RADIATION DETECTOR FOR GAMMA RAYS
RADIATION DETECTOR
GAMMA RAYS
The first device that springs to mind when thinking about
measuring radioactivity is the Geiger-Müller tube. However, these counter tubes
are getting hard to find and expensive, and even if you do manage to get hold
of one, you will still need to find a way to generate its operating voltage of
several hundred volts. It is less well known that even a humble photodiode such
as the BPW34 can be used to detect X-rays and gamma radiation.
Ionising radiation is potentially harmful to health, and it
is important to minimize one’s exposure to it as far as possible. A simple
Geiger counter with a small glass mantle tube will not usually be adequate to
detect possibly harmful radiation. The semiconductor sensor we describe below
also has a relatively low sensitivity, only being able to detect fairly intense
sources of radiation, but it is nevertheless an interesting device for carrying
out experiments and measurements. An advantage of using a photodiode is its small
sensitive area. The background rate due to cosmic rays is very low and signals from
small samples are easier to detect than with a counter tube.
Radiation
When considering protection from radiation it is gamma rays that are the most important. They can penetrate walls and it is difficult to block them. Hard gamma rays are present in the environment all around us and are also not stopped even by a thick wall. Alpha particles, on the other hand, only have a short range and generally cannot even penetrate a sheet of paper: this is the reason that many counter tubes cannot detect them, unless they have a very thin mica window. Beta particles have a longer range and can penetrate thin sheets of metal. Most counter tubes are mainly designed for detecting gamma rays while, within certain limitations, also being sensitive to beta particles.
When considering protection from radiation it is gamma rays that are the most important. They can penetrate walls and it is difficult to block them. Hard gamma rays are present in the environment all around us and are also not stopped even by a thick wall. Alpha particles, on the other hand, only have a short range and generally cannot even penetrate a sheet of paper: this is the reason that many counter tubes cannot detect them, unless they have a very thin mica window. Beta particles have a longer range and can penetrate thin sheets of metal. Most counter tubes are mainly designed for detecting gamma rays while, within certain limitations, also being sensitive to beta particles.
Diode as detector
The behaviour of a type BPW34 PIN photodiode is similar to that of a low-cost counter tube. Alpha particles will be stopped by the plastic enclosure of the device, whereas gamma rays pass through without problem and create many electron-hole pairs in the diode’s depletion layer. If the diode is reverse-biased, almost all of the charge carriers will be drawn away: this corresponds to a small current pulse which can be amplified and processed. Beta particles can also generate such a signal if they are sufficiently energetic to reach the depletion layer.
The behaviour of a type BPW34 PIN photodiode is similar to that of a low-cost counter tube. Alpha particles will be stopped by the plastic enclosure of the device, whereas gamma rays pass through without problem and create many electron-hole pairs in the diode’s depletion layer. If the diode is reverse-biased, almost all of the charge carriers will be drawn away: this corresponds to a small current pulse which can be amplified and processed. Beta particles can also generate such a signal if they are sufficiently energetic to reach the depletion layer.
The amplitude of the signal produced by the photodiode is
considerably smaller than that normally obtained from a counter tube, and so a
very low-noise instrumentation amplifier circuit is needed. Another requirement
when using a photodiode as a beta and gamma radiation detecto is that light must be completely excluded, as
otherwise the photocurrent will overwhelm the signal we are looking for. In our
prototype we used ordinary aluminium kitchen foil as a screen. The difference
between PIN diodes and PN diodes is that the former include an extra very lightly
N-doped region called the ‘intrinsic’, or ‘i’ region. This high-resistance region
lies between the ‘n’ and ‘p’ regions. The result is a wider depletion layer in
the diode, and hence a greater volume of semiconductor that can interact with
photons.
The structure is used in a photodiode in order to obtain as many charge
carriers as possible per photon, optimising the device’s sensitivity. Another
way to increase sensitivity is to increase the sensitive area of the device.
However, this has the disadvantage of increasing its capacitance, which reduces
the (voltage) amplitude of its output signal. Commercially-available semiconductor
radiation detectors have a large area and a wide intrinsic region. Simple PIN
photodiodes such as the BPW34 are less sensitive than these devices, but also
of course somewhat cheaper. The BPW34 and BPX61 photodiodes are
practically identical apart from their enclosures. The (cheaper) BPW34 comes in a plastic package, whereas the BPX61 comes in a TO-5 metal enclosure with a glass window. It is possible to remove this window (carefully!) to expose the chip: this will
make the diode capable of detecting alpha particles.
practically identical apart from their enclosures. The (cheaper) BPW34 comes in a plastic package, whereas the BPX61 comes in a TO-5 metal enclosure with a glass window. It is possible to remove this window (carefully!) to expose the chip: this will
make the diode capable of detecting alpha particles.
The rays or particles must first make it through a 15 µm
thick piece of aluminium (the thickness of ordinary kitchen foil). This is no
obstacle to gamma rays and beta particles, and alpha particles with an energy
of 4 MeV or more will also pass through. When the particle enters the plastic
of the photodiode package, deceleration radiation (German: ‘bremsstrahlung’) will
be produced in the form of brief flashes of light, which can also sometimes be
detected by the sensor. It is therefore not impossible for even the BPW34 to
have some sensitivity to alpha particles.
In principle any semiconductor is sensitive to ionising radiation. It is
perhaps less surprising, then, that a photodiode is sensitive to radiation than
that the effect has not been widely remarked on before. The effect is however
well known in dynamic RAMs, whose stored data can be corrupted by incident radiation.
The problem of building electronics to withstand the higher levels of radiation
found in space is becoming increasingly difficult, because as structures get
smaller it becomes increasingly likely that a single energetic particle can
interfere with the operation of a circuit.
Amplifier
In the literature charge amplifiers are usually constructed using a low-noise FET-input opamp as the input stage. Here we take an alternative approach: Figure shows the circuit of the sensor amplifier. Two transistors are used to amplify the signal from the photodiode. The direct-coupled amplifier automatically sets itself to a mid-range operating point, which gets a good signalto-noise ratio from the low-noise BC549C transistors.
In the literature charge amplifiers are usually constructed using a low-noise FET-input opamp as the input stage. Here we take an alternative approach: Figure shows the circuit of the sensor amplifier. Two transistors are used to amplify the signal from the photodiode. The direct-coupled amplifier automatically sets itself to a mid-range operating point, which gets a good signalto-noise ratio from the low-noise BC549C transistors.
The transistor input of the amplifier has a comparatively low impedance, which
gives good noise matching. As a result of its base-collector capacitance the
first stage also operates as an integrator: this turns the brief pulses from
the photodiode into longer pulses which can then more easily be amplified.
Sensitivity can also be increased by increasing the reverse voltage on the
diode. This reduces the capacitance of the diode and increases the size of the
depletion layer. The voltage can be as high as 32 V, although the optimum value
probably lies somewhat lower: the diode already operates well at 9 V. It is also
possible to wire two or more photodiodes in parallel, and that way it is possible
to achieve a sensitivity on a par with that of a small counter tube such as the
ZP1310. An oscilloscope can be connected to the output of the circuit to view
the signal. Readers who yearn for the clicking sound of a ‘real’ Geiger counter should consult the text
box ‘From radiation to sound’ for a suitable solution.
Construction
The circuit can be built on a piece of breadboard, with the photodiode on the underside . To keep light out of the sensor the whole circuit is wrapped in aluminium foil . As mentioned above, ordinary kitchen foil is ideal for this as it is thin enough to let beta particles through. The foil also functions as electrical screening. To avoid the foil causing short circuits, wrap the board first in insulating tape, leaving a gap for the window of the photodiode. Then wrap the assembly in foil, not forgetting to connect the foil to ground.
The circuit can be built on a piece of breadboard, with the photodiode on the underside . To keep light out of the sensor the whole circuit is wrapped in aluminium foil . As mentioned above, ordinary kitchen foil is ideal for this as it is thin enough to let beta particles through. The foil also functions as electrical screening. To avoid the foil causing short circuits, wrap the board first in insulating tape, leaving a gap for the window of the photodiode. Then wrap the assembly in foil, not forgetting to connect the foil to ground.
AMPLIFIER CIRCUIT FOR THE DETECTOR
From
radiation to sound
