The Energy of Disintegration of Radio-Phosphorus [P30]

Size: px

Start display at page:

Download "The Energy of Disintegration of Radio-Phosphorus [P30]"

Timothy Underwood
6 years ago
Views:

1 714 The Energy of Disintegration of Radio-Phosphorus [P30] By C. D. E llis, F.R.S., and W. J. H enderson, Ph.D. (Exhibition of 1851 Student, Queen s University, Kingston, Canada) ( Received October 23, 1935) 1 Introduction The exact masses of the nuclei are quantities of great interest depending directly on the forces of cohesion between the nuclear particles. Already much valuable information has been obtained about the lighter elements both by mass-spectrographic methods and by the study of atomic disintegrations. The discovery of the new radioactive elements has extended greatly the number of nuclei open to investigation, but since nearly all of these disintegrate by emitting either positrons or electrons forming a continuous spectrum we meet here the same difficulty in determining the total energy change in the disintegration as with the natural [3-ray bodies. In this latter case Henderson* has proved the correctness of the suggestion of Ellis and Mottf that the difference of energy of two nuclei, apart from y-emission, is given by the upper limit of the [3-ray spectrum. However, as was emphasized by Cockroft at the British Association Meeting at Norwich, in September, 1935, this is a point which needs verification in the region of low atomic numbers and particularly for positron disintegration. We have attempted to obtain some information on this point by investigating the disintegration of radio-phosphorus [P30] formed from aluminium by a-particle bombardment. The disintegration of radio phosphorus has already been investigated several times, but there is such a notable disagreement between the values given by different observers for the energy of the upper limit that we felt fresh experiments were needed. Further, it is necessary to determine whether the upper limit corresponds to the formation of the ground state or of an excited state of the product nucleus. Our final results are that the energy of the upper limit of the positrons from radio phosphorus is close to 2-9 x 106 volts and that this corresponds to the formation of the product nucleus Si30 in its ground state. * ' Proc. Roy. Soc., A, vol. 147, p. 572 (1934). f Proc. Roy. Soc., A, vol. 141, p. 502 (1933).

Energy o f Disintegration o f Radio-Phosphorus 715 Adopting the hypothesis that this energy determines the difference of internal energy of Si30 and P30 we find, on the basis of existing information,

2 Energy o f Disintegration o f Radio-Phosphorus 715 Adopting the hypothesis that this energy determines the difference of internal energy of Si30 and P30 we find, on the basis of existing information, that there is the same difference of energy between Al27 and Si30, whether Si30 is formed directly or by the intermediate stage of P30 13A F + 2He4-14Si30 + 1H1 13A He4 -> 15P30 + o 15P30-14Si30 + t Finally, if the mass of Si30 be written as 30 + jr, then the masses of Al27 and P30 are respectively 27 + x an 2 Previous Work In describing the previous work on radio-phosphorus we shall also refer to data on the low energy portion of the spectrum, since in our former experiments we thought we obtained evidence of a definite lower limit to the curve. Curie and Joliot* found that most of the positrons from radio-phosphorus were stopped by 1 gm/cm2 of copper, suggesting an upper limit slightly greater than 2-4 x 106 volts. This was confirmed approximately by cloud chamber measurements which gave a value of 3-0 x 106 volts. No information was obtained about the form of the spectrum below 200,000 volts. The present authorst investigated the absorption of the positrons in aluminium and found an end point at about 1-25 gm/cm2 corresponding to an upper limit of 2-75 x 106 volts. A rough magnetic analysis of the spectrum was also made which gave a value of about 2-8 x 10u volts for the upper limit. A feature of the absorption curves found with both aluminium and copper, was a flat initial portion, and this was taken to indicate the absence of any very slow positrons. It was suggested that the distribution curve had a lower limit at about 200,000 volts. MeitnerJ used a cloud chamber in a magnetic field to investigate the emission, and concluded that the upper limit lay between 2 and 2-4 x 106 volts. While in some experiments she found a relatively large number * J. Phys. Rad., vol. 5, p. 153 (1934). t Proc. Roy. Soc., A, vol. 146, p. 206 (1934). + Naturwiss., vol. 22, p. 388 (1934).

3 716 C. D. Ellis and W. J. Henderson of short tracks she showed that many of these must have been due to the y-rays of the polonium used to form the radio-phosphorus and which, in some experiments, was kept in position during the expansion. However, after allowing for this effect, Meitner left it an open question whether the spectrum continued to zero energy or not. Frisch* measured the absorption curve of the positrons in copper and did not observe any initial flat portion of the curve. He found a maximum range for the positrons of 0-8 gm/cm2 corresponding to an upper limit of about 2 x 106 volts. Nishina, Sagane, Takenchi, and Tomitaf give a value for the upper limit of 4 x 106 volts based on the measurement of 1500 tracks in an expansion chamber. Only 10 of the tracks had energies greater than 3 x 106 volts. They observed positrons with energies as low as 100,000 volts present in appreciable numbers, but their experimental arrangement was not suitable for investigating the lower end of the spectrum. Alichanow, Alichanian, and DzelepowJ analysed the spectrum by the semi-circular magnetic focussing method and obtained a value 3-75 x 106 volts for the upper limit. The low energy part of the spectrum was not investigated. The lack of agreement between these various measurements of the upper limit may be seen from Table I ; clearly further measurements are necessary. Table I Observer Upper limit of the positron spectrum of P30 in volts Curie and Joliot... 3 X 10 Meitner x 106 Frisch... 2 x 106 Nishina et alii... 4 X 106 Alichanow et alii X 106 Ellis and Henderson X 10 3 Experimental Arrangement The arrangement of Geiger-Mueller counters used to investigate the absorption curve is shown drawn to scale in fig. 1. Only coincidences were counted using the circuit recently described by Barrasch. It was * Nature, vol. 133, p. 721 (1934). t Sci. Pap. inst. phys. chem. Res. Tokyo, vol. 25, p. 1 (1934). t Z. Physik, vols. 5, 6, p. 350 (1934). Proc. Phys. Soc., vol. 47, p. 824 (1935).

Energy o f Disintegration o f Radio-Phosphorus hoped initially to investigate the absorption by inserting absorption sheets between the counters at B, but using the shape of source most suitable for

4 Energy o f Disintegration o f Radio-Phosphorus hoped initially to investigate the absorption by inserting absorption sheets between the counters at B, but using the shape of source most suitable for activation, a small cylinder of aluminium foil, it proved difficult to canalize the beam sufficiently well to make the numbers of particles in the front and back counters comparable. As will be seen later, however, useful information was obtained by placing absorbers between the counters when there were initially absorbers in front of the first counter. The advantage of the arrangement depended really on the type of source available. The most powerful and convenient source of a-particles was a glass tube, containing radon, whose walls were sufficiently thin to allow the particles to escape. To use fully such an a-ray tube it was necessary to place over it an aluminium tube about 2 mm B F ig. 1. in diameter and about 2 to 3 cm long. Now any single counter which was of sufficient size to receive a reasonable number of the particles emitted by such a source would have a very large natural effect, whereas with our arrangement, as can be seen from fig. 1, we took in quite a large fraction of the particles but, owing to coincidence counting, the natural effect was small. Actually the natural effects of the front and back counters alone were 50 and 70 per minute respectively, whereas, using coincidence counting, they were only 5 per minute. Even this number was largely true coincidences of (3-particles passing through both counters and due to the y-rays from the laboratory supply of radium which was only 100 feet away. For example, an absorber of 0-7 gm/cm2 placed between the counters reduced this natural count to 1-4 per minute. The radon tubes employed contained usually 60 to 100 millicuries of radon and their walls had a stopping power equivalent to 1-5 to 2-5 cm of air. The counters had sides, as shown, formed of brass grids covered

718 C. D. Ellis and W. J. Henderson with mica of about 2 cm air equivalent. The front counter had a crosssection 1-9 x 2-7 cm and the sensitive counting region was 7 cm long.

5 718 C. D. Ellis and W. J. Henderson with mica of about 2 cm air equivalent. The front counter had a crosssection 1-9 x 2-7 cm and the sensitive counting region was 7 cm long. The back counter was made from one-half of a brass tube of 4 cm internal diameter and had a counting region 10 cm long. The two counters were mounted 1 cm apart and the source of radio-phosphorus was placed at S, 1 cm from the face of the front counter. 4 Experimental Results The absorption curve was taken with copper, chiefly because it was less bulky than aluminium, and our measurements are shown in fig. 2. We should like to call attention particularly to the initial convex portion gm/cm2 Fig. 2. of the curve which extends up to about 0-09 gm/cm2. With the (3-ray bodies of the ordinary radioactive series no such initial convex portion is observed. This is the same result that we obtained in our previous experiments ;* and it appears to indicate;that there can be but few positrons emitted with less energy than about 200,000 volts. The curve as a whole agrees excellently with our previous measurements of the absorption in copper which were extended only to an absorption of 0 50 gm/cm2. The end of the curve is shown on an enlarged scale in fig. 3, from which it will be seen that there is an end point at 1-30 ± 0-05 gm /cm2. While there is some doubt about the relation between the end point measured in copper in such an arrangement as this and the maximum * Proc. Roy. Soc., A, vol. 146, p. 256 (1934).

Energy o f Disintegration o f Radio-Phosphorus 719 energy to which it corresponds we can assign an upper energy limit to the main positron emission of (2-9 ±0-1) x 106 volts.

6 Energy o f Disintegration o f Radio-Phosphorus 719 energy to which it corresponds we can assign an upper energy limit to the main positron emission of (2-9 ±0-1) x 106 volts. This assumes that the range-energy relation is the same for positrons as for electrons. This agrees with our previous measurements and with those of Curie and Joliot.* We cannot at once, however, assume that this energy is really the energy of disintegration of radio-phosphorus [P3n] into silicon [Si30]. From the work of Duncanson and Millerf we know that Si30 can be formed in at least three excited states of 2-23, 3-60, and 4-74 x 10 volts excess energy, and the upper limit we have measured might correspond to any of these Absorption in gm/cm2 F ig. 3. states of excitation, leading to a correspondingly greater energy of disintegration. However, we can be sure that this upper limit corresponds to the most frequent mode of disintegration, so that if, for example, it involved excitation to the 2-23 x 10 volt state of Si30 we should find a y-ray emission of 2-23 x 10 volts for practically every disintegration. There is, in fact, as will be seen from fig. 3, a noticeable y-ray effect after the end point at 1-30 gm/cm2 and we investigated carefully whether this was simply the annihilation radiation of the positrons or whether there *.1. Phys. Rad., vol. 5, p. 153 (1934). f Proc. Roy. Soc., A, vol. 146, p. 396 (1934).

7 720 C. D. Ellis and W. J. Henderson was in addition a y-ray emission of about 2-23 x 10 volts per disintegration. The following two tests were made on the hardness of the radiation beyond the end point. Extra absorbing sheets of lead up to 5 gm/cm2 were inserted between the source and the front counter and a mass absorption coefficient of about 0-2 was obtained. This is in fair agreement with other determinations of the absorption of the annihilation radiation of positrons, viz., Thibaud* 0-19, Joliotf 0T9-0-30, Crane and LauritsenJ 0T3, and Klemperer 0T5. Using a source of Th C" under exactly the same conditions, we obtained a mass absorption coefficient of about Now per disintegration Th C" emits 1 quantum of 2-62 x 10 volts, 0-73 quantum of x 105 volts, 0-20 quantum of x 105 volts, and 0-08 quantum of X 105 volts. We should not expect this radiation to be so markedly harder than the radio-phosphorus radiation if the latter really consisted of 2 quanta of X 105 volts (annihilation radiation) and also 1 quantum of 2-23 x 10 volts. It would appear, therefore, that excitation of Si30 is not frequent. In another experiment the source was placed inside a closed lead cylinder whose walls had a thickness of 2-05 gm/cm2. The coincidences of the two counters are due to electrons ejected from this lead cylinder by the y-rays. An estimate of the energy of these electrons, which are heterogeneous since they originate at various depths in the lead, was obtained by inserting absorbing sheets between the counters. It was found that 0-1 gm/cm2 of copper produced a diminution of 20% which, for homogeneous rays, would indicate an energy of about 400,000 volts. The number of counts was too small to permit of a more detailed analysis. It will be seen that both experiments suggest that the y-radiation is mainly the annihilation radiation of the positrons and there is no indication of any large amount of hard radiation of over 2x10 volts energy. In view, however, of the importance of this point, we attacked the problem in another way. We have already mentioned that the question is whether radio-phosphorus emits per disintegration only 2 quanta of x 10 volts or these two quanta and one quantum of 2-23 X 10 * * C.R. Acad. Sci., Paris, vol. 197, p (1933). t C.R. Acad. Sci., Paris, vol. 197, p (1933); vol. 198, p. 81 (1934). t Phys. Rev., vol. 45, p. 430 (1934). Proc. Camb. Phil. Soc., vol. 30, p. 347 (1934). These intensities are slightly different from those given recently by Ellis ( Int. Conf. Nuclear Phys. London (1934)), and are based on some unpublished work of F. Oppenheimer.

8 Energy o f Disintegration o f Radio-Phosphorus 721 volts. In this latter case the comparison with the y-radiation of Th C" should be fairly close. We therefore attempted to compare the y- radiation of these two bodies in a manner that would determine the relative number of quanta emitted. We prepared by recoil a source of Th C" on the inside of an aluminium cylinder identical with those used for the production of radio-phosphorus. With this source we took an absorption curve in copper similar to that shown in fig. 2. The count due to y-rays, shortly after the end point, was 10% of the initial intensity, whereas from radio-phosphorus the y-rays were only about 1%. This difference is decisive and we can conclude that the end point of the positron spectrum at 2-9 x 106 volts corresponds to the formation of silicon in its ground state. It may seem a little surprising that the difference between the y-ray intensities of Th C" and P30 was so great. This was due to two causes. Our experimental arrangement slightly favoured high-frequency y-rays, since we only counted those electrons ejected from the absorbing sheets which passed through both counters. Secondly, the annihilation radiation from P30 is produced mainly in the copper absorbing sheets at the end of the range and represents, therefore, a diffuse source in contrast to the Th C" y-radiation which comes from the small activated aluminium tube. It is interesting to recall that Duncanson and Miller* found that in the proton emission leading directly to Si30 the 2-23 x 10;1volt excited state is formed three times as often as the ground state. We must conclude that there is a difference in the relative frequency of formation of these two states by a factor of the order of 10 for the two reactions Al27 + He4-* Si30 + H1 and P30 -> Si30 + s. It may be noted that when excitation does occur in the latter reaction it will give rise to a continuous (3-ray spectrum ending at 0-7 x 106 volts. 5 D iscussion As suggested already by Meitner and Jaeckel,t we can obtain an important check on our application of energy relations to nuclear disintegrations by considering the dual disintegrations of aluminium under the action of a-particles. This may be written as follows. The equations will also represent the energy balance if the symbols are taken to represent the masses of the neutral atoms, electrons, and positrons, etc., and the * Proc. Roy. Soc., A, vol. 146, p. 396 (1934). t Z. Physik, vol. 91, p. 493 (1934). VOL. CLII. A. 3 c

9 722 C. D. Ellis and W. J. Henderson Q s represent in mass units the algebraic sum of the kinetic energies of the particles 13AI27 + 2He4 = 14Si30 + 4H* + Q4 ( 1 ) whence 1 3 A F + 2He4 = 15P30 + 0/P + Q2 ( 2 ) 15P30 = i4si30+ s + +s + Q3, (3) 1.H1 + Qi = s + + o^1 Q2 "i~ Q,3* (4) From Duncanson and Miller s* work on the production of protons from aluminium we obtain a value of Q4of Jaeckel s measurement on the maximum energy of the neutrons ejected from aluminium gives for Q2 a value of Our measurement of the P i0 disintegration gives Q3 = Hence 0 i _ jh1 = This agrees excellently with the masses published recently by Oiiphant, Kempton, and Rutherford,! viz., q/i1 = , 4HJ = However, Aston sj new preliminary measurement of deuterium , in conjunction with Chadwick and Goldhaber s measurement of the binding energy of the deuteron yields 0 n1= If it would mean that there was roughly one million volts too much energy emitted in the neutron and positron branch. The measurement of neutron energies is admittedly difficult, but as Jaeckel pointed out his value was more likely to be too small than too large. Our measurement of the energy of disintegration is shown with an uncertainty of 100,000 volts which is probably an over-estimate of the error. It would tend to be too low from the inherent characteristics of the range method, but, on the other hand, we have made a generous allowance for the difference between the range-energy curves in aluminium and copper, which, if anything, has put the value too high. Provisionally it seems reasonable to accept the disintegration masses for the neutron-proton difference and hence conclude that it is correct to deduce the energy of a positron disintegration from the energy of the upper limit of the spectrum. It may be convenient to state here the relation masses of Al27, P30, and Si30. If the, at present, unknown mass of Si30 be written as 30 + x then Al27 = 27 + x and P30 = 30 + x * Proc. Roy. Soc., A, vol. 146, p. 396 (1934). f Proc. Roy. Soc., A, vol. 150, p. 241 (1935). t Nature, vol. 135, p. 541 (1935). Proc. Roy. Soc., A, vol. 151, p. 479 (1935).

Energy o f Disintegration Radio-Phosphorus 723 In conclusion, we should like to thank Mr. G. R. Crowe for preparing the radon tubes, and Mr. R. Cole for help in the experiments.

10 Energy o f Disintegration Radio-Phosphorus 723 In conclusion, we should like to thank Mr. G. R. Crowe for preparing the radon tubes, and Mr. R. Cole for help in the experiments. S u m m a r y The maximum energy of the positrons emitted from radio-phosphorus P3U has been determined by measuring the end point of the absorption curve in copper. The value found is 2-9 ± 0 1 x 10fi volts in good agreement with a previous determination. A careful search was made to see if any y-rays accompanied the disintegration but none was found besides the annihilation radiation of the positrons. This energy of 2-9 x 10 volts therefore determines the difference of energy of the ground states of P30 and Si30. Combining this value with other published data, the difference in mass of Al27 and Si30 is found to be the same when calculated from 13A F + 2He4 -> 14S P + XH \ or from the two stages 13A F + 2He4-15P30 + 0«4 15P S P + t

The production and properties of positrons

C ARL D. AN D E R S O N The production and properties of positrons Nobel Lecture, December 12, 1936 Information of fundamental importance to the general problem of atomic structure has resulted from systematic