Sec. 13.6] STANDARDIZATION OF RADIOACTIVE SAMPLES 365 



the same as that of the daughter isotopes. Thus in accordance with the 

 scheme given in Chap. 7 (uranium-radium series), the rate of UX 2 betas 

 emitted is very nearly the same as the rate of UI alpha particles per unit time. 

 For the actual values, see Table 35. In practice the prepared uranium 

 sample is carefully weighed and its rate of disintegration by alpha emission 

 accurately measured. From these data the rate of UX2 beta emission is 

 found directly. The level scheme of UX2 gives rise to some complication 

 because the beta spectrum is not simple. A recent level scheme proposed 

 by Bradt and Scherrer [38] is shown in Fig. 102. The beta radiations of 

 UX 2 represent 99.85 per cent of all disintegrations, the rest, 0.15 per cent, 

 being gamma rays which produce UZ. Furthermore, 98 per cent of UX2 

 beta particles is of 2.32 mev maximum energy, while 2 per cent consists of 

 one or more beta particles of 1.5 mev in coincidence with gamma rays. If a 

 known quantity of uranium is deposited on a foil and allowed to equilibrate 

 with UXi and UX 2 , a combined alpha-beta standard source can be obtained. 

 The alpha particles of UI, UII, and AcU and the beta particles of UXi 

 have to be absorbed if the UX 2 beta emission is to be used for standardization 

 purposes. This is readily done with 25 mg per cm 2 aluminum foil which 

 absorbs all alpha and UXi beta particles but only about 5 per cent of the UX 2 

 beta particles. 



A uniform deposit of uranium on foil may be prepared with a colloidal 

 suspension of uranium oxide. Pure uranium oxide is ground in a quartz 

 ball mill for 12 hr. The material is taken up by a suspending agent such as 

 chloroform or ethyl alcohol and centrifuged to precipitate the coarse particles. 

 The remaining colloidal suspension is then evaporated in a standard holder 

 by the radiant heat from a General Electric heat lamp. The amount of 

 material deposited is determined by weighing the aluminum foil before and 

 after deposition. Some of the uranium standards are still inconvenient when 

 they are used with Geiger counters because there may be appreciable scatter- 

 ing of the beta rays in the bulk of the standard sample itself. If the mass 

 of the sample is reduced so that this effect is negligible, the emission rate of 

 beta particles also decreases to only a few times that of the background, 

 thus making the determinations less accurate. These points have been 

 noted by Broido et al. [15], who made use of methods for quantitative separa- 

 tion of UXi and UX 2 from aged uranium solutions, using lanthanum carrier. 

 One of the methods used is as follows: 



To 25 ml of an aqueous solution containing 0.1 gm of uranium per milliliter 

 add 25mg of lanthanum carrier and 1.5 ml of 27 TV hydrofluoric acid. Cen- 

 trifuge and wash the resulting lanthanum fluoride and UXi precipitate with 

 10 ml of 6N hydrofluoric acid. Collect the supernatants and carry them 

 through a second lanthanum fluoride precipitation to check for losses. Dis- 

 solve each precipitate in a mixture of 8 parts nitric acid and 1 part boric 



