Biomedical Engineering Reference
In-Depth Information
Tab l e 9 . 1
Reactions Used to Produce Monoenergetic Neutrons
with Accelerated Protons (p) and Deuterons (d)
Reaction
Q
Value (MeV)
3
H(d,n)
4
He
17.6
2
H(d,n)
3
He
3.27
12
C(d,n)
13
N
-0.281
3
H(p,n)
3
He
-0.764
7
Li(p,n)
7
Be
-1.65
Tab l e 9 . 2
(
α
, n) Neutron Sources
Source
Average Neutron
Energy (MeV)
Half-life
210
PoBe
4.2
138 d
210
PoB
2.5
138 d
226
RaBe
3.9
1600 y
226
RaB
3.0
1600 y
239
PuBe
4.5
24100 y
For a given ion-beam energy, neutrons leave a thin target with energies that depend
on the angle of exit with respect to the incident beam direction.
An alpha source, usually radium, polonium, or plutonium and a light metal,
such as beryllium or boron, can be mixed together as powders and encapsulated
to make a “radioactive” neutron source. Neutrons are emitted as a result of
(
α
,n)
reactions, such as the following:
4
2
He +
4
Be
12
6
C+
0
n.
(9.2)
→
Light metals are used in order to minimize the Coulomb repulsion between the
alpha particle and nucleus. The neutron intensity from such a source dies off with
the half-life of the alpha emitter. Neutrons leave the source with a continuous en-
ergy spectrum, because the alpha particles slow down by different amounts before
striking a nucleus. The neutron and the recoil nucleus [e.g.,
1
6
C in (9.2)] share a
total energy equal to the sum of the
Q
value and the kinetic energy that the alpha
particle has as it strikes the nucleus. Some common (
,n) sources are shown in
α
Table 9.2.
Similarly, photoneutron sources, making use of (
,n) reactions, are also avail-
able. Several examples are listed in Table 9.3. In contrast to (
γ
,n) sources, which
emit neutrons with a continuous energy spectrum, monoenergetic photoneutrons
can be obtained by selecting a nuclide that emits a gamma ray of a single energy.
Photoneutron sources decay in intensity with the half-life of the photon emitter.
All the sources in Table 9.3 are monoenergetic except the last;
226
Ra emits gamma
rays of several energies. It is important for radiation protection to remember that
α
