TG-51 PROTOCOL
D. SILICON DIODES
122 Part I Basic Physics
advantage is in measuring doses in regions where ion chamber cannot be used. For example, TLD is extremely useful for patient dosimetry by direct insertion into tissues or body cavities.
Since TLD material is available in many forms and sizes, it can be used for special dosimetry situations such as for measuring dose distribution in the buildup region, around brachytherapy sources, and for personnel dose monitoring.
ChaPter 8 Measurement of absorbed Dose 123
of electronic current flow is from the n- to the p-region (which is opposite to the direction of conventional current).
D.2. Operation
Figure 8.16A shows schematically a radiation diode detector, which essentially consists of a sili- con p–n junction diode connected to a coaxial cable and encased in epoxy potting material. This design is intended for the radiation beam to be incident perpendicularly at the long axis of the detector. Although the collecting or sensitive volume (depletion zone) is not known precisely, it is on the order of 0.2 to 0.3 mm3. It is located within a depth of 0.5 mm from the front surface of the detector, unless electronic buildup is provided by encasing the diode in a buildup material.
Figure 8.16B shows the diode connected to an operational amplifier with a feedback loop to measure radiation-induced current. There is no bias voltage applied. The circuit acts as a current-to-voltage transducer, whereby the voltage readout at point B is directly proportional to the radiation-induced current.
Diodes are far more sensitive than ion chambers. Since the energy required to produce an electron–hole pair in Si is 3.5 eV compared to 34 eV required to produce an ion pair in air, and because the density of Si is 1,800 times that of air, the current produced per unit volume is about 18,000 times larger in a diode than in an ion chamber. Thus, a diode, even with a small collecting volume, can provide an adequate signal.
D.3. Energy Dependence
Because of the relatively high atomic number of silicon (Z = 14) compared to that of water or air, diodes exhibit severe energy dependence in photon beams of nonuniform quality. Although some diodes are designed to provide energy compensation through filtration (49), the issue of energy dependence never goes away and, therefore, their use in x-ray beams is limited to rela- tive dosimetry in situations where spectral quality of the beam is not changed significantly, for example, profile measurements in small fields and dose constancy checks. In electron beams, however, the diodes do not show energy dependence as the stopping power ratio of silicon to water does not vary significantly with electron energy or depth. Thus, diodes are qualitatively similar to films so far as their energy dependence is concerned.
+Ve bias voltage (typically 10 to 500 V) R
To signal amplifier
Thickness in range
∼10µ to ∼5 mm Depleted region
Motion of electrons
Motion of holes
Direction of electric field Extremely thin n-type region
(typically ∼0.1−µ thick)
+ + +
+ +
+ + +
+ +
+ +
+ +
+ −
−
−
−
−
− +
− +
−−
−−
−−
−−
−−
−
−−
−−
−−
− −
−−
−−
−−
−
− −
− − −
−−
−
− −
− −
+ + + + +
+ + +
+ + +
−−
−−
−−
−−
−− +
− −+ + +
Incident ionizing par ticle
p-type single crystal of silicon
Figure 8.15. a schematic diagram showing basic design of a silicon p–n junction diode. (From attix Fh. Introduction to Radiological Physics and Radiation Dosimetry. New York, NY: John Wiley & Sons; 1986, with permission.)
124 Part I Basic Physics
Some diodes exhibit greater stability and less energy dependence than others. It is therefore incumbent upon the user to establish dosimetric accuracy of a diode by comparative measure- ments with an ion chamber.
D.4. Angular Dependence
Diodes exhibit angular dependence, which must be taken into account if the angle of beam incidence is changed significantly. Again, these effects should be ascertained in comparative mea- surements with a detector that does not show angular dependence.
D.5. Temperature Dependence
Diodes show a small temperature dependence that may be ignored unless the change in tempera- ture during measurements or since the last calibration is drastic. The temperature dependence of diodes is smaller than that of an ion chamber. Moreover, their response is independent of pressure and humidity.
D.6. Radiation Damage
A diode can suffer permanent damage when irradiated by ultrahigh doses of ionizing radia- tion. The damage is most probably caused by displacement of silicon atoms from their lattice positions. The extent of damage will depend upon the type of radiation, energy, and total dose.
Summing point
Feedback loop
Diode O.A.
Output A
B 5 mm
Copper contact
∼2 mm Thin coaxial cable
Epoxy potting material Silicon diode
A
B
Figure 8.16. Schematic diagrams showing (A) silicon p–n junction diode and (B) basic electronic circuit using operational amplifier with a feedback loop. (From Gager LD, Wright ae, almond Pr. Silicon diode detectors used in radiobiologic physics measurements. Part I: development of an energy compensating shield. Med Phys. 1977;4:494-498, with permission.)
82453_ch08_p097-132.indd 124 1/7/14 6:55 PM
ChaPter 8 Measurement of absorbed Dose 125 Because of the possibility of radiation damage, especially after prolonged use, diode sensitivity should be checked routinely to ensure stability and accuracy of calibration.
D.7. Clinical Applications
As previously mentioned, diodes are useful in electron beam dosimetry and in limited situations in photon beam measurements. Most often their use is dictated by the requirements on the detec- tor size. For example, dose profiles or output factors in a small field may pose difficulties in the use of an ion chamber. So a film or a diode response is checked against an ion chamber under suitable benchmark conditions.
Diodes are becoming increasingly popular with regard to their use in patient dose monitor- ing. Since diodes do not require high voltage bias, they can be taped directly onto the patient at suitable points to measure dose. The diodes are carefully calibrated to provide a check of patient dose at a reference point (e.g., dose at dmax). Different amounts of buildup material can be incorporated to make the diode sample the dose close to the peak dose for a given energy beam.
Calibration factors are applied to convert the diode reading into expected dose at the reference point, taking into account source-to-detector distance, field size, and other parameters used in the calculation of monitor units.
For further details on diodes and their clinical applications, the readier is referred to some key articles in the literature (50–53).