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22 06-2011 elektor
TesT & MeasureMenT
Measure Gamma Rays
with a Photodiode
Radiation detector
using a BPW
Ionising radiation is potentially harmful to health, and it is important to minimise one’s exposure to it as far as possible. A sim- ple Geiger counter with a small glass mantle tube will not usually be adequate to detect possibly harmful radiation. The semicon- ductor sensor we describe below also has a relatively low sensitivity, only being able to detect fairly intense sources of radia- tion, 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 radia- tion 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 can- not even penetrate a sheet of paper: this is the reason that many counter tubes can- not 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 sensi- tive to beta particles.
Diode as detector
The behaviour of a type BPW34 PIN photo- diode is similar to that of a low-cost coun- ter tube. Alpha particles will be stopped by the plastic enclosure of the device, whereas gamma rays pass through without prob- lem and create many electron-hole pairs in the diode’s depletion layer. If the diode is reverse-biased, almost all of the charge car- riers will be drawn away: this corresponds to a small current pulse which can be ampli- fied 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 photodi- ode as a beta and gamma radiation detector is that light must be completely excluded, as otherwise the photocurrent will over- whelm 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 semi- conductor that can interact with photons. The structure is used in a photodiode in
by Burkhard Kainka (Germany)
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.
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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 disad- vantage of increasing its capacitance, which reduces the (voltage) amplitude of its output signal. Commercially-avail- able 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 enclo- sures. The (cheaper) BPW34 comes in a plastic package, whereas the BPX61 comes in a TO-5 metal enclosure with a glass win- dow. 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 par- ticles, and alpha particles with an energy of 4 MeV or more will also pass through. When the particle enters the plastic of the photo- diode package, deceleration radiation (Ger- man: ‘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 sensi- tive to ionising radiation. It is perhaps less surprising, then, that a photodiode is sen-
sitive 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 build- ing electronics to withstand the higher lev- els 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 usu- ally constructed using a low-noise FET-input opamp as the input stage. Here we take an alternative approach: Figure 1 shows the circuit of the sensor amplifier. Two tran- sistors are used to amplify the signal from the photodiode. The direct-coupled ampli- fier automatically sets itself to a mid-range operating point, which gets a good signal- to-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 increas- ing 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 suit- able solution.
Construction
The circuit can be built on a piece of bread- board (see Figure 2 ), with the photodi- ode on the underside ( Figure 3 ). To keep light out of the sensor the whole circuit is wrapped in aluminium foil ( Figure 4 ). As mentioned above, ordinary kitchen foil is
D
BPW T
BC549C
T
BC549C
R 4k
R 220k
R 1k^
C 10u16V
C 100u16V
R 2M
R 220k
C 100n
K
+9V
110372 - 11
Figure 1. The amplifier circuit.
Figure 2. Prototype of the sensor amplifier. Figure 3. The sensor is on the underside of the board.
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pulses with an amplitude of over 100 mV than in the case of the luminous watch. This shows that, unlike a Geiger tube, this detector can determine the energy of the individual particles. This in turn lets us make deductions about the types of nuclei that are disintegrating. In the case of pitch- blende these will be elements in the ura- nium decay series; in the case of the lumi- nous watch the decaying nucleus is likely to be radium.
The possibility of accumulating readings over a long time period lets us examine samples where we would expect little or no radioactivity. Here the photodiode works better than the counter tube as the back- ground rate is practically zero. With a Gei- ger counter there are almost always pulses arising from cosmic rays: these hard gamma rays also affect the photodiode sensor, but because of its much smaller sensitive area these events are much rarer, and so it is eas-
ier to separate the wanted signal from the background. Figure 8 shows readings from a sample of galena, a mineral that we would expect not to be radioactive at all. After half an hour, however, we see two clear peaks. We obtained a similar result from a sam- ple of granite, which is known to be slightly radioactive.
Certain components and pieces of appara- tus manufactured before stricter modern
Figure 5. Circuit output in the quiescent state.
Figure 6. Readings over 30 seconds from an old watch with luminous hands.
Figure 7. Readings over 30 seconds from a sample of a mineral containing uranium.
anzeige
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controls were in place can turn out to be radioactive sources. A well-known example is that certain gas discharge lamps and volt- age regulator tubes rated under 100 V con- tain radioactive substances. The author had already had suspicions about an old Russian gas discharge lamp rated at 75 V/3 mA ( Fig- ure 9 ). There is a small metal cap welded on to the outer cover, beneath which a strange pill is visible. Beneath this is a tiny hole.
Taking readings over half an hour ( Fig- ure 10 ) revealed impulses with a particu-
larly high energy: all the more surprising given that the radiation had had to pene- trate the glass envelope of the tube.
Outlook
We have described the sensor and a simple amplifier for it. If the circuit is built into an enclosure, along with the comparator cir- cuit and loudspeaker described in the text box, the result is a device that can be used in the field, for example to test minerals in a quarry. Combine the comparator with a digital counter, and the overall level of
radioactivity can be measured. A sample- and-hold circuit could be added to record energy levels, and the results could be dis- played as a kind of energy spectrogram.
Another possibility is to look at taking measurements from other samples over a long period. For example, potassium chlo- ride is a very weak beta emitter. It would be interesting to see if the photodiode can detect it. (110372)
Radon decay products Radioactive samples for testing can be obtained directly from the environment: we are constantly surrounded by radioactive materi- als. For example, radon is continuously escaping from the ground. This radioactive gas decays (with a moderate half-life) producing further radionuclides, which can be collected. Take a thin (0.2 mm diameter) piece of enamelled copper wire and stretch it out indoors. Apply a negative potential of between –5 kV and –10 kV to the wire and leave it for ten minutes. Disconnect the high voltage and then run a strip of paper along the length of the wire: you will pick up a dark line of dust that was attracted to the wire by the high voltage. These dust particles are particularly rich in the radioactive decay products of radon. The reason for this is interesting: when the ra- don decays the new nuclei are moving rapidly, and are therefore
stripped of some of their electrons. This means that they have a positive charge and so are attracted to the negatively charged wire. When this ‘dirty’ strip of paper is held up to the radiation detector, a high level of activity will be recorded. There is no danger, how- ever: if the isotopes had not been picked up by the wire, you would probably have breathed them in instead. This method will let you determine which rooms in your house have more radon in them. Normally levels will be higher in a cellar or basement as the source of the radon is the earth below. A suitable high-voltage generator was described as the ‘Air Ioniser’ mini project in the June 2009 edition of Elektor (http://www.elektor. com/071072). It is necessary to extend the design by a further two stages (two capacitors and two diodes) to obtain an output voltage of 5 kV.
Figure 10. Readings taken from the gas discharge tube shown in Figure 9.
Figure 9. A gas discharge tube with a radioactive ioniser to aid starting.
References and Internet Links
- Maxim Application Note 2236: Gamma-Photon Radiation Detector http://pdfserv.maxim-ic.com/en/an/AN2236.pdf - Erhan Emirhan and Cenap S. Özben: PIN photodiode-based X- and gamma-ray detectors
http://thm.ankara.edu.tr/tac/YAZOKULU/yazokulu6/ dersler/06-09-2010/erhan-emirhan-cenap-ozben-pin- photodiode.pdf
- C. W. Thiel: An Introduction to Semiconductor Radiation Detectors www.physics.montana.edu/students/thiel/docs/Detector.pdf
Figure 8. Readings over 30 minutes from a sample of galena.
