Environmental Engineering Reference
In-Depth Information
considered by emin [13]. As
10
B is an excellent absorber of thermal neutrons, they cannot penetrate (for much more than
~1 mm) into a
10
B-enriched material before being absorbed. Upon absorbing a neutron,
10
B decays into
7
Li and
4
He, thereby
heating the material in the immediate vicinity of the neutron absorption. A temperature difference is thereby established bet-
ween the faces of a boron carbide sample. it can be monitored using boron carbide's large Seebeck effect. All told, boron car-
bides may serve as simple, small, and robust (as the neutron-radiation damage arising from the decay of
10
B isotope in the
icosahedral boron-rich solids is minimized by the self-healing that characterizes such structures) neutron detectors.
real-time solid-state neutron detectors were also fabricated [14] from different semiconducting boron-carbon alloys pre-
pared by the plasma-enhanced chemical vapor deposition (PeCVD) method. Single neutrons were detected and signals induced
by gamma-rays were determined to be insignificant. The source gas closo-1,2-dicarbadodecaborane (ortho-carborane) was used
to fabricate the boron-carbon alloys with only the natural isotopic abundance of
10
B. Devices made of thicker boron-carbon
alloy layers enriched in
10
B led to increased detection efficiency. Active diodes could use the inherent micron-scale spatial
resolution, increasing the range of applications.
radioisotopes and particle accelerators are widely utilized as neutron sources: neutron therapy, radiography, and damage
evaluation of fusion reactor materials are typical examples of these up-to-date applications. With such an increase in neutron
facilities, personal dosimetry, environmental monitoring, and evaluation of leakage neutrons become more important. in a study
by Oda et al. [15], a plastic track detector was applied to thermal neutron dosimetry by combining it with a ceramic boron
nitride converter using (n, α) reactions. BN seems to be a promising converter because of its high boron concentration and
smooth surface. The efficiency was 1.0 × 10
−3
pits n
−1
, corresponding to the sensitivity of 9.6 × 10
2
pits mm
−2
mSv; the linear
response was between 0.035 and 0.7 mSv; and the minimum detectable dose equivalent was estimated to be approximately
0.005 mSv. Boron neutron capture therapy uses high intensity neutron radiation (up to 10
9
cm
−2
s
−1
). To evaluate the prescribed
dosage, it is important to separately detect the thermal neutron flux and the gamma dose. Tanaka et al. [16] proposed a method
using a glass-rod dosimeter (grD) with thermal neutron shielding. in order to evaluate the thermal neutron fluence, the
combination with grD and BN as a thermal neutron converter was also developed.
recently, epitaxial layers of hexagonal boron nitride (h-BN) synthesized by metal organic chemical vapor deposition
(MOCVD) have been proposed [17] for neutron-sensor applications. Measurements indicated that the thermal neutron absorption
coefficient and absorption length of h-BN epilayers with natural boron-isotopic composition are approximately 0.0036 µm
−1
and
277 µm, respectively. To partially address the key requirement of long carrier lifetime and diffusion length for a solid-state
neutron detector, microstrip metal-semiconductor-metal (MSM) detectors were fabricated on the same basis and were tested.
A good current response was generated in these detectors using continuous irradiation with a thermal neutron beam corresponding
to an effective conversion efficiency of approximately 80% for absorbed neutrons. graphene-oxide—a derivative of the single-
layer graphite—exhibits an ability to provide sufficient change in luminescent properties when exposed to neutron radiation.
Utilizing this property, robinson et al. [18] investigated the integration of hexagonal boron nitride (h-BN) with some graphene-
based structures to evaluate the radiation-induced conductivity in nanoscale devices and discussed the successful integration of
h-BN with large-area graphene electrodes as a means to provide the foundation for large-area nanoscale radiation sensors.
Boron phosphide (BP) single crystals were also studied [19] to develop refractory electric devices, such as solid-state neutron
detectors utilizing a large cross section of the
10
B isotope. The isotopic composition of the wafer used was (
10
BP)
0.95
(
11
BP)
0.05
.
Thermo-luminescent dosimeters (TLD), which are one of the few known types of thermal-neutron dosimeters commonly
used for radiation-protection purposes of personnel, utilize (n, α) reaction of
10
B nucleus [20]. it should be emphasized that
these dosimeters are sensitive to γ-rays as well. This decreases the accuracy of neutron dosimetry to some extent, because at
many working places thermal neutrons coexist with γ-rays. This type of neutron dosimetry can be performed, for example, by
the spark counting of tracks in the boron-doped film [21]. For this purpose, thin cellulose nitrate films are doped with a boron
compound (0.1%). After thermal neutron irradiation, the tens of millimemeter-thick films are etched in an aqueous solution of
NaOH and then the etch-pits caused by
10
B (n, α)
7
Li reactions are punched and counted at high voltages. The ratio of the spark
density to the thermal neutron fluence was found to be 1.0 × 10
−4
for a boron concentration of 1%. A thermal neutron dose up
to 3 × 10
−6
Sv can be measured with this system. As a highly sensitive, simple, and nonradioactive neutron dosimeter, the plastic
plates (Cr-39) doped with another boron compound—ortho-carborane—were prepared [22]. After thermal neutron irradia-
tion, the plates were etched in an aqueous solution of KOH. The etch-pits generated by
10
B (n, α)
7
Li reactions were then
counted using an optical microscope or an automatic track-counting system. When the etching time is kept constant, the etch-
pit density is proportional to the irradiated thermal neutron fluence. The proportional constant is termed “sensitivity,” which
was found to be 4.2 × 10
−4
for a plate containing ortho-carborane at a concentration of 0.5 wt.% for an etching time of 16 h. By
considering background counts, it was established that a thermal neutron dose of 0.025 mSv can be measured with this plate.
These plates are insensitive to visible, UV, X-, β-, and γ-rays and are easy to handle because here the detector and converter are
incorporated together. Alternative boron compound, lithium tetraborate Li
2
B
4
O
7
, was also considered [23] as promising con-
verter for neutron dosimeters. The poor reliability of sintered crystals as dosemeter was overcome by making it forms of glass
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