ThePhysicsofRadiologychapter53放射物理英文版教学课件

1、Measurement of Radiation: Instrumentation&TechniquesPart ThreeBME College Sherman Sheen(辛学刚)The Physics of Radiology第1页，共18页。solid state detectorsthe diodeShort response timeEnormous dynamic rangeHigh sensitivityFlexible forms第2页，共18页。Thermoluminescent Dosimetry (TLD) PRECISION OF TLD AND DOSE RANGE

2、 MATERIALS FOR TLD USES OF TLD-CLINICAL DOSIMETRY PERSONNEL MONITORING DOSE CALIBRATION SERVICES 第3页，共18页。Chemical Dosimetry A is the change in absorbance at following a dose D to the ferrous sulphate is the molar extinction coefficient of the ferrous ion is the density of the solution in kg/m3 G is

3、 the yield in number per 100 eV 第4页，共18页。Example. It is required to determine the average dose to a sample of water in a complexly shaped radiation container near a cobalt source. This is most easily done using the Fricke dosimeter. The container is filled with Fricke solution and exposed. A sample

4、of the well-mixed irradiated solution is then placed in a quartz cuvette of 2 cm path length and its absorbance measured against the stock solution at 304 nm. Suppose the measured absorbance is 0.360. Calculate the dose.第5页，共18页。Film as a Dosimeter radiant energycrystals of silver bromidelatent imag

5、eFile developedfilm fixednegative film resultsareas that have been exposed to x rays appear dark第6页，共18页。Direct Measurement of Absorbed Dose The Calorimeter 第7页，共18页。Summary of Calibration Procedures Exposure Rate DeterminationsSelect a suitable treatment applicator or reference field size (for exam

6、ple 10 by 10 cm) and direct the beam in such a way as to avoid scattering from nearby objects.b. Select filter, tube current (mA), and voltage (kV) and determine the half-value layer for these settings of the machine.第8页，共18页。c. Select a suitable practical dosimeter that has an exposure calibration

7、factor traceable to a standardization laboratory (see section 4.09) for the half-value layer of the radiation beam to be measured. The ionization chamber should be one that has been designed specifically for the energy of radiation being measured. It may be necessary to interpolate between two calib

8、ration factors to obtain the required one.第9页，共18页。d. Fasten the dosimeter in a clamp so that its position is accurately known and can be reproduced for repeated readings. The center of the sensitive volume of the ion chamber should be at the center of the field and even with the end of the treatmen

9、t cone if there is one, or else at the reference distance chosen for the calibration. Sometimes this is not possible, in which case an inverse square correction will have to be made第10页，共18页。e. Set the timer to an appropriate value, turn on the machine, and bring it to the operating point selected i

10、n b. Open the shutter, or in the case of a cobalt unit simply turn the beam on. If the ion chamber is operated as an exposure rate measuring instrument, its reading can now be taken directly. If it is to be operated as an exposure meter, timer, settings should be chosen so that a set of readings are

11、 obtained at both relatively high scale readings (say about 3/4 scale) and relatively low scale readings. The exposure rate may then be obtained by taking differences of sets of readings to determine the slope of the reading versus time curve. 第11页，共18页。Several sets of readings should be averaged to

12、 give M/t, the reading per unit time. Alternatively, since many computers and calculators are equipped with “least squares routines,” it is convenient to take a series of readings Mi at times t extending across the range of the dosimeter and determine the slope, M/t, of the M versus t line. This lat

13、ter procedure gives information about the precision of determining this slope; it can also give information about the shutter timing error.第12页，共18页。f. During the exposure, for an x ray machine it may be necessary to adjust either the kV or mA. If this is not possible the average tube current and vo

14、ltage over the time of the exposure should be recorded.g. Measure temperature and pressure. The exposure calibration factor as issued by the standardization laboratory is valid only for the temperature and pressure stated by them. It is usually 22oC and 101.3 kPa but this should be confirmed.第13页，共1

15、8页。h. The corrected exposure rate is:where M/t is the average uncorrected exposure rate as obtained in paragraph e, f is the SSD, x is the distance from the SSD to the center of the chamber, P and T are the pressure and temperature of the gas in the ion chamber when the measurement is made, and Tx i

16、s the temperature for which the calibration factor, Nx, applies.第14页，共18页。i. A number of other small corrections may be required. Efficiency of ion collection and the problems of leakage, stem ionization are discussed. A dosimeter should also be checked for constancy of sensitivity, and if it has ch

17、anged since the calibration was performed, allowance must be made. Some instruments are equipped with a device containing a long-lived radioactive isotope for this purpose. Alternatively, careful measurements of a cobalt unit can be used. 第15页，共18页。j. These procedures, carefully carried out, should

18、allow an output calibration for the conditions measured, to an accuracy of better than 2.5%（I7).第16页，共18页。Determination of Absorbed Dose in a Phantom The phantom should be clamped to the treatment unit or located in such a way that its position can be reproduced for repeated measurements and the distances from the source to the ion chamber should be known with a precision of at least 2 mm An appropriate phantom must be chosen. These procedu