RULES OF THUMB FOR ALPHA PARTICLES
1. An alpha particle of at least 7.5 MeV energy is needed to
penetrate the nominal protective layer of the skin
(7 mg / cm or 0.07 mm).
2. The alpha emissions and energies of the predominant
particles from 1 ìg of several common materials are:
238
Pu
239
Pu
240
Pu
242
Pu
a
Natural U
Oralloy (93%
b
Natural Th
D-38 (DU, tuballoy)
a
234
Includes
b
228
Includes
chemical separation,
disintegration rate lower than 0.5.
c.
With 2p (50%) geometry, the surface of a thick uranium metal
(tuballoy) source gives ~ 2400 alpha counts/min per cm .
Depleted uranium (D-38) gives ~ 800 alpha cpm/cm .
3. Alpha particles lose about 0.8 MeV per mg/cm density
thickness of the attenuating material.
4. Detector window thicknesses cause alpha particles to lose
energy at about 0.8 MeV per mg/cm of window thickness.
Therefore, a detector with a window thickness of 3 mg/cm
(such as sealed gas-proportional pancake alpha/beta
detectors and pancake GM detectors) will not detect alpha
emitters of less than 3 MeV.
2
DPM per ìg
39,000,000
140,000
500,000
8,700
1.5
235
U) 160
0.5
1
U in equilibrium.
Th in equilibrium. Depending upon the time since
228
Th can decrease to give a net
Alpha Energy (MeV)
5.50 (72%)
5.15 (72.5%)
5.16 (76%)
4.90 (76%)
4.20 (37%), 4.77 (36%)
4.39 (~ 80%)
4.01 (38%), 5.43 (36%)
4.20 (~ 60%)
2
2
2
2
2