Environmental Engineering Reference
In-Depth Information
The chemical element boron has two stable isotopes:
10
B and
11
B. The natural abundance of boron stable isotopes
nat
B is
approximately 19.6%
10
B and approximately 80.4%
11
B. These isotopes differ by a single neutron in the 1
p
3/2
shell in the
nucleus. However, this difference makes the neutron capture cross section of
10
B significantly exceed that of
11
B. The total
absorption cross section of
10
B,
11
B, and
nat
B for “room-temperature” neutrons (with velocity ~2200 m s
−1
) equals 3835, 0.0055,
and 767 barns, respectively. Thus, boron has two principal isotopes, which are chemically almost identical, but have quite dif-
ferent neutron-absorbing characteristics. The effectiveness of boron as a neutron absorber is due to the cross section of the
10
B
isotope, while
11
B is essentially nonreactive with neutrons. This provides a great deal of flexibility in the use of boron-contain-
ing materials in nuclear systems, since the mixture of boron isotopes can be adjusted. The value of an absorbing component can
be increased by a factor of approximately 4.5 by substituting with enriching natural materials without changing the sample's
dimensions, which is not possible with other candidate materials. especially, boron enriched in the
10
B isotope to greater than
its natural isotopic abundance is useful for neutron irradiation detection equipment, as well as neutron fluence-measuring
instruments.
it is worth mentioning that neutrons were discovered by producing them from boron nuclei by bombardment with α-particles:
B + He → N + n [1]. Since neutrons are not ionizing particles, their detection depends on the ionizing properties of the products
of neutron capture reactions, the most important of which is thermal neutron capture by
10
B nucleus [2]:
(
)
+
6
%:
10
BnB i eV
+→→
1
11
7
1 01
.
4
He
( .
178
MeV
),
5
0
5
3
2
(
)
94
%:
10
Bn
+→
11
1
B i eV
→
7
084
.
+
4
He
( .
1 47
MeV
)
+ γ
0
(.
048
MeV
).
5
0
3
2
0
As for the general method of measuring intermediate and fast neutrons, one must first moderate their energies to thermal
values so that they can be detected with a thermal neutron detector. One of the principal measurement systems that moderate
intermediate and fast neutrons is so-called long counter, containing a boron fluoride (BF
3
) tube, surrounded by an inner paraffin
moderator. it responds uniformly to neutron energies from approximately 10 keV to approximately 5 MeV. incident neutrons
cause a direct response after being thermalized in the inner paraffin layer. Those from other directions are either reflected or
thermalized by the outer paraffin jacket and then absorbed in a layer of boron oxide (B
2
O
3
). The disadvantage of this arrange-
ment is that the probability that a moderated neutron will enter the BF
3
tube and be counted is not dependent on the initial
energy and, therefore, no information on the spectral distribution of neutron energies is obtained.
in the early review [3], a brief description of the applications of the nuclear properties of boron isotopes was provided.
These included counters of neutrons and energy sensors for neutron detectors. Some neutron-capture applications of boron
cluster compounds were also mentioned [4, 5]. Neutron-sensor device structures based on boron-containing materials were also
described in the newer review [6] devoted to the isotopic effects of boron in solid materials. recently, in the special issue of
the SPie Proceedings [7], devoted to the achievements in radiation detectors physics, boron-containing epitaxial layers pro-
posed for neutron-sensor application have been described. it should be emphasized that, in all the solid-state neutron sensors,
the working body is a nanolayer because
10
B is an excellent neutron absorber and neutrons are rapidly stopped in a
10
B-enriched
material.
Crystalline semiconducting boron was used in neutron thermometers [8]. Such neutron sensors offered advantages over the
conventional (e.g., gas-filled) ones. Neutron detectors constructed from the boron-rich semiconductors could be particularly
effective due to the easily detectable products of the neutron reaction with the
10
B nucleus. A boron-based neutron detector
was first described by gaulé et al. [9]. A pair of thermoresistors were matched with respect to their semiconductor properties,
but with different nuclear properties: one thermistor was made of
10
B, and the other of
11
B. Both isotopes were stable, but,
when exposed to neutrons with thermal energies, only
10
B nuclei underwent transformation according to reaction
10
1
7
4
+→+ + . . The significant amount of the averaged energy (2.79 MeV) released caused the
10
B-thermistor to become warmer than the
11
B-thermistor. This temperature difference can be converted into an electrical signal.
The formation of the rectifying contact on the
10
B sample allowed its application in a different type of solid-state neutron
detector [10]. its principle of operation is based on the generation charge carriers due to absorbing
4
He, that is, α-particles,
ejected in the process of neutron interaction with boron. in the neutron detector [11] based on semi-insulating boron-containing
semiconductor crystal with an external bias voltage, a neutron interaction with
10
B nucleus produces
4
He and
7
Li nuclei. Both
are rapidly stopped (within ~10 µm) in the semiconductor, exciting thousands of electron-hole pairs. These free charges then
drift in the electric field on the crystal, generating a measurable current pulse.
in a study by emin and Aselage [12], a boron carbide-based thermoelectric device for the detection of a thermal-neutron flux
was proposed. it exploited, on the one hand, very high melting temperature and the radiation tolerance of boron carbides, mak-
ing them suitable for use within hostile environments, for example, within nuclear reactors, and, on the other hand, boron car-
bides' anomalously large Seebeck coefficient, by proposing a relatively sensitive detector of the local heating that follows the
absorption of a neutron by a
10
B nucleus. The possibility of elaboration of the boron carbide-based neutron detectors was also
Bn Li
He
279
MeV
5
0
3
2
