The thern~oluminescence curve, obtained by plotting the intensity of light emission against temperature for a constant rate of heating, is valuable for finding the distribution of electron traps in a phosphor. Each maximum in the light intensity corresponds to the emptying of a trap whose energy level is a function of the temperature a t the n~aximum. This temperature is an easily measured experimental quantity but it has hitherto not been possible to evaluate the trap depth from this alone. Accessory data were required, such as the decay rate or the light sum stored a t the peak (4), which might be clifficult to obtain or imprecise. I t will be shown here how the trap depth can be calculatecl using only the positions of the maxima a t two different rates of heating.Randall and Wilkins (3) derived the equation for the tl~ermolun~inescet~ce or "glow" curve for a single trap depth, assuming first order kinetics and no re-trapping, in the formwhere I is the brightness, C and ?to are arbitrary constants, E is the trap depth, s is an atomic frequency factor, and B is the rate of heating. From this they did not solve for the conditions for the maximum (the glow peak) directly, but wrote E = T* [ I + f (s, B ) ] k log, s where T* is the temperature a t the maximum; and showed by plotting a numerical example of equation [I], s sing Biinger and Flechsig's (1) values for s and E in KCI(T1) phosphor, that j'(s, B) is small compared with unity when B is in the range 0.5 to 2.5 degrees/sec. This linear relationship becomes, with the same value of s(2.9 X lo9 set.-l) E = 25 k T* (for B in the range given).This last result has been widely quotecl as a general rule of thumb; but the numerical constant depends on s, and this will be different for each trap, even in the same substance. 111 very simple cases where s can be obtained from the intercept on the plot of 1/T against the logarithm of the phosphorescence decay constant, E will be known from the slope, so that the glow curve data are then redundant.I t may have escaped attention that, even though the integral in the first exponential bracket cannot be expressed in closed form, an exact solution for the inaximunl in equation [I] can be had by setting the derivative equal to Can. J. Chem. Downloaded from www.nrcresearchpress.com by 54.213.206.170 on 05/10/18For personal use only.
Controlled reductive assembly of capped Keggin anions [PMo(12)O(40)(ML(m))(n)](3-) has been achieved by reduction of [PMo(12)O(40)](3-) with sodium-mercury amalgam in the presence of metal halides, as exemplified by the rational syntheses of mono-capped [PMo(12)O(40){Co(MeCN)(2)}](3-) and bi-capped [PMo(12)O(40)(VO)(2)](3-) and [PMo(12)O(40)Sb(2)](3-).
Evidence from typical examples (Iead and cadmium in NaCl) suggests that the formation of trace-scale " anomalous mixed crystals " is caused by adsorption of the foreign ions on specific crystal planes. The process is thus related to crystal habit modification, but not all habit modifiers form such systems. Among a group of metal ions, all of which modify the habit of NaCl, cadmium and lead co-crystallize strongly with NaCl from solutions containing the foreign ions in radioactive trace amounts, whilst zinc, manganese, mercury and bismuth do not. Apparently the trace-scale phenomenon occurs only when the adsorption is exceptionally strong, for the exceptional behaviour of cadmium and lead is paralleled in other crystallogIaphic effects which have been attributed to adsorption. Lead and cadmium chlorides strongly inhibit crystallization of NaQ, thicken the crystallization layers and form oriented overgrowths on the (100) face. The other metal salts exhibit these effects weakly or not a t all. The sharp distinction between the two groups of habit modifiers is compatible in some respects with the lattice-fitting hypothesis of the adsorption mechanism, but the possible importance of other factors is not excluded.
A method for extracting P32 from neutron-irradiated sulphur has been developed. The sulphur is melted under a mixture of acetic acid and acetic anhydride at boiling temperature, and the P32 is recovered from the residue left after distilling the acid. An apparatus for carrying out the process by remote control is described. It has been used for production of 100 millicurie amounts of carrier-free P32 at the Chalk River laboratories.
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