Diodes
The image below shows several PIN photodiodes of the BPW34 family and BPX61 diodes on a black piece of foam:
Both kinds of diodes, the BPW34 series in different plastic cases and the BPX61 with metal/glass encasing, have the same sensitive area (~7 mm^2) and contain a very similar silicon chip - if not the same. The physics of these semiconducting solid-state silicon sensors when used with ionizing radiation is explained in detail in the article corresponding to this repository. The section about figure 1 discusses why their general efficiency for detecting gamma photons is quite low.
Considering the original packaging of the diodes, alpha particles from natural sources of radioactivity (up to 8 MeV energy) are not able to cross the passive plastic or glass windows without being fully absorbed before reaching the actual sensitive area.
Removing the BPW34's plastic casing is quite difficult without destroying the tiny bond wire connecting the anode of the diode.
For enabling alpha-spectrometry, the glass window of the BPX61 diode can be easily removed as explained below.
The easiest way to operate the detector is to put everything in one large and dark metal box:
- the circuit board with its mounted diode(s)
- the battery
- the object to be measured
The metal box must be absolutely light-tight. Otherwise, the photodiodes will measure what they are supposed to do. If the oscilloscope shows regular 50 or 100 Hz sinus waveforms, it is most likely artificial stray light from fluorescent neon tubes or LED lamps. If stray light is not the problem (put additionally thick black cloth on top of the detector case to be sure) 50 Hz and higher frequencies can disturb the signals because of electromagnetic interferences from household appliances, for example, nearby mains connected refrigerators or machinery consisting of electronic motors. Changing the room may help if indoors.
The detector circuit is extremely sensitive to electromagnetic interferences (EMI) from radio waves/RF radiation and must be operated within a metal case acting as a Faraday cage.
A metal case completely blocks alpha particles from outside (apart from radon gas potentially sneaking inside) while other types of ionizing radiation like electrons/beta particles will be only partially absorbed which leads to reduced detection rates (additionally affected by an increased distance between detector and specimen).
The rightmost BPX61 diode in the picture at the top is covered with a piece of very thin metal foil using superglue. This metal foil stems from an old broken down foil capacitor shown next to the diodes on the very right in the same picture. This is an advanced technique that can be used to protect the diode partially from light (100% light blocking cannot be achieved this way) and have it peek through a hole of the detector's metal case into the outside. If used in this way, the diode should be still operated by facing downwards on the specimen and not within a bright light environment. Some further light-shielding methods like a ring of dark foam or a towel/dark piece of cloth covering everything can be applied.
Pictures from @Matthias32 showing how foil capacitors can be opened with enough force and the help of strong pliers or cutters:
Microscope image of Osram BPX61 diode below on the left, the green scale indicates 2 mm. The sensitive area of the silicon chip is 2.65 x 2.65 mm^2. A bond wire from the anode pin on the right connects the top of the chip (this side is also marked with a notch in the metal case, lower right corner).
In order to detect alpha particles, the glass window of the diode must be removed. Care must be taken such that the bond wire stays in place when the glass window is removed: The right picture shows the mounted BPX61 diode after its glass window was removed: cutting small dents into the border of the TO metal case with small pliers easily crack the glass. Tipp: keep the diode upside down above a trash can during this procedure such that the glass pieces fall immediately down and away from the silicon chip!
Another view of a BPX61 diode without glass window here mounted on the backside of the circuit board:
Four small dents, carefully cut using small pliers is enough to crack the glass window completely. Turning the diode upside down afterwards and hitting it with or against a piece of hard metal/tool on the corner/side will remove the glass bit by bit. Poking inside the diode with tweezers is not advised since that can easily destroy the tiny wirebond. Loose chips of the glass can be carefully removed with some canned air, although too much pressure might as well impact the bond wire. As long as none of the glass chips or too much glass dust rests on top of the silicon chip, it will work fine.