DIY_particle_detector_wiki/Enclosures.md

General Recommendations

The detector enclosure must be made from metal, acting as a Faraday cage.

Important aspects when choosing a metal case:

• large enough to house a 9 V battery, the circuit board, signal output connector and on/off switch
  minimum size recommendation:
  • 8 x 4.5 x 3 cm³ (9V battery upright)
    or
  • 8 x 5.5 x 2 cm³ (9V battery flat)
• measuring objects inside the box (easier; proximity to the diodes is useful) or outside (more advanced)
  For measuring the energy spectra of alpha particles with the alpha-spectrometer variant, using a larger box with everything inside, including the radioactive object, is the recommended method!
• 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.

Creating a "radiation window"

If you choose a small enclosure where radioactive objects cannot fit inside, it is a good idea to add a "window for radiation" 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.

Candy Tin Boxes

Some tin box enclosure examples below. More in the picture Gallery.

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.

Diecast Aluminum Enclosures

Two small enclosures that are just big enough for housing everything (besides most specimens):

• Multicomp G102MF, fits the circuit board precisely using edge mount screws, BUT very tight fit together with the battery (shown below on the left)
• Deltron 480-0010, more available space but no existing mounting holes for the board (shown below on the right)

These diecast aluminum cases are so small that only tiny objects like a small stone or piece of uranium glass (e.g. a marble) will fit inside. If you want to measure barely radioactive objects, larger metal cases are much better. For measuring objects of low radioactive intensity, the diodes should be in direct contact with the specimens' surface which is possible if the metal case is large enough to house everything - including the specimen. Alternatively, consider adding a radiation window in order to improve the sensitivity of the detector.

(Note: The hole drilled into the lid on the left is a radiation window covered with metal tape.)

An example of mounting the diodes on the backside of the PCB and adding a radiation window to the underside:

This is the best method to get the detector as sensitive as possible (since the diodes are very close to the windows this way). It may be a bit more difficult to create a hole in the underside of those small die-cast cases, but it doesn't have to be square-shaped - it can be round as well, drilled with a larger diameter drill bit.

The switch is optional and just fits into the free space next to the battery.

Left side: diodes facing down towards radiation window, right side: diodes facing up (radiation window in the lid):

(Note: The right picture is featuring a previous version of the circuit board, V1.1, where the capacitor below U1 was called C6 instead of C5.)

Important remarks when using one of these two very small cases:

• 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 rechargeable NIMH accumulators that come in plastic housings
• 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.
• route the connection wires along the edges/walls of the enclosure (they can pass along the curved edges of the battery)
• the on/off switch can be mounted on the second short side of the main case, on the opposite side to the signal output connector. It will be somewhat in the way above the diodes, but that does not matter when building the electron-detector variant.
• In the case of the alpha-spectrometer variant, the BPX61 diode should be mounted on the opposite side of the circuit board, facing downwards. A hole with the outer diameter of the diode's metal case should be drilled into the bottom, serving as the radiation window. Instead of covering that hole with sticky metal tape, consider shielding the BPX61 diode directly using an even thinner foil as explained here. This is the most difficult build - using a bigger case that can fit the radioactive object is much easier.