267Techniques forSmall Signatures
Gamma-rays and x-rays are electromagnetic radiation and have a much lower proba-
bility
o
f
i
nteracting
w
ith
m
atter
t
han
d
oes
c
harged-particle
r
adiation.
A
lthough
c
harged
p
articles
i
nteract
d
irectly
t
o
p
roduce
e
xcited
a
toms
a
nd
i
ons,
p
hotons
m
ay
i
nteract
i
n
s
everal
d
ifferent
w
ays.
B
ecause
b
oth
γ-
rays
a
nd
x
-rays
a
rise
i
n
t
ransitions
be
tween
d
iscrete
s
tates,
t
hey
a
re
e
mitted
a
t
d
iscrete
e
nergies.
T
he
i
nteraction
o
f
a c
ollimated,
monochromatic beam of high-energy photons incident on an absorbing medium is not
c
haracterized
b
y
a r
ange,
b
ut
r
ather,
b
y
a c
haracteristic
h
alf-thickness
(T)
.
T
he
i
nten-
sity
(I) o
f
t
he
p
hoton
be
am,
a
fter
p
assing
t
hrough
a t
hickness
X o
f
a
bsorber,
i
s
g
iven
b
y
I
= I
0
exp[−(ln 2) X/T],