Sr90 Geiger Counter/Gamma Ray Spectrometry

Sr90 spectrum

Sr90 emits beta ray, not gamma ray.
Most radiation detectors sense gamma rays only, so Sr90 is not likely detectable.

However, in an environment with 0.05μSv/h background, Sr90(0.1μCi)attached closely to iMetry or HORIBA PA-1000 shows raise of radiation dose up to around 2μSv/h. What is this?

Normal background

iMetry with closely attached Sr90, dose and spectrum

HORIBA PA-1000 with closely attached Sr90, dose

Strontium90, undergo beta decay with 28years half-life, emits 546keV beta ray and decays to Yttrium90, Yttrium90, undergo beta decay with 64hours half-life, then emits 2280keV beta ray and decays to Zirconium90. Bremsstrahlung due to these strong beta rays may be emitted from iMetry's aluminium closure. Being a bremsstrahlung, it has white, continuous spectrum.

Spectrum with iMetry and Sr90 closely attached

According to wikipedia, a structure first covers beta ray source with less bremsstrahlung emitting low-Z (proton number) nuclear material, such as plastics, and then shields the X ray with lead is recommended for beta ray shielding.

Beta ray is less penetrating, easily blocked with an aluminium plate of a few mm thickness or a plastic plate of 1 cm thickness. But beta ray produces X ray when blocked and slowed down (This X ray is called bremsstrahlung), so shielding for this secondary X ray is needed.

The larger the Z (proton number) of the blocking material becomes, the stronger the bremsstrahlung gets, so beta ray shielding needs two-step process of first using low-Z (proton number) and then blocking secondary X ray with high-Z material such as lead.


  • 最終更新:2013-06-16 22:25:11