Enclosures
Important questions when choosing a metal case:
- large enough to house a 9V battery, the detector PCB, the signal output connector (optionally an on/off switch)
- measuring objects inside the box (easier; proximity to the diodes is useful) or outside (more advanced)
- the circuit board should be firmly fixed in place with at least one screw
- mobile or stationary usage of the detector?
For beginners building the DIY particle detectors, a candy tin box and rather stationary usage on a table is generally advised - the case can be easily upgraded later.
Tin boxes are easy to work with since the metal is typically very flexible and soft, but this can be also a downside. The larger the tin box, the more it is sensitive towards picking up vibrations ("microphony effect").
Therefore, if the detector is enclosed in a candy tin box, it should not be touched or moved during measurements.
In particular, the alpha-spectrometer variant may be disturbed even in loud environments due to its larger amplification factor.
If choosing instead sturdy and thick-walled metal enclosures like diecast aluminum, the detectors will be immune to vibrations.
The enclosure must block all light completely from reaching inside. In case of doubt about light-tightness, put a thick and dark towel or piece of cloth on top of the enclosure during measurements. Further context in the Diodes section.
If you choose a small enclosure where radioactive objects cannot fit inside, it is a good idea to add a "radiation window" in front of the diodes. This allows more particles to directly reach the diodes with less interaction/absorption in the dense metal walls of the enclosure. See below for some examples where holes have been added opposite to the diodes. In order to keep light still blocked, those holes must be covered again with thin layers of tape. A good combination is one layer for sticky metal tape (copper or aluminum) plus one layer of black electrical insulation/gaffa tape. If metal tape is not at hand, 2-3 layers of electrical tape may be required - depending on the tape and intensity of the surrounding light. Note thin invisible cracks may occur in the metal tape if it is being touched, the additional layer of plastic tape helps to prevent that.
Some tin box enclosure examples:
Use a 9 mm hole puncher to quickly and safely create a hole for the signal output BNC connector. Use a smaller hole puncher for the on/off switch and a bigger one to create a radiation window if desired. Note that the hole will be always bigger than the nominal diameter of the hole puncher as tin box walls are usually quite soft. Put a piece of hard wood below the tin box when punching holes. If physical access with the puncher is diffcult because the tin box is small, bend the tin box metal outwards, punch the hole and bend the metal back inwards when finished.
Two different small enclosures that are just big enough for housing everything are shown below. Important remarks:
- consider ordering thinner circuit boards than the standard 1.6 mm PCB thickness,
better 1 or 0.8 mm as the available space is very tight. - use thin 9 V batteries, like rechargeable NIMH accumulators that come in plastic enclosures
- cut the component traces, also the ones of the amplifier chip U1, as short as possible and orient the capacitor C5 flat on the board as shown below (or use an SMD version).
- if the 9 V battery has itself a metal enclosure, make sure to prevent short circuits from the sharply cut pins.
e.g. apply enough insulation tape around the battery or add a thin sheet of plastic between board and battery. - It is no problem if the lid of the enclosure doesn't close completely as long as no light reaches inside. About 1 mm of a slit between the lid and main part of the enclosure is tolerable. Put the soft white or black rubber seal in place, it helps with blocking all light. If all 4 screws are in place, that is enough for the metal case acting as a Faraday cage and provide electromagnetic shielding (blocking EMI) - a thin remaining slit between the lid and main part doesn't matter.
(This is an image of the previous version of the circuit board, V1.1, where the capacitor below U1 was called C6 instead of C5.