Patent GB-800636-A (GB)
Title: Improvements in or relating to methods and devices for the sensing of neutrons
IPC: G01T3/08
Filed: 1956-03-15
Abstract: 800,636. Automatic control systems for nuclear reactors. SIEMENS-SCHUCKERTWERKE A.G. March. 15, 1956 [March 18, 1955], No. 8217/56. Class 38 (4). [Also in Groups XXXVI, XL (a) and XL (b)] A neutron detector comprises a semi-conductor body, the electrical properties of which change upon exposure to neutrons, the body consisting of a compound of an element of the third periodic group with an element of the fifth group and including boron and/or nitrogen. Favoured compounds are B with one of the elements N, P, As or Sb, and N with one of Al, Ga and In. When a body of such a boron compound is subjected to a thermal or slow neutron radiation, a boron atom is converted into lithium with the emission of a particle of 2.3 MeV energy which produces numerous electron-hole pairs. During their short lifetime the electron-hole pairs modify the resistivity of the body to provide a current-voltage pulse. The conversion of the B<SP>10</SP> nucleus into the Li<SP>7</SP> nucleus provides an impurity centre in the crystal lattice which results in an additional carrier (electron or hole) and so produces a permanent change in the resistivity of the body. This process provides an indication of the total number of neutrons captured over a period of time. Higher energy neutrons may be detected by embedding the body in paraffin to retard the neutrons. In the case of nitrogen compounds, the following reactions are utilized: (i) N<SP>14</SP>(n,p)C<SP>14</SP> for slow and medium velocity neutrons and (ii) N<SP>14</SP>(n,y)B<SP>11</SP> for fast neutrons. Other reactions involved include Al(n,γ)Si and Ga(n,γ)Ge. Nitrides, as for borides may be used to indicate neutrons by either the temporary effect due to electron-hole formation caused by a-particles or protons emitted from the N nuclei under the effect of the neutrons, or the permanent, irreversible change of resistivity due to nitrogen nuclei being converted into carbon nuclei, and other transformations such as Al and Ga to Si and Ge, to form lattice imperfections. The semi-conductor body, preferably monocrystalline, may consist of a PN or a PIN structure. Borides and nitrides may be made by doping with S, Se, or Te. Electrodes may consist of indium or gold vaporized or fused on to the semi-conductor. Fig. 2 shows the semi-conductor body 3 consisting of an A III  Bv nitride or boride forming a variable resistor in a circuit having a battery 2, amplifier 6 and indicator 7. Neutrons impinge on the body 3 after penetrating a jacket of paraffin 8. Indicator 7 is adapted to respond to the pulses of electron-hole formation and thus to show the instantaneous value of neutron flow. An alternative circuit arrangement is described which responds to the steady state resistance of the body, thus showing the integrated value of neutron flow; this arrangement may also be modified so that the Hall effect is measured to indicate the change carrier concentration. Fig. 6 shows a nuclear reactor regulating system. A uranium-graphite charge 43 with cadmium control rods 45, is enclosed in a graphite reflector 42 and shield 41. A neutron detector comprises a boride or nitride semi-conductor body 46 connected in series with a battery and a resistor 48. The voltage across resistor 48 is utilized to control the movement of control rods 45 into the uranium 43. Specification 719,873, [Group XXXVI], is referred to.