## Occupational Safety and Health Administration OSHA

1 - Inverse Square Law

I2 =  I1 [ d1 / d2 ] 2
I2, I1 = Intensity at d2, d1

d2, d1= Distance to source

2 - Activity

• a) Ai = 0.693 / T1/2 Ni
•

• b) A = Ai(0.5) t / T1/2
•

• c) A = Aie  -0.693t / T1/2
• Ai = Initial Activity in Becq. (ps)
• Ni= Initial Number of Atoms
• T1/2 = Half - Life in seconds
• A = Activity (dps) in Becq.
• t = time elasped in seconds
•
• e = natural log

3 - Exposure & Dose

• a)  Rad = 0.869 (f-factor) (Roentgens)
• b)  rem = rad x Quality Factor
• c)  Life Accumulation = 5 x (N-18) in rem      N = Age in years

4 - Exposure Rate

D = ΓA / d2

D = Exposure rate (R/hr)

Γ = Specific gamma ray constant in units of R · cm2/mCi · hour

d = distance from source in cm

A = Activity in mCi of the isotope

5 - Shielding

• a) I = (1/2) A I0
•

• b) I = (1 / 10) B I0
•

• c) I = I0 / 2 ( x / HVL )
•

• d) x = 3.32 Log [I0 / I ]   [ HVL ] or x = Log ( I0 / I ) HVL / Log 2
• I = Intensity on other side of shield
• I0 = Intensity without shield (R/hr)
• A = Number of Half-Value-Layers (HVL)
• HVL = Shield thickness needed to reduce exposure by half.
• B = Number of Tenth-Value-Layers (TVL)
• TVL = Shield thickness needed to reduce exposure by 1/10th.
•
• x = Shield thickness

6 - Effective Half - Life

• 1 / T 1/2 eff = 1 / T 1/2 rad + 1 / T 1/2 bio
• or
• T 1/2 eff = T 1/2 rad  x  T 1/2 bio / T 1/2 rad   +   T 1/2 bio
• T1/2eff = Effective Half - Life