Patent GB-1425574-A (GB)
Title: Methods and apparatus for determining the spatial distri bution of a radioactive material
IPC: G01T1/164
Filed: 1972-03-28
Abstract: 1425574 Photovoltaic devices R W TODD B A GODDARD D B EVERETT and J S FLEMING 23 March 1973 [28 March 1972] Heading H1K [Also in Divisions H3-H5] In apparatus for determining the spatial distribution of a gamma-ray emitting radioactive substance, a three dimension detector 1, Fig. 1 is used within which a gamma-ray photon from source 2 undergoes a series of Compton collisions 3 and 4, the line 3-4 of the photo between the collisions being used as the axis of a cone of probability 5 having a half apex angle equal to the Compton scattering angle at 3 and intersecting an object plane under investigation in an ellipse 6 somewhere along which the source 2 probably lies. Similar elliptical probability curves are produced from Compton collisions of other photons from the source, the curves all tending to cross at the position of source 2 (Fig. 2). The detector 1 comprises a stack of 100 silicon wafer discs each 0À5 mm. thick and having 50 aluminium X coordinate strips 42, Fig. 8, on one surface and 50 gold Y co-ordinate strips orthogonal to the first set on the other surface. Each wafer defines a Z co-ordinate and gives X and Y coordinates within that Z co-ordinate. If a Compton collision occurs within a wafer the accelerated electron will produce a voltage pulse in that pair of X and Y strips which cross over at the collision point. The X and Y strips are connected in corresponding pairs (e.g. X 1 , Y 1  and X 2 , Y 2 ) to transformers 30 with such numbers of turns in the transformer windings that e.g. a collision at X 1  gives a voltage pulse of + 2 units from the X 1 , Y 1  transformer, a collision at Y 1  gives a voltage pulse of - 1 unit and a collision at the crossover co-ordinate X 1 , Y 1  gives a + 1 unit pulse from the transformer. The transformer outputs are fed, after digitizing, to a computer for the calculation of the coordinates of collisions 3 and 4. The Compton equation #<SP>1</SP>-# = h/MoC (1-Cos #), #<SP>1</SP> and # are photon wavelength before and after collision, h is Planck's constant, Mo is restmass of electron, C is velocity of light and # is Compton scattering angle, gives a relation between the loss of energy by the photon (or the acquirement of energy by the electron) and the scattering angle. The energy aquired by the collision electron is given by the amplitude of the voltage unit supplied by the transformers 30 and so enables the calculation of #. Knowledge of the energy of the source 2 is obtainable if three consecutive Compton collision points are used, various energies being assumed and the scattering angles being determined and investigated as to whether they agree with the energy asumptions. The co-ordinates of collisions 3 and 4 and the scattering angle # enable the cone 5 and ellipse 6 to be plotted. The ellipses are displayed on a cathode ray-tube at such a brightness level that only the intersections of the ellipses at the indicated position of source 2 shows up. If source 2 is not in the plane under investigation the ellipses will not intersect at a single point and no bright indication will be displayed. Each wafer is cut from a 5 cm ingot of 20 K/ cm n-type silicon with the axis along the 1, 1, 1 crystallographic direction. The edges of each surface are recessed, Fig. 9, so that leads 41 can there be connected to the respective strips by means of silver paste. One wafer surface is lapped and the other surface is lapped and polished and the wafer then etched at 0‹ C for 24 hours in a 5/3/3 mixture of HNO 3 /HF/CH 3  COOH until the etching is quenched with HF. The mixture is then diluted with deionized water until a resistance of 1 meg/cm is reached. After 2 weeks storage in deionized water, gold is evaporated onto one surface of the wafer through a glass grid and aluminium is evaporated onto the other surface, also through a glass grid. The wafers are stacked with the aluminium strips of one wafer being placed adjacent the aluminium strips of the next wafer.