I'm wondering about including a mu-metal shield in the enclosure design. If we're committed to an all solid-state detector system it shouldn't be necessary. The Al tube will be a fine EMI shield on its own.
If a scintillator+PMT approach is to be an option, then, yes, mu-metal will be needed for that.
OK - I agree. We can keep one in mind if need be, but not include it for now.
I am thinking also of the divided box. It has some advantages.
Simplified Digression about Electromagnetics
Let us have a bit for HM members who are not into RF engineering.
I knew that the complete aluminum covering would stop all real electric fields. That alone also shuts off all incoming radiated EM (meaning electromagnetic) fields, but won't hold out against LF (low frequency like 60Hz mains frequency) magnetic fields. These come from house wiring, power lines, etc. Do NOT expect to use this gadget while someone is welding in the shop!
For those who don't know the difference, a (very) condensed explanation is that EM fields are like light. They are the same stuff as light. They propagate at the speed of light. They are provoked into life only by
changing real electric or magnetic fields, which we often optimize with antenna structures, but in fact, all the house wiring is acting as less efficient antennas.
To get a real electric field to change needs movement of electrons, which is an electric current, which in turn is accompanied be the making of a magnetic field. All currents make magnetic fields around the wires! If you manage to vary one kind of field, you will generate some of the other. Electromagnetic (light-like) fields are stopped completely by the aluminum shielding, which makes what is known as a Faraday Cage. You cannot make an electric field inside a conducting surface that will still exist outside the surface, and vice versa. The electronics inside the metal covers will be completely screened from EM stuff. I know we call the light-like radiation "electromagnetic", but that is only because of the varying fields we use to make it. It's not the kind of thing that would twitch a compass needle.
That still leaves the ability of 60Hz
magnetics from house wiring and power lines (like the cord to your beer refrigerator) to just waltz right in, and start inducing unwanted sine-wave power line noise into the (very) sensitive low noise amplifier electronics. This is more of a problem for photo-multiplier tubes, which have an electron beam that is very readily messed up by magnetic stuff, but less so for solid state kit. For this gadget, the only vulnerable parts are the connections from the photodiode to the amplifier, and if we have them close together, to minimize the loop current area, then not much can happen to to couple in unwanted 50/60Hz noise.
50/60Hz suppression
I say that hopefully, but given that we have multi-stage electronics gain, we can, if need be, make one of the stages also reject 50/60Hz, reducing it by a factor of perhaps about 1000. To start with, I am making the amplifier straight - no suppression. If it works, then OK. Otherwise, AC mains filters get included.
Filters
I am completely sold on having the ability to mount physical elements discrimination filters. The next CAD will show them as push-on tubes that fit over the aperture to the photodiode.
[Somewhere, on this floor, are 0603 decoupling capacitors that will only ever have a future involving a vacuum cleaner!
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