Disintegrations of light elements

This work is divided into three parts. Part A, discusses some of the focusing properties of magnetic fields used for the deflection and analysia of ion beams; part B describes the design and construction of magnets employing these theoretical principles; and Part C is an account of some experimental work done using focussed beams of deuterons. The work was all alone at the Clarendon Laboratory, between Michaelmas Term 1948 and Michaelmas Term 1953, and the three parts form a roughly chronological sequence.
Part A: The Focusing Properties of Magnetic Analysers.
The 1 M.V. and 500 KV. acaelerators of the Clarendon Laboratory provide relatively high current beams of positive ions which are very homogeneous in energy. For separating the ions of different masses a magnetic analyser is clearly necessary, since it can be shown that the alternative, an electrostatic analyser, gives energy dispersion only.
The simple magnetic analyser which was customarily used until recently, being a sector shaped uniform field, has strong focussing properties in the plane containing the beam and perpendicular to the field lines (the "perpendicular" plane), but none in a plane containing the emergent beam and parallel to the field lines (the "parallel" plane). This focussing becomes stronger as the sector angle Φ is increased, until at 90°, which is frequently a convenient angle, an incident collimated beam of circular cross-section converges to a line focus at the exit boundary.
This is a great inconvenience in mnny cases where care has been taken with the accelerator to provide an intense ion beam for nuclear bombardment studies. The properties of such uniform sector fields are discussed briefly, since they serve to demonstrate the formal analogy between ion trajectories in magnetic fields (ion optics) and light optics.
The power of the lens in the two planes can be changed by using a field which in non-uniform, varying with radial distance as H α r-n. Such a field focusses in both planess but it is shown that coincident foci can only be obtained at image distances too close to the magnet for practical convenience in many cases.
The inherent non-uniformity of the "fringe" field at the pole boundaries can be used to change the power of the lens in both planes if the boundaries make an angle with the beam at entrance or exits. Expressions are derived for the image distances in both planes and the conditions for equality are then discussed. The equation has a variety of possible solutions which are to some extent limited by practical considerations. In particular, the magnification of the systme will in general increase with the increase of image distance and this proves to be the principal limiting factor, since the object for the ion-optical system has a finite size (initially the ion source canal).
The theoretical results for a 90° analyser are compared with experiment and reasonable agreement is found. However it is found that there is a great loss of intensity in the analysed beam. the whole question of abberations is discussed and it is found that the loss of intensity is likely to be due to spherical abberation in the electron lens system, a circumstance that can probably not be improved in the existing type of accelerator column, and which makes 90° analyser of the type discussed of no great advantage over the uniform-sectpr field for many purposes.
For an accelerator with large spherical aberration the only way out of this difficulty is to sacrifice the large dispersion and mechanical convenience of 90° deflection and use deflection through small angles. this is seperately considered as a special case of a field with oblique non-uniform boundaries, and the optical properties of such a magnet as is used in Part C are discussed.
Part B: The Design of Magnets for Ion Analysers.
The general considerations which guide the design of an analysing electromagnetic are presented, and details are given of the construction of a magnet and pole-pieces designed to give 90° deflection of l M.V. deutrons in either of the two opposite directions. The pole-pieces have faces which give the beam a 19 cm. radius path in the field, and freedom to choose the boundary angle (in accordance with the principles of Part A) is provided by the adjustable segments at the pole tips.
The limitations on operation imposed by heating of the windings, non-uniformity of the field due to magnetic saturation and the proximity of the coils , and the effect of the hysteresis on field stabilisation are mentioned
The vacuum box the carries the beam between the magnet poles is made of brass wave guide with a rigid supporting structure of Yorkshire tubing which also serves as a high speed pumping by-pass.
The field is stabalised by referring the voltage developed by the magnet current across a resistance to that of a standard cell. The unbalance current is amplified by means of a galvanometer, modulated light beam, photocell and AC amplifier, and corrects the magnet current by a moter driven variac in the power supply before rectification.
It is shown that a satisfactory magnet design can be based on triangular cores and windings. The dimensions of a magnet to deflect 1.4 M.V. tritons through 90° in either of the two opposite directions are given, and also details of the tests, on a ¼ scale model, to find the temperature rise in the windings and the expected uniformity of the field in the full-size magnet. The field at various points in space near a triangular winding has been worked out.
Part C: A Further Study of the D- D Reactors
The angular distribution of the products of the two reactions:
(a) D + D &rightarrow; He3 + n
(b) D + D &rightarrow; H3 + p
has been investigated in the energy range 150 KV. to 450 KV. (deuteron bombarding energy). Although other workers have made measurements of (a) in this region, the technique used was different, and the present work partly bridges the gap between the work of Eliot (0 to 50 KV.) and that of Blair (1 to 315 MV.), both of whom used similar methods.
The apparatus consisted of a cylindrical target chamber filled with deuterium at a pressure of about 0.1 mm. Hg., and attached to the end of the accelerator column of a 500 KV Phillips generator. the beam was carefully collimated by a canal system, and the charged reaction products from a small volume at the centre of the target box were counted by ion counters operated in the proportional region. Two counters were arranged to swivel about an axis through the centre of the reaction volume, and a third, which was used to monitor conditions, was fixed to the lid of the box. The counter windows were of very thin (about 0.5 μ) quartz. The counter gas was heptane vapour at a pressure (about 1 cm.) determined by a fixed temperature bath. Counts were made of the number of He3 and H,sup>3
particles penetrating the counter window at various angles. These particles were distinguished by a careful choice of bias applied to a phase height disoriminato, and by the use of absorbing mica screens in front of the window. The principle is that the three charged particles from the reaction, as well as scattered deuterons, give in general different amounts of ionization in the counter materials and absorbers, because of their difference of energy and charge. the results to be expected are discussed in detail.
The results are conveniently expressed in terms of the 'asymmetry' coefficients A and B in the expression for the angular distribution of the yields: Y α 1+ Acos4Ρ + Bcos4Ρ
There is a significant difference in the values of A and B for reactions (a) and (b), which is in agreement with the results of other workers. The values are:
He3 H3
E(KV) A B A B
150  1.334 &pm; 0.267   -0.087 &pm; 0.294   0.773 &pm; 0.117   0.031 &pm; 0.130 
200  1.355 &pm; 0.169   0.155 &pm; 0.183   0.687 &pm; 0.161   0.297 &pm; 0.130 
260  1.718 &pm; 0.212   0.186 &pm; 0.245   0.978 &pm; 0.122   0.149 &pm; 0.150 
350  1.427 &pm; 0.126   0.711 &pm; 0.145   0.637 &pm; 0.116   0.839 &pm; 0.130 
450  1.342 &pm; 0.138   1.371 &pm; 0.157   0.949 &pm; 0.110   0.506 &pm; 0.130 
The difficulties of interpretation of these results on the basis of the only published theoretical treatment of the D - D reaction is briefly discussed.