Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

466 CHAPTER 9
TABLE 9-3. Foil Reactions with Thermal Neutrons
200 m/s
FOIL TARGET TARGET % NUCLEAR PRODUCT PRODUCT ACTIVATION
MATERIAL NUCLEUS ABUNDANCE REACTION NUCLEUS HALF-LIFE CROSS-SECTION (b)
Ag 107 Ag 51.35 n, γ 108 Ag 2.3 min 44
Ag 109 Ag 48.65 n, γ 110 Ag 25 s 110
In 115 In 95.77 n, γ 116m In 54 min 145
U (nat.) 235 U 0.715 n, f F.P. T −1.2 582
Dy 164 Dy 28.18 n, γ 165 Dy 2.32 h 2100
V 51 V 99.76 n, γ 52 52 V 3.77 min 4.5
Mn-Cu 55 Mn 80.00 n, γ 56 Mn 2.56 h 13.4
Cu 63 Cu 69.1 n, γ 64 Cu 12.8 h 4.3
Cu 65 Cu 30.9 n, γ 65 Cu 5.1 min 1.8
Al 27 Al 100 n, γ 28 Al 2.3 min 0.21
Au 197 Au 100 n, γ 198 Au 2.7 days 96
Rh 103 Rh 100 n, γ 104m Rh 4.4 min 12
Rh 103 Rh 100 n, γ 104 Rh 44 s 140
Co 59 Co 100 n, γ 60 Co 5.27 yrs 36.3
NaCl 23 Na 100 n, γ 24 Na 15 h 0.53
Lu-Al 175 Lu 97.5 n, γ 176m Lu 3.7 h 35
B-Al 10 B 19.6 n, α 7 Li Stable 4010
Eu-Al 151 Eu 47.77 n, γ 152 Eu 9.2 h 1400
Pb 208 Pb 52.3 n, γ 209 Pb 3.2 h 6E−5
Nb 93 Nb 100 n, γ 94m Nb 6.6 min 1.0
No adjustment was made for composition of alloys or compounds. This table lists the most useful thermal neutron reactions.
(Courtesy of Reactor Experiments, Inc.)
are calibrated by exposing the pack to a known beam of neutrons and then measuring
the induced activity. (A thermoluminescent dosimeter may be added to measure the
gamma-ray exposure from a criticality accident.)
Neutron Counting with a Proportional Counter
Counting in the proportional region makes it simple to measure neutrons in the
presence of gamma radiation. A neutron counter used for this purpose uses BF3 gas
to take advantage of the n, α reaction on 10 B:
10 B(n,α) 7 Li.
Boron-10 has a high cross section for this reaction, 4010 barns for thermal neutrons,
and consequently makes a very sensitive detecting medium. The alpha particles
resulting from this reaction are produced inside the detector. Because of the great
difference between the output pulses resulting from the alpha particles and Li ions
and those due to gamma rays, it is a simple matter to discriminate electronically
against all pulses except those due to the alphas and Li ions when a neutron is
captured by 10 B.
The sensitivity of a BF3 neutron detector may be increased by using 10 B-enriched
BF3 gas. Naturally occurring boron contains about 19.8% 10 B. However, it is possible
to concentrate the 10 B isotope to the extent that BF3 gas containing 96% 10 Bis
routinely available from commercial suppliers. The BF3-filled counter is a sensitive
and simple thermal-neutron detector and is widely used by health physicists for
measuring thermal-neutron flux. Furthermore, since 680 thermal neutrons/cm 2 /s