Ordinary Silicon PIN photodiodes can serve as detectors for X-ray and gamma ray photons. The detection efficiency is a function of the thickness of the silicon wafer. For a wafer thickness of 300 microns (ignoring attenuation in the diode window and/or package) the detection efficiency is close to 100% at 10 KeV, falling to approximately 1% at 150 KeV(3).
For energies above approximately 60 KeV, photons interact almost entirely through Compton scattering. Moreover, the active region of the diode is in electronic equilibrium with the surrounding medium--the diode package, substrate, window and outer coating, etc., so that Compton recoil electrons which are produced near--and close enough to penetrate--the active volume of the diode, are also detected. For this reason the overall detection efficiency at 150 KeV and above is maintained fairly constant (approximately 1%) over a wide range of photon energies. Thus, a silicon PIN diode can be thought of as a solid-state equivalent to an ionization-chamber radiation detector.
DC Current-Mode Operation
The DC-current response to gamma radiation incident on a PIN diode detector can be estimated as follows:
Then the average current produced in the diode is given by:
I = N A r Ê e / s
Example--Consider a 10 mCi point-source of ß+- emitting activity (gamma-ray energy = 511 KeV annihilation radiation) at a distance of 10 cm from a type PDC 24S PIN diode detector (Detection Technology, Inc., Micropolis, Finland). The diode area is .057 cm2. The flux at the detector, N = (2 x 10 x 3.7 x 107) / (4 pi x 102 ) = 588,870 gamma photons / cm2-second. This is equivalent to an exposure doserate at the face of the detector of 0.54 roentgens / hour.
An exact expression for the average energy of a Compton recoil electron may be found in (6). An approximate formula -- accurate enough to be useful over a wide range of gamma ray energies* -- is given by Ê = 1/2 Emax, where Emax is derived from Compton's formula for the energy of the 180o (backscattered) photon:(4)
In this example hv = 511,000 eV, so that Emax = 340,000 eV, and Ê = 170,000 eV.
The radiation-induced current in the diode is therefore 588,870 x .057 x .01 x 170,000 x 1.6 x 10-19 / 3.6 = 2.53 x 10-12 amperes. Thus the scale factor relating current to exposure doserate in this example is (2.53 picoamperes) / (0.54 roentgens per hour), or 1.68 x 10-8 coulomb / roentgen.
One roentgen produces, by definition, 3.33 x 10-10 coulombs in 1 cm3 of standard air(5). Thus, our .057 cm2 diode has the same radiation-induced-current sensitivity as a (1.68 x 10-8) / (3.33 x 10-10 ) = 50 cm3 standard air-ionization chamber.
*The estimate, Ê, is ~ 2% high at 80 KeV, dropping to ~3% low at 511 KeV, dropping further to ~10% low at 1000 KeV.
Pulse-Mode Operation--Radiation Survey and Monitoring Applications
Stable, reliable operation at low-to-medium exposure doserates in general radiation survey and monitoring applications is enhanced by operating the PIN diode detector in AC-coupled pulse-mode. This essentially eliminates drift and instability due to changes in system parameters, such as diode leakage current, with time and temperature.
In this mode of operation the diode is closely coupled to a charge-integrating preamplifier of our own design (USA patents 5,990,745 and 6,054,705), so that individual x-ray or gamma-ray photon interactions are detected as discrete pulses of current. The preamplifier gain, expressed in units of "volts per unit charge" is 1 / Cint, where Cint is the value of the integrating capacitor which, in this implementation, is of the order of 2 x 10-12 farads.
Example: Assume that an incident 511 KeV photon produces a 340 Kev recoil electron in the diode. This, in turn, produces a charge of 340,000 x 1.6 x 10-19 / 3.6 = 1.51 x 10-14 coulombs deposited in 2 picofarads, or a voltage pulse whose amplitude = 7.55 millivolts. Individual voltage pulses are then further amplified, thresholded, and integrated.
We eliminate system noise by introducing a low-energy threshold before the input to a bipolar junction transistor - charge-pump. This, in turn, is followed by an RC-integrating filter with a time-constant nominally = 1 second. The overall system gain beyond the preamp is set so that a 1.33 mV preamp-output pulse (60 KeV photon energy) just exceeds the threshold.
The input current to the charge-pump is set by a series resistor. The output of the charge pump / filter is a DC current proportional to doserate in the detector which may be read by a meter, chart recorder, or computer data-acquisition system. In addition, a current-to-pulse-rate converter provides a TTL-compatible output for convenient interfacing to computer process-control and monitoring systems.
In addition to general survey and monitoring applications, the same basic system detector and amplifier concept can be used for spectroscopy applications, where the detector diode is optically-coupled to a scintillating crystal such as CsI(Tl) for gamma-ray spectroscopy, or where the detector diode is used directly to detect x-rays below 60 KeV.
Spectroscopy applications demand the best in low-noise performance from the detector and its amplifying system. The circuit components and active devices must be selected specifically to match the application at hand. In particular, the amplifying transistors (JFET's) at the input of the charge-integrating preamplifier must be chosen to match the junction capacitance of the detector diode in order to achieve the lowest possible noise and best pulse-height resolution. In addition, the time constant(s) in the shaping amplifier must be optimized to minimize line-broadening effects due to noise and from "ballistic deficit" effects due to non-uniform charge-collection times in the diode detector.
Below is an example of a gamma-ray / x-ray spectrum from an 241Am check-source attached to the surface of a 2.7 mm x 2.7 mm diode (Cj = 15 pF, Vbias = 24 VDC). The prominent source peaks are:
The large peak at 8 KeV is due to x-ray fluorescence from a thin copper foil attenuator placed between source and detector.
Spectral measurements are done at room temperature. The shaping-time is 8 µsec. The pulse-height resolution is 2.07 KeV (FWHM) at 59.5 KeV. The second trace, whose peak is shown centered at 43 KeV, is from a pulser and shows the line-broadening effects of electronic noise in the diode detector and preamplifier. The pulser-peak is 1.80 KeV wide (FWHM), which corresponds to an equivalent noise charge in the system "front end" of 212 e- rms.
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