NEUTRON EFFECTS OX ANIMALS 21 



The maximum neutron energy was 13.5 million electron volts, these being 

 emitted from the target in the direction of the incident deuterons (arrow- 

 in Fig. 1). The main intensity of radiation emitted in this direction was 

 due to neutrons of approximately 6 million electron volts energy. This 

 was deduced from the energy determinations of Aebersold and Anslow (1) 

 for a similar neutron beam, and from comparison with the neutron spec- 

 trum reported by Staub and Stephens (2). 



Positions 1 to 6 were as close to the line of maximum energy neutrons 

 as room dimensions permitted. For positions 1 , 2, 3, 4 and 6 the deviation 

 was not sufficient to decrease the energy of the peak neutron radiation in- 

 tensity. Position 7 was almost directly behind the target. The cage in 

 this position received neutrons with a main intensity lower than 6 million 

 electron volts but not lower than 2 million electron volts. It did not, how- 

 ever, receive any greater proportion of slow neutrons than the cage in 

 position 4. Cage 5 received a main neutron radiation intensity lower than 

 that of cage 4 but higher than that of cage 7. The energy of the main 

 intensity neutrons was estimated as 3 million electron volts for position 7 

 and 5 million electron volts for position 5. Care was also taken to arrange 

 the cages so that no cage was interposed between the Be target and any 

 other cage. 



As positions 3 and 2 were near the water walls, they received thermal 

 (very slow) neutron radiation formed by slowing down of fast neutrons in 

 the water walls. These neutrons were removed by covering the water 

 walls with cadmium absorbers. 



The neutron intensity spectra in positions 4, 3, and 2 were compared. 

 This was done by surrounding a .25 r Mctoreen ionization chamber by 

 successively thicker layers of paraffin and measuring the amounts of cham- 

 ber discharge caused by equal neutron dosages emitted by the cyclotron. 

 The results are shown graphically in Fig. 4. 



Fast neutrons striking the paraffin are slowed and deflected so that they 

 produce an increase of ions in the chamber. A peak intensity is reached 

 when sufficient paraffin has been used to slow down all the fast neutrons. 

 Further addition of paraffin cuts down the neutron intensity as the extra 

 paraffin absorbs the slow neutrons. Hence the paraffin thickness at which 

 maximum intensity occurs is greater if fast neutrons predominate in the 

 radiation field. 



Measurements made before the installation of the cadmium are shown in 

 Fig. 5. Here cage 2 has a maximum intensity point for a very thin paraffin 

 absorber, indicating a high slow neutron intensity previous to installation 

 of the cadmium shielding. 



The curves of Fig. 4 are approximately identical. Position 4 was close 

 to the target and presumably received most of its neutron radiation in- 



