Selenium 75

With an energy profile lower than Iridium 192, Selenium is becoming an industry standard. The lower energy profile allows large sources to be used, up to 80 Curies for type A quantities, and still giving reduced controlled areas compared to Iridium. The long half life, 4 months, allows fewer change outs of the source and with good physical sizes, 3 mm sphere for up to 100 curies, a resolution for a given exposure time comparable to Iridium can be achieved.

Half Life:

119.78 days

Exposure Rate (1Ci at 1 metre):

2.05 mSv/hr

Mode of decay:

Gamma 66 - 401 keV (avg 217 keV)

Optimum working thickness:

2mm to 30mm steel

Attenuation Value Layers:

Material	1/2	1/10	1/100
Lead (Pb)
U-238
Tungsten (W)

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