.
Farmer-type chambers have a cylindrical cavity with a nominal volume of 0.6 mL. The cavity radius is approximately 0.3 cm There are three electrodes: Outer electrode- thimble wall Central electrode- 1mm diameter aluminum rod Guard electrode- a cylindrical conductor that wraps around the insulator surrounding the central electrode in the stem of the chamber. |
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Describe the Extrapolation Chamber and it's uses.
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An ionization chamber that measures the surface dose in an irradiated phantom.
By measuring the ionization per unit volume as a function of electrode spacing, one can estimate the superficial dose by extrapolating the ionization curves to zero electrode spacing. Used for special dosimetry such as the measurement of dose in the superficial layers of a medium and the dosimetry of electrons and Beta particles. |
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Describe the Parallel-plate Chamber and it's uses.
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Used for the dosimety of electron beams.
Similar to the extrapolation chambers except for the variable electrode spacing. The electrode spacing is small (~2mm) but fixed. By adding layers of phantom material on top of the chamber window, one can study the variation in dose as a function of depth . |
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What is an electrometer?
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The electrometer is a charge-measuring device.
The most commonly used electrometers are negative-feedback operational amplifiers. They are connected to the ion chamber a shielded cable. |
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What is meant by ions saturation?
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As the voltage difference between the electrodes of an ion chamber exposed to radiation is increased, the ionization current increases at first almost linearly and later more slowly. The curve finally approaches a saturation value for the given exposure rate.
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What is chamber polarity?
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If the chamber reading changes with change in polarity of its bias voltage, it is exhibiting polarity effect.
It is sometimes found that for a given exposure the ionic charge collected by an ion chamber changes in magnitudes as the polarity of the collecting voltage is reversed. Polarity effect should be less than 0.5% for a well-designed chamber. |
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Write out the Ctp formula and solve it for 20 C and 755mmHg
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Ctp = (760 / P) X (273.2 + t / 295.2)
P = pressure in mm Hg t = temperature in degrees Celsius Ctp = (760 / 55) X ((273.2 + 20) / 295.2) Ctp = 0.999 |
