STIDIKS l.N DK'lFXrrLNC; RADIO-AC I INl, FAI.l.-OUT 



sample (100 days in this case) can be found. By averaging tlic ^-energies in 

 the appropriate proportions it is found that the average ^-ray energy is 

 approximately 0-8 Me\'. This has a range of approximately 1-3 mm 

 aluminium, which is in good agreement with the order of half-value layer 

 obtained. 



Comparisons of Collection Methods 

 (7) Earthed and insulated screens 



In a single observation, some American workers^ noted that the dust 

 collected on earthed and insulated screens was approximately equal in 



300 



0) 



I 200 



c 



O 



o 



100 



Earthed ^ 

 Insulated 



June Dec June 



1957 1958 



Figure 6. Comparison of earthed and insulated screens — sample 



activities 



activity, but the mass of dust collected on the insulated screen was nearly 

 double that on the earthed. This matter was further investigated here, as 

 it v^ould be interesting to know just how great a part electrostatic attraction 

 plays in the settling out of airborne particles. 



A series of measurements w^as carried out with one piece of cheesecloth 

 on an earthed frame and the other piece on an insulated frame. The results 

 were compared with those for a series when both frames were earthed 

 {Figure 6). 



From the graph, litde significant difference can be seen between the 

 continuous and broken lines except on one occasion (24.10.57) when the 

 insulated cheesecloth fell off the frame before the week was up. However, 

 when the results from two earthed screens are examined, it is seen tliat 



134 



