<< Back to Introduction to Ionizing Radiation


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
 
 
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
b)
 
A
 
=
 
Ai(0.5)
 
t
T1/2
 
c) A = Aie -0.693t
T1/2

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 = Γ A
d 2
  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   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
 
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

6 - Effective Half - Life

1
T 1/2 eff
= 1
T 1/2 rad
+ 1
T 1/2 bio
  T1/2eff = Effective Half - Life
T1/2rad = Half-Life of internal radioisotope or source
T1/2bio = Time for the body to eliminate half of the source.
 
or
 
T 1/2 eff = T 1/2 rad   x   T 1/2 bio
T 1/2 rad   +   T 1/2 bio