180 CPM as Minimum Background radiation

[mickrose]

Hi!
Background radiation in my place is ~2000 CPM, using a Scintillation crystal NaI.
I could reach 180 CPM after designing/testing an amateur Lead Castle (bricks + plates).
What's stopping my progress?

50 Kg of Lead not enough?
Muons?
Pb210 of recent unpure Lead ? (I am working on a spectrogram of this noise)

thanks a bunch!

[sgt_bear]

Hi!

Could you provide some information about your detector model and voltage?
A Spectrum of the shielded background would be helpful.

[mickrose]

Sure!
Portable 25x25 NaI(Tl) FWHM 10% resolution at 662keV
http://www.rhelectronics.net/store/port ... scopy.html

Regards from Geneva

[MaxGaspa]

The answers are...yes , yes and yes. Moreover there are other sources of gamma background....

You got a shielding factor of 10 that is the expected shielding factor for an amateur shield of few centimeters. My DIY shield was made using 83 kg of lead (commercial) , 3.5 cm thick. It has a shielding factor (ratio of total counts) of 18 against my "normal" natural background using a 30x10 mm NAI and an Hamamatsu R9420.

You should consider that "professional" shields made by Camberra or Ortec are using much more lead..... thousands of Kg......50 kg is just an amateur shield. Anyway a factor of 20 or 10 is already a good result and helps the gamma spectroscopy a lot.

[Geoff]

Not to mention that your lead is likely contributing to your shielded background. Contamination in commercial lead is not uncommon.

[Sesselmann]

I think it is unrealistic to achieve a near zero background, almost everything around us has traces of radioactive isotopes. The glass in the PMT is likely to contain some K40, then there is the lead itself, and I am pretty sure that some random electrons will cascade down the PMT even with no photons striking the cathode.

180 CPM should be considered a pretty good result, and I think spending time and money on a copper/tin lining is going to be more rewarding than adding more lead.

[luuk]

Hi Mike,
You should try 5 to 10mm copper inside your leadcastle then a 10mm perspex layer that will reduce bgrnd to a minimal reachable.
Luuk

[stamasd]

My background averaged over a long time (a couple of hours) is 10.75CPS. That's with a lead castle made of 170 pounds commercial lead (77Kg) 3cm thick in all directions and lined with 0.7mm copper inside. The detector is a 2.5*2.5" NaI(Tl). Without the castle the background count is 80-90cps depending on time of the day and weather. I think it's pretty OK for shielding; maybe I'll add some tin later on, perhaps some thicker copper (I still see the Pb XRF lines through this amount of copper)