NUCLEONICS DATA SHEET No. 29 
X-ray Production with Linear Accelerators 
By MALCOLM H. MacGREGOR 
Applied Radiation Corp., Walnut Creek, California 
Linear electron acclerators are parti- 
cularly useful in their ability to pro- 
duce intense bremsstrahlung beams. 
A knowledge of this photon production 
is important for both irradiation work 
and for shielding purposes when the 
electron beam is used directly. The 
graphs and table presented here are de- 
signed for evaluation of photon produc- 
tion and associated shielding problems. 
Radiation lengths. If target thick- 
nesses are expressed in radiation 
lengths, formulas for bremsstrahlung 
production are essentially the same for 
all elements. One radiation length is 
defined as the distance along the beam 
in which the energy of the typiéal elec- 
tron is reduced to 1/e of its original 
value (1). Figure 1 gives radiation 
lengths as a function of atomic number. 
Forward intensity. The most in- 
tense photon radiation occurs directly 
in line with the electron beam when a 
target of optimum thickness is placed 
in the beam. Figure 2 shows these 
maximum intensities (2). The forward 
intensity as a function of target thick- 
ness is shown in Fig. 3. The curve is 
for gold at 17 Mev (3), but it applies to 
all high-Z targets and all electron 
energies above 5 Mev (4). For low-Z 
targets and low electron energies, ion- 
50 
L 
(2) 
w 
{o) 
20 
Radiation Length (gm/cm?) 
20 40 60 80 
Atomic Number 
FIG. 1 shows radiation length as function of atomic number. 
ro} 
oO 
Forward Intensity (r/min/ma at | meter) 
103 
102 
4 6 10 15 20 
2 Electron Energy (Mev) 
5 10 
2a a © — q 
2 06 
2 04 
a Ol 03 10 
0.03 
Target Thickness (radiation lengths) 
Expressing target thickness 
in radiation lengths makes bremsstrahlung-production calculations essentially the same 
for all targets. 
for a high-Z target of optimal thickness. 
with target thickness 
ization losses become increasingly im- 
portant (see Fig. 6), and the peak in 
Fig. 3 shifts somewhat toward smaller 
target thicknesses. Figures 2 and 3 
Photons Produced per Unit Energy Range (relative) 
Photon Energy 
Electron Energy (Mev) 
(Mev) 6.6 8.61 10.61 14.61 16.61 20.51 
2 45.3 130 208 418 550 868 
4 10.6 50.1 86.2 183 245 396 
6 a 22.8 46.6 108 147 241 
8 os 143 25.2 71.4 99.1 167 
10 2.10 49.1 71.4 123 
12 31.0 52.0 95.5 
14 2.84 34.1 75.5 
16 3.22 58.8 
18 40.6 
20 3.97 
248 
FIG. 2 gives maximum photon intensity as a function of electron energy 
FIG. 3 indicates variation of forward intensity 
together enable one to calculate for- 
ward photon intensity for a high-Z tar- 
get and any combination of target 
thickness and electron energy over the 
range covered. For a low-Z target, 
forward intensity is lower. The effect 
of lower Z can be inserted by using 
Fig. 5. 
Angular distribution. The curves 
of Fig. 4 enable calculation of intensi- 
ties in other directions from the forward 
intensity. In the ranges covered these 
curves are in good agreement with ex- 
periment (3). There is an absence of 
data for energies above 5 Mev, particu- 
larly for thick targets. Shielding re- 
quirements are difficult to estimate. 
Probably a safe rule of thumb is that 
