X-rays are ionizing radiation comparable to radioactivity (in fact, low energy gamma rays and high energy X-rays are the same). X-rays are characterized by two independent values: their "energy", expressed in terms of accelerating voltage, and their intensity, directly related to the current. Penetration power is determined by energy, i.e. the percentage of a given intensity that is let through a certain material depends on the energy. The higher, the more penetrating. This means, that with an accelating voltage below 10kV, most X-rays don't even get through the tube glass. Higher energy also means higher ionisation power. One consequence of this is that X-rays become more dangerous the higher their energy. Another consequence is, that geiger counters are not sensitive to X-rays below perhaps 30kV. This of course depends on the geiger tube you're using and may even be higher. Their remains a dangerous gap: at 20-30kV, the X-rays are already penetrating the tube glass (and therefore are dangerous to your body too!), but you can't measure them properly. MIND THE GAP!! Not measuring an increased count rate doesn't necessarily mean there are no X-rays and you're safe! (Personal experience :-( )
For X-rays coming from a near point size source (this is the case with a tube) the square-distance law applies:
This means that e.g. at 2m distance, the intensity is 1/4 of that at 1m, or 1/400 (!) of that at 10cm distance.
For the absorption by material, an exponential law applies:
where
The following table gives some values (mu in mm^-1, d2 in mm). Wood and water only approx. Data taken from McMaster et al., see http://www.csrri.iit.edu.
|
voltage(kV) |
Al | Fe |
Cu |
Pb |
Wood (0.6g/cm^3) |
Water | ||||||
|
mu |
d2 |
mu | d2 |
mu |
d2 |
mu |
d2 |
mu | d2 | mu | d2 | |
| 70 | 0.0607 | 11.4 | 0.59 | 1.18 | 0.93 | 0.74 | 3.71 | 0.186 | 0.011 | 63 | 0.02 | 35 |
| 60 | 0.0726 | 9.55 | 0.87 | 0.801 | 1.40 | 0.50 | 5.52 | 0.125 | 0.012 | 58 | 0.021 | 33 |
|
50 |
0.0954 |
7.27 |
1.41 | 0.452 |
2.29 |
0.30 |
8.87 |
0.078 |
0.013 | 53 | 0.023 | 30 |
|
45 |
0.115 |
6.01 |
1.89 | 0.367 |
3.07 |
0.23 |
11.67 |
0.059 |
0.014 | 50 | 0.025 | 28 |
|
40 |
0.147 |
4.73 |
2.64 | 0.263 |
4.28 |
0.162 |
15.89 |
0.044 |
0.015 | 46 | 0.027 | 26 |
|
35 |
0.198 |
3.49 |
3.86 | 0.179 |
6.24 |
0.111 |
22.54 |
0.0307 |
0.017 | 41 | 0.031 | 22 |
|
30 |
0.292 |
2.38 |
6.03 | 0.115 |
9.66 |
0.072 |
33.76 |
0.0205 |
0.02 | 35 | 0.037 | 19 |
| 20 | 0.904 | 0.77 | 19.4 | 0.036 | 30.3 | 0.023 | 97.5 | 0.007 | -- | -- | -- | -- |
| 10 | 7.13 | 0.097 | 132.5 | 0.005 | 196.1 | 0.004 | 150.6 | 0.005 | -- | -- | -- | -- |
Examples: At 50 kV, 1mm lead will reduce the intensity to 0.5^(1/0.078) = 0.014% of the one without lead. 5cm wood only halves the intensity. 28cm water reduces the radiation to 0.1%.
Rule of thumb:
Every half-life thickness halves the intesity (hence the name :-),
10 times half-life thickness = 0.1% intensity.
NOTE: If you operate a tube at, say, 50kV, this does NOT mean that the X-rays coming from it all have an energy of 50kV. It means the x-rays cover all energies BELOW 50kV, with only a fraction of the total intensity being near 50kV. However, if you take the absorption coefficient for 50kV from the table, you're usually on the safe side.
A very general rule that should keep you quite safe is: