ThePhysicsofRadiologychapter53放射物理英文版

1、Measurement of Radiation: Instrumentation&TechniquesPart ThreeThe Physics of Radiologysolid state detectorsthe diodeShort response timeEnormous dynamic rangeHigh sensitivityFlexible formsThermoluminescent Dosimetry (TLD) PRECISION OF TLD AND DOSE RANGE MATERIALS FOR TLD USES OF TLD-CLINICAL DOSI

2、METRY PERSONNEL MONITORING DOSE CALIBRATION SERVICES Chemical Dosimetry graysGlAD)1065. 9(9A 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 the yield in number per 100 eV Ex

3、ample. 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 of the well-mixed irradiated solution is th

4、en 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.GyDto8 .495 .1510240 . 221961065. 9360. 09sol Fricke GywaterD9 .490296. 00297. 08 .49)(Film as a Dosimeter radiant energycr

5、ystals of silver bromidelatent imageFile developedfilm fixednegative film resultsareas that have been exposed to x rays appear darkDirect Measurement of Absorbed Dose The Calorimeter kgcalorieskgJGy18. 4111CCcalkgkgcalToo431039. 210118. 412 . 11039. 210100545RSummary of Calibration Procedures Exposu

6、re Rate Determinationsa.Select a suitable treatment applicator or reference field size (for example 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

7、 the machine.c. Select a suitable practical dosimeter that has an exposure calibration 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 e

8、nergy of radiation being measured. It may be necessary to interpolate between two calibration factors to obtain the required one.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 cham

9、ber should be at the center of the field and even with the end of the treatment 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 madee. Set the timer to an appropriate value,

10、 turn on the machine, and bring it to the operating point selected in 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 m

11、eter, timer, settings should be chosen so that a set of readings are 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

12、time curve. Several sets of readings should be averaged to 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

13、determine the slope, M/t, of the M versus t line. This latter procedure gives information about the precision of determining this slope; it can also give information about the shutter timing error.f. During the exposure, for an x ray machine it may be necessary to adjust either the kV or mA. If this

14、 is not possible the average tube current and voltage 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 an

15、d 101.3 kPa but this should be confirmed.h. The corrected exposure rate is:XXNTTPfxftM)2732733 .101()(2where 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

16、of the gas in the ion chamber when the measurement is made, and Tx is the temperature for which the calibration factor, Nx, applies.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

17、 be checked for constancy of sensitivity, and if it has changed 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. j.

18、These procedures, carefully carried out, should allow an output calibration for the conditions measured, to an accuracy of better than 2.5%（I7).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

19、 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 procedures, carefully carried out, should yield an absorbed dose determination in the phantom to an accuracy of approximately

20、 2.5 Summary Solid state detectorsthe diodeThermoluminescent dosimetryChemical dosimetryDirect measurement of absorbed dosecalorimeterFilm as a dosimetryDirect Measurement of Absorbed Dose The Calorimeter kgcalorieskgJGy18. 4111CCcalkgkgcalToo431039. 210118. 412 . 11039. 210100545Rd. 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 t