The results are shown graphically in [Fig. 96], by the curves EE and FF, in which the ordinates represent the number of β and α particles expelled per second by the products D and F respectively. The complete calculation for three changes shows that the number of β particles soon reaches a practical maximum, and then decays nearly exponentially with the time, falling to half value in 40 years. The number of α particles expelled per second increases for several years, but reaches a maximum after 2·6 years and then diminishes, finally falling off exponentially with the time to half value in 40 years.
The experimental curve of the rise of α ray activity, shown in [Fig. 93], as far as it has been determined, lies accurately on this curve, if the maximum is calculated from the above theory. The observed activity after a period of 250 days is marked by the point X on the curve.
237. Experiments with old radium. Since the substance radium D is produced from radium at a constant rate, the amount present mixed with the radium will increase with its age. The writer had in his possession a small quantity of impure radium chloride, kindly presented by Professors Elster and Geitel four years before. The amount of radium D present in it was tested in the following way:—The substance was dissolved in water and kept continuously boiling for a period of about six hours. Under these conditions the emanation is removed as rapidly as it is formed, and the β rays from the radium, due to the product radium C, practically disappear. A newly prepared specimen of radium bromide under these conditions retains only a fraction of 1 per cent. of its original β radiation. The old radium, however, showed (immediately after this treatment) an activity measured by the β rays of about 8 per cent. of its original amount. The activity could not be reduced any lower by further boiling or aspiration of air through the solution. This residual β ray activity was due to the product radium E stored up in the radium. The β ray activity due to radium E was thus about 9 per cent. of that due to radium C. Disregarding the differences in the absorption of the β rays, when the activity of the product E in radium reaches a maximum value, the β ray activity due to it should be the same as that due to C. Since the parent product D is half transformed in forty years, the amount present in the radium after four years should be about 7 per cent. of the maximum amount; i.e. it should show a β ray activity of about 7 per cent. of that due to radium C. The observed and calculated values (7 and 9 per cent. respectively) are thus of the same order of magnitude. The amount of β rays from radium E present in pure radium bromide about one year old was about 2 per cent. of the total.
The amount of radium F present in old radium was measured by observations of the activity imparted to a bismuth disk left for several days in the solution, and was found to be of the same order as the theoretical value. Radium F is not deposited to an appreciable extent on the bismuth from a water solution of radium bromide. If, however, a trace of sulphuric acid is added to the solution, the radium F is readily deposited on the bismuth. The addition of sulphuric acid to the radium solution practically effected a separation of radium D, E and F from the radium proper; for the latter was precipitated as sulphate and the products D, E and F remained in solution. After filtering, the solution contained the greater proportion of the products D, E, and F and very little radium.
238. Variation of the activity of radium with time. It has been shown that the activity of freshly prepared radium increases at first with the time and practically reaches a maximum value after an interval of about one month. The results already considered show that there is a still further slow increase of activity with the time. This is the case whether the activity is measured by the α or β rays. It will be shown later that radium is probably half transformed in about 1000 years. From this it can readily be calculated that after a lapse of about 200 years the amount of the products radium D, E and F will have reached a maximum value. The same number of atoms of each of the products C and E will then break up per second. If each atom of these products in disintegrating throws off an equal number (probably one) of β particles, the number of β particles thrown off per second will be twice as great as from radium a few months old. The number will increase at first at the rate of about 2 per cent. a year.
Similar considerations apply to the α ray activity. Since, however, there are four other products of radium besides radium itself which expel α particles, the number of α particles emitted per second from old radium will not be more than 25 per cent. greater than the number from radium a few months old. The activity measured by the α rays will thus not increase more than 25 per cent., and probably still less, as the α particles from radium F produce less ionization than the α particles expelled from the other radium products. The activity of radium will consequently rise to a maximum after 200 years and then slowly die away with the time.
239. Presence of these products in pitchblende. The products radium D, E and F must be present in pitchblende in amounts proportional to the quantity of radium present, and should be capable of separation from the mineral by suitable chemical methods. The radio-active properties of these substances, if obtained in the pure state, are summarized below.
Radium D when first separated, should give out very little α or β radiation. The β ray activity will rapidly increase, reaching half its maximum value in 6 days. The α ray activity will at first increase nearly proportionately with the time, and will reach a maximum value after an interval of about 3 years. The α and β ray activity, after reaching a maximum, will finally decay, the activity falling to half value in about 40 years. Since radium D is half transformed in 40 years, and radium in 1200 years, the maximum β ray activity of radium D, weight for weight, will be about 300 times that of radium.
The α ray activity, at any time, will be removed by placing a bismuth disk in the solution.