GENERAL PROCEDURES FOR RADIO ASSAY 187 



Hard-beta Counting. The more energetic the beta particle, the easier 

 the counting measurement. With energies higher than 1 Mev, very con- 

 venient methods may be used without any great loss of sensitivity. At 

 energies between 0.5 and 1 Mev the choice of method will usually be the 

 most convenient one that will still give adequate counts for the particular 

 samples being used. 



If soft-beta measurements are being made in the laboratory, then the 

 thin-mica-window tube can also be used for the hard-beta emitters. 

 Otherwise it is preferable to use a more rugged counter tube. Solid sam- 

 ples can be prepared and counted in the usual way. Although sample 

 uniformity is not so critical as for the soft-beta emitters, nevertheless a 

 reasonably flat and uniform surface is required, and self-absorption must 

 be taken into account. 



Solution counting offers many advantages (13, 45). The preparation 

 of the sample for assay is simple, and the geometry is easily reproducible. 

 Many biological liquids can be measured directly. If the samples contain 

 too much activity, they can be dihited or an ali(}uot taken for assay. 

 Solution counting may be accomplished either with a tube that dips into 

 the solution (dipping counter) or by an external measurement from the 

 surface of the liquid. Dipping measurements are usually more sensitive 

 but have the disadvantage that the counter tube comes in contact with 

 the radioactivity and must be cleaned and brought to background 

 between each measurement. In some cases the radioisotope tends to be 

 adsorbed onto the glass surface of the tube, and cleaning becomes a diffi- 

 cult problem, especially if there are both high- and low-counting samples 

 in a series of measurements. This difficulty, however, may be overcome 

 to a large extent by the use of a silicone coating on the counter. Special- 

 purpose counter tubes are available which can be used for continual 

 measurement of flowing liciuids. 



The thin-mica-window counter can be used for external measurement 

 of the licjuid surface. However, these measurements cannot usually be 

 made with such a counter inside a closed shield because of vapor effects. 

 The usual cylindrical counters having glass walls of about 30 mg/cm- are 

 also convenient for this purpose. Attention is called to a cylindrical 

 metal thyrode tube which is rugged and inexpensive. The solution can 

 be placed in any suitable dish, although various sizes of petri dishes have 

 been found convenient. The thickness of the solution layer in relation 

 to the surface area will have a considerable bearing upon the sensitivity 

 obtained. Some comparisons with various isotopes have been reported 

 by Comar (13). In general, with beta energies greater than 1 Mev the 

 sensitivity of a solution count compared favorably with that of a solid 

 count. 



With aqueous solutions there is usually no difficulty due to evaporation 



