Friday, February 19, 2021

SE International Radiation Alert Ranger - making "protective cap accessories".

One thing I really like about the design of the SE International Radiation Alert Ranger is the protective cap on the back for the "pancake" detector.

This cap is great to protect the delicate mica window from contamination and mechanical events that can destroy the detector.

I realized that this cap can easily be turned into a useful  "charged particle filter" or even a check-source.

I went ahead and ordered a few spare caps from SE International at $1.50 a piece.

The  assortment of "cap accessories I made - a check-source, an Alpha filter and an Alpha + Beta filter. The Alpha filter is used for Beta + Gamma measurements and the Alpha + Beta filter is used to measure Gamma only.
The "Alpha Filter" (the cap with the black insert) is made by cutting a disk of self-adhesive aluminum foil (thickness 0.1mm) and applying it on the inside of the cap. The foil in combination with the plastic of the cap will stop all alphas, while letting most betas come trough. Adding the thin aluminum foil is not "a must" as the plastic of the cap will be completely sufficient to block Alphas but it can double as a "soft beta" filter as well.

For the "Alpha filter", thicker cooking foil can be used as well and attached with double-sided adhesive tape or glue.

The "Alpha + Beta" filter (gray) is made by cutting a disk out of aluminum. The disk is a tad small than 1" 3/4 and thickness is 3mm. This disk is secured inside the cap with a piece of double-sided adhesive tape.

The "Alpha" filter (top) is placed for measuring Beta + Gamma activity only. The "Alpha + Beta" filter (bottom) in addition shields all Betas up to 2 MeV and attenuates Gamma <7 % @662 keV (Cs-137)

For the "Alpha + Beta" filter, I cut the disk out of equipment rack blank panel. The aluminum of the blank is 3 mm thick which stops all beta particles with energy up to 2 MeV and has very low Gamma attenuation (less than 7%) for the Cs-137 isotope (662 keV).

Cutting a perfect, large (1" 3/4) disk out of 3mm aluminum is a bit of a chore but nothing that can't be solved by a drill press, a grinding wheel and a file.

As an alternative, pure 1 mm Lead (Pb) sheet will stop betas up to 2.3 MeV and will attenuate Cs-137 gamma by 10%, Co-60 Gamma by 5% and not only the math is easier but it is much easier to cut than 3mm aluminum - one can use regular scissors for such thin lead sheet.
The plastic of the cap should slow down beta particles a bit perhaps reducing the generation of secondary X-Rays (from the Bremsstrahlung effect)
Inserts made out of other materials can be made to fit these protective caps - I'll be making another cap with a 3 mm thick lead sheet insert for measuring extremely high activity gamma sources (normally the Ranger overflows at 350K CPM). 
By calculating the attenuation factor of the Lead in the cap, I can correct the measurements which will extend the maximum range of the instrument. For example 1/8" Lead (3.1mm) will attenuate Cs-137 Gamma rays by 28% and Co-60 by 14% and this can be used as a crude dose rate correction.

The Check-Source Cap using an epoxy sealed tiny Autunite crystal.
The Autunite crystal is placed on a piece of aluminum foil substrate and suspended inside a drop of epoxy. I made a shallow cavity in the plastic to accommodate the epoxy drop and after trimming the foil it is attached on the inside of the cap with Kapton tape. The source produces around 600 CPM when the cap is in place.

It is very important for the Autunite crystal to be completely sealed inside the epoxy drop so no radon can quickly escape and contaminate the mica window of the pancake detector. In addition, this cap is normally stored in the carry pouch and only used as needed.
I used N.O.R.M. for the source for 2 reasons - I don't have to worry about half-life and changes of the activity over time and secondly, I wanted to have a very low activity check-source that I have no problem carrying around and traveling with. Unfortunately, the lowest activity Cs-137 disk source I have at hand is 1 uCi so I just went the NORM route instead.
Alternatively (and it actually would be even better), one can attach a Spectrum Technique sealed Cs-137 disk source on the inside of the cap - something like 0.05 uCi or 0.1 uCi or even a tiny slab of uranium glass - the larger cap is just more difficult to lose and easier to work with acting as a "holder' for the source.

1 comment:

Unknown said...

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