Biomedical Engineering Reference
In-Depth Information
Tab l e 9 . 3
(
γ
, n) Neutron Sources
Source
Neutron Energy (MeV)
Half-life
24
NaBe
0.97
15.0 h
24
NaD
2
O
0.26
15.0 h
116
InBe
0.38
54 min
124
SbBe
0.024
60 d
140
LaBe
0.75
40 h
226
RaBe
0.7 (maximum)
1600 y
all photoneutron sources have gamma-ray backgrounds of >1000 photons per neu-
tron.
Some very heavy nuclei fission spontaneously, emitting neutrons in the process.
They can be encapsulated and used as neutron sources. Examples of some impor-
tant spontaneous-fission sources are
254
Cf,
252
Cf,
244
Cm,
242
Cm,
238
Pu, and
232
U.
In most cases the half-life for spontaneous fission is much greater than that for al-
pha decay. An exception is
254
Cf, which decays almost completely by spontaneous
fission with a 60-day half-life.
The Spallation Neutron Source (SNS), shown in Fig. 9.3 at the Oak Ridge
National Laboratory, produced its first neutrons in 2006. This cooperative effort
among a number of laboratories operates as a user facility and offers an order-of-
magnitude improvement in neutron beam intensity compared with other sources.
Neutrons are produced by bombarding a target module containing 20 tons of circu-
lating mercury with 1-GeV protons from a linear accelerator, using superconductor
technology, and an accumulator-ring system. The SNS will make research possible
in a number of heretofore unreachable areas in instrumentation, materials proper-
ties and dynamics, high-temperature superconductivity, biological structures, and
nanoscience.
9.3
Classification of Neutrons
It is convenient to classify neutrons according to their energies. At the low end of
the scale, neutrons can be in approximate thermal equilibriumwith their surround-
ings. Their energies are then distributed according to the Maxwell-Boltzmann for-
mula. The energy of a thermal neutron is sometimes given as 0.025 eV, which is
the most probable energy in the distribution at room temperature (20
◦
C). The aver-
age energy of thermal neutrons at room temperature is 0.038 eV. Thermal-neutron
distributions do not necessarily have to correspond to room temperature. “Cold”
neutrons, with lower “temperatures,” are produced at some facilities, while others
generate neutrons with energy distributions characteristic of temperatures consid-
erably above
20
◦
C. Thermal neutrons gain and lose only small amounts of energy
