spectroscopic identification and manometric measurement...

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Spectroscopic Identification and Manometric Measurement of Artificially Produced Helium*

By F. A. P aneth , E. G lu ck a u f , and H. L oleit

{Communicated by J. C. Philip, F.R.S.— Received 3 June, 1936)

The evidence for artificially produced elements has so far been based entirely on radioactive methods (fluorescent screen, electrometric device, or Wilson chamber); either the rays accompanying the process of transmutation have been observed, or, in the case of artificial radio-elements, the rays emitted by the products of transmutation have been used for their detection and the study of their behaviour. The amount of new matter is usually so small that there is no possibility of discovering it by any non-radioactive method; attempts to detect hydrogen spectroscopically have failed,! but with helium where the limit of identification is so low {see I) there was more hope of success.

There are at present several processes of artificial transmutation of which helium is known to be a product. In high-voltage apparatus it is difficult completely to exclude the possibility of contamination by helium from the air, or from glass walls;! but bombardment by radioactive sources can be carried out under conditions much better suited to our purpose. Chadwick and Goldhaber§ and Fermi and co-workers|| have

* ‘ Helium Researches XIII.’ As previous numbers of the ‘ Helium Researches ’ are frequently quoted, we give the references here: I, II, and III, Paneth and Peters, ‘ Z. phys. Chem.,’ vol. 134, p. 353 (1928); ibid., B, vol. 1, pp. 170, 253 (1928); IV, Paneth, Gehlen, and Peters, ‘ Z. anorg. Chem.,’ vol. 175, p. 383 (1928); V, Paneth, Gehlen, and Gunther, ‘ Z. Elektrochem.,’ vol. 34, p. 645 (1928); VI, Paneth, Petersen, and Chloupek, ‘ Ber. deuts. chem. Ges.,’ vol. 62, p. 801 (1929); VII, VIII, and IX, Paneth and Urry, ‘ Mikrochem,’ Emich-Festschrift, p. 233 (1930); ‘ Z. phys. Chem.,’ A, vol. 152, pp. 110, 127 (1931); X, Paneth and Koeck, ‘ Z. phys. Chem.,’ Bodenstein-Festband, p. 145 (1931); XI, Gunther and Paneth, ‘ Z. phys. Chem.,’ A, vol. 173, p. 401 (1935); XII, Holmes and Paneth, ‘ Proc. Roy. Soc.,’ A, vol. 154, p. 385 (1936); referred to later as I, etc.

t Paneth and Gunther, ‘ Nature,’ vol. 131, p. 652 (1933). See also XI.+ See Paneth and Thomson, ‘ Nature,’ vol. 136, p. 334 (1935).§ Chadwick and Goldhaber, ‘ Nature,’ vol. 135, p. 65 (1935); ‘ Proc. Camb. Phil.

Soc.,’ vol. 31, p. 612 (1935); Taylor and Goldhaber, ‘ Nature,’ vol. 135, p. 341 (1936); see also Taylor, ‘ Proc. Phys. Soc.,’ vol. 47, p. 873 (1935); Taylor and Dabholkar, ibid., vol. 48, p. 285 (1936); Kurtchatow, Kurtchatow, and Latyshew, ‘ C.R. Acad. Sci. Paris,’ vol. 200, p. 1199 (1935).

|| Amaldi, D’Agostino, Fermi, Pontecorvo, Rasetti, and Segre, ‘ Proc. Roy. Soc.,’ A, vol. 149, p. 522 (1935).

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Artificially Produced Helium 413

found that under the impact of slow neutrons lithium and boron produce helium, the latter according to the reaction:

510B + f n - 24He + 37Li. (1)

We decided to try boron because this element can be obtained in the form of volatile liquid esters from which it is easy to drive out the helium; the presence of hydrogen atoms in the esters is an additional advantage, since it is essential to slow down the neutrons before they collide with the boron atoms. The methyl ester of boric acid which we used, B (OCH3)3, contains nine hydrogen atoms for every boron atom; these hydrogen atoms, as well as the oxygen and carbon atoms, have, compared with boron, a negligible absorbing power for slow neutrons.* 1 cc. of ester, d20 = 0-915, contains 0-095 gm. boron and 0-080 gm. hydrogen.

A ppa ra tu s a n d E xperim ental P rocedure

To avoid contamination by helium from glass, the ester was in contact exclusively with metal during the whole irradiation, which had to be continued for months. It was contained in a spherical copper flask (A in fig. 2) with a gun-metal stopcock. The radius of the flask in its final form was 10 cm .; the neutron sources could be placed at the centre of the flask by means of a cylindrical pocket of 1 - 2 cm. radius. The flask was inserted in a cylindrical water tank of 40 cm. diameter. As neutron sources we used mainly glass tubes filled with finely powdered beryllium and radon, supplied to us from time to time from three different sources: St. Bartholomew’s and the Middlesex Hospitals in London, and the Institute for Radium Research in Vienna.

Before the experiment starts the boron ester has to be freed from atmospheric air and the Ne and He contained therein. This is achieved by opening the stopcock a of the copper flask and the glass stopcocks b, c d, and i to the high-vacuum pump and keeping the ester boiling for about 15 minutes; the distillate is collected in the traps B and B' which are cooled by liquid air. The same operation is then performed as described below for the collection of helium from the ester; in this blank test the quantity of neon -f helium observed in the capillary on top of the bulb L and measured by the manometric device N must not exceed 10~9 cc. We found that with methyl borate this point can be reached fairly quickly, and, further, that a second test, carried out weeks or months later, gives

* Fermi and co-workers, loc. cit.; Bjerge and Westcott, ‘ Proc. Roy. Soc.,’ A, vol. 150, p. 709 (1935).



414 F. A. Paneth, E. Gliickauf, and H. Loleit

the same result: no release of any detectable amount of helium from the walls of the copper vessel takes place. The interval between the two tests was in one experiment 9 weeks.

The actual experiment, the bombardment of the ester with neutrons, was carried out four times. In the first three experiments we used a somewhat smaller copper flask (radius 7-5 cm.) with a larger pocket (1 -5 cm.). The rest of the apparatus was in all cases identical, but the neutron sources were of very different strength. In addition to the radon- beryllium tubes mentioned above, we employed also a radium-beryllium, and a radiothorium-beryllium, source which happened to be at our disposal. The efficiency of the various preparations, as neutron sources, was determined by comparing the intensity, measured by a Geiger- Miiller counter, of the artificial radioactivity they produced in silver and in rhodium. We found no appreciable variation in the neutron efficiency of the numerous radon tubes, although the fineness of the powder and the free space left was slightly different. Our radium-beryllium mixture had only 0*75 of the neutron efficiency of a radon-beryllium tube of equal y-ray intensity; our radiothorium-beryllium mixture, on the other hand, being prepared from a radiothorium source of high emanating power, was 1 • 8 times as efficient a neutron source as a y-equivalent radon- beryllium tube. As neither radium nor radiothorium decays appreciably during the time of one experiment, the intensity of their radiation, taking for radiothorium the mean value, can be considered as constant. As to the radon, since its period of average life is 5-52 days, the total radiation emitted from 1 millicurie (me.) radon, during its complete decay, is equivalent to a constant radiation of 5-52 me. days or 4-77 x 103 me. seconds.

The quantities of radon which were allowed to decay in our four experiments, and the strengths of the constant sources, together with the duration of their application, are given in Table I.

After the irradiation by neutrons, the analysis of the methyl borate for helium was carried out, the procedure being the same in all four experiments. It was based on previous work on the detection and measurement of small quantities of helium (see especially I and VIII), but could be somewhat simplified since little hydrogen had to be removed. It was unnecessary, therefore, to use the calcium furnace or the palladium capillary, the palladium furnace being sufficient. As the new model of this furnace, which has been in use in our laboratory for some time, has so far not been described in detail it is shown in a special diagram, fig. 1. For delicate helium analyses, where the complete removal of hydrogen is essential, it can be strongly recommended. The innermost part A, made of soda




glass and sealed to the rest of the analysing apparatus, contains at the bottom about 0-2 gm. of palladium sponge. It is inserted in a jacket B so that it can be heated while the latter is cooled by water. B, made of Jena glass, carries inside a cylinder supporting a flat “ Nichrome ” wire by means of which the heating of the palladium to a dull red heat can be effected with an energy of 50 w att; B is evacuated through a side tube c. As can be seen from the drawing, part A contains an inner glass tube , reaching almost to the bottom, and a second inlet b ; this makes it possible to let the hydrogen-oxygen mixture pass over the hot palladium as many

T able IExperiment

1 2 3N̂,

4Radon-beryllium sources—

Me. decayed ...................................... 57 446 1975 1858Equivalent to 108 me. sec.................... 0-27 2-13 9-42 8-86

Radium-beryllium source—y-equivalent to me. radon .............. — — — 50-9Neutron-equivalent to me. radon .. — — — 38Time of irradiation in days .............. — — — 26-7Effect equivalent to 10s me. sec. radon — — — 0-88

Radio-thorium beryllium source—y-equivalent to me. radon .............. 18 — 21 —Neutron-equivalent to me. radon .. 32-4 — 38 —Time of irradiation in days .............. 14-5 — 47-6 —Effect equivalent to 108 me. sec. radon 0-41 — 1 -56 —

Total activity applied—In me. radon decayed ...................... 140 446 2300 2040In 108 me. sec. radon ...................... 0-68 2*13 10-98 9-74

times as seems desirable (see description below), thus ensuring complete combustion.

In order to drive out the helium formed in the methyl borate, the latter is kept boiling just as was done initially for the removal of air p. 413); but this time the outlet of the traps B and B' (fig. 2) is closed to the pump and opened through stopcocks k, /, and n to the three tubes E, F, J (each containing 12 gm. charcoal), to the palladium furnace G, and to the bulb L (volume 600 cc.). As B and B' are cooled by liquid air the gas filling the space just described consists, apart from the traces of helium, almost exclusively of hydrogen and methane which have been evolved as a conse-



416 F. A. Paneth, E. Gliickauf, and H. Loleit

quence of the irradiation of the boron ester by y-rays and neutrons.* For the elimination of hydrogen and, at the same time, the transport of all the helium contained between c and / into the analysing apparatus, a surplus of oxygen is admitted through stopcock h. This oxygen has

cF ig. 1 (half natural size).

* Very littlejis known so far about the chemical influence of neutron bombardment; according to Hopwood and Phillips (‘ Nature,’ vol. 136, p. 1026 (1935)), the chemical effects of the neutrons of a radon-beryllium source are of the same order of magnitude as those due to its y-rays.




r i

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418 F. A. Paneth, E. Gluckauf, and H. Loleit

been electrolytically prepared,* freed from hydrogen and a possible contamination of helium and neon by condensation in charcoal C, and afterwards stored in bulb D. It is first admitted into B' and B, and then, by opening k for short intervals, adsorbed in E which is cooled by liquid oxygen, so acting as a carrier for the helium II, p, 173). After repeating this process six times all the helium should be present in the apparatus behind k. To ensure that no helium is retained by the comparatively large quantity of ester condensed in B, we made it simmer by surrounding B with hot water, B' remaining in liquid air; during this operation tap d is closed and oxygen of about 10 cm. Hg pressure present.

From now onwards k remains closed. To separate the helium contained in E from the bulk of the oxygen the temperature of E is slightly raised by removing the liquid oxygen bath for about 1 minute, while / is closed; by repeatedly opening / for short intervals all this helium is carried beyond /. In the last and most important experiment 4 we made sure that no helium was left in A, B, B', and E by repeating the whole process onwards from boiling the methyl borate; no measurable amount of helium was recovered.

The oxygen released into the analysing apparatus should have a pressure of not more than about 12 cm. and is now used for the combustion of the hydrogen. The palladium in furnace G is heated and the mixture of gases, the major part of which is contained in the large volume L, passed over the palladium by cooling charcoal F in liquid air and thereby adsorbing the oxygen. Subsequent heating of F by means of hot water causes the oxygen to expand again and so to return through the palladium furnace to bulb L. Eight repetitions of this operation of alternate cooling and heating are always sufficient to burn all the hydrogen present to water. This water and the surplus of oxygen is removed by cooling first charcoal F and finally J for about 30 minutes each.

At the end of this cooling all gases with the exception of helium should have been removed. In order to test this, mercury is now raised into L, and the gas compressed into the capillary on the top of it and spectroscopically examined. If there has been any leakage of air into the vessel A during the weeks of irradiation, or into the glass parts of the apparatus during the separation of the helium, this contamination reveals itself immediately by the presence of neon lines in the helium spectrum. This is a very valuable check (see I and II) and it is, therefore, essential to use as cooling agent liquid oxygen and not liquid nitrogen or liquid air, for charcoal cooled to the temperature of liquid nitrogen retains a very considerable part of the neon. As has been shown previously, less than

* See I, fig. 3, p. 365.




0-01% of neon in helium is spectroscopically easily visible, and a comparison of the relative strength of the lines permits of a fairly accurate estimate of the percentage of neon in the helium (see XI). While in the first three of our experiments the neon present amounted to about 20% of the helium, in the last experiment we succeeded in keeping the boron ester air-free to such an extent that, according to spectroscopic examination, the neon was certainly less than 10% of the helium.

The measurement of a minimum of 10~8 cc. of helium can be carried out by means of a Pirani gauge (see VIII). For this purpose the mercury in bulb L is lowered below the tube leading to M ; then with the help of stopcock y the mercury in M is withdrawn till the gas gains access to one side of the Pirani gauge N. For the calibration of the latter known quantities of helium from bulb He (volume measured in the capillary s-sl, pressure in the McLeod gauge S) are distributed through exactly the same volumes, P, N, M and upper part of L. (The cooled charcoals R and P are applied in order to remove all other gases, especially tap grease, and to make sure that only helium and neon are measured in the Pirani gauge.)

A certain quantity of the helium has, of course, not been included in the bulb L when the mercury was raised, but has remained in the palladium furnace G, the charcoals, and the connecting glass tubes up to stopcock /, according to the respective volumes and temperatures. The necessary correction for this part of the helium is found by introducing into the whole apparatus from L to / a known quantity of helium (measured by means of the capillary o-o1 and the McLeod gauge S) and treating this in exactly the same way. The helium measured in the Pirani gauge has to be multiplied by the factor so determined; in our case this was 1 -33.

A few other details in fig. 2 are self-explanatory, or their purpose can be found from our previous publications.

R esults*

In experiment 1 the quantity of helium was sufficient only for a qualitative identification but not for a measurement in the Pirani gauge. From the lines visible in the spectroscope 5875-63, 5015-68, 4921-93, and, faintly, 4471-48 A., we could roughly estimate! the amount of helium as of the order of 10-9 cc. The experiment is interesting as showing that

* The results of the second and third experiment have already been briefly communicated ‘ Nature,’ vol. 136, p. 950, (1935).

t See table on p. 371 in I; or ‘ Mikrochem.,’ vol. 7, p. 425 (1929).



420 F. A. Paneth, E. Gluckauf, and H. Loleit

a quantity of not more than 140 me. radon, mixed with beryllium, is sufficient for a production of detectable amounts of helium from boron; but here we are near to the limit of sensitivity of our micro-method.

In the second experiment, carried out with 446 me. radon, the helium quantity produced was still too small for an exact measurement, although in this experiment the helium line 4471 *48, hardly visible in the first experiment, was very distinct; this is in good agreement with the increase in the strength of the radon source. In experiments 3 and 4 two more lines, 6678*15 and 4713*15, could be easily seen, and here the Pirani gauge could be applied. We found in experiment 3, making allowance for the 20% neon present ( see formula in XI), 1 *4 ± 0*2 x 10~7 cc. helium, and in experiment 4, where the quantity of neon was negligible, 2 • 4 ± 0*2 x 10~7 cc. helium (N.T.P.).

Although the activity in experiment 3 exceeded that applied in experiment 4 by almost 13%, the helium found was only about 0*6 as much. The reason for the greater yield in the last experiment is no doubt the increased volume of the copper vessel A filled with boron ester, and it is very likely that a further increase of the volume would result in a still higher value. It is hardly possible to calculate exactly the proportion of neutrons which is still not caught even in the bigger vessel. A fraction of the neutrons produced in the beryllium source is certainly not slowed down by the hydrogen of the boron ester, but only by the water of the tank outside; some of them, however, get back by diffusion from here into the vessel A and are finally caught by boron atoms. One has also to consider that during the experiments the vessel A was not completely filled with the liquid ester; not only the dome on top but also a part of the spherical vessel was empty because, as described above, some of the ester had to be distilled into B in order to get it air-free; as, however, the surface of the boron ester was still 4 cm. above the centre of the sphere in experiment 4 (and 2 cm. in experiment 3), the influence of the incomplete filling on the final result can only be small and by a slightly changed arrangement in a repetition of the experiment this source of error could easily be avoided. It will be more troublesome to make the vessel A large enough to ensure capture of practically all the neutrons produced in the radon-beryllium source, but if the experiment is deemed sufficiently important, it could no doubt be done. An increase of the radius of A from 7*5 cm. to 10 cm., together with a reduction of the radius of the pocket and a rising of the level of the ester, enabled us to catch 1 • 9 times as many neutrons in experiment 4 as in experiment 3; from this fact, as well as from theoretical considerations based on work on the diffusion of neutrons in water, it seems that another substantial increase might be expected if



all neutrons could be used up by reaction (1) inside a still larger copper vessel.

Although, therefore, the helium found is only a minimum value it is quite interesting to make use of this figure for a calculation of the minimum number of neutrons generated in the beryllium source. In experiment 4, 9-74 x 108 me. sec. radon produced 2-4 x 10-7 cc., or 6-5 x 1012 atoms of helium; according to equation (1) the same number of neutrons has been used up, from which it follows that 1 me. sec. of a radon beryllium source emits at least 6-7 x 103 neutrons. This figure is considerably higher than the value 1 x 103 deduced from early experiments.* More recent observations in a Wilson chamber made 104 neutrons per me. sec. seem a likely value.f Our figure is obtained by a quite different and very direct way; being a minimum value, it is consistent with this result. It may be pointed out that in experiment 4, on which our figure is based, only 10% of the activity was provided by a radium beryllium source, so that any uncertainty in determining the neutron equivalent of such a source to radon beryllium is of very little influence.

From a minimum neutron efficiency of 6-7 x 103 neutrons per me. sec. it follows that one a-particle from a (Rd + RaA + RaC') source by its impact on beryllium produces at least 6 x 10-5 neutrons, more than one neutron per 17,000 a-particles; but as the three a-particles have very different energies, not much physical significance can be attached to such an average value.

As to the origin of the helium, the objection could be made that possibly the y-rays and not the neutrons were the efficient agent. Chadwick and Goldhaber,J however, were able to detect nuclear disintegration as a product of y-radiation only in deuterium and in beryllium and not in any of the elements contained in the vessel A (boron, carbon, oxygen, copper). And in a control experiment in which 193 me. radon, without beryllium, decayed in the centre of vessel A, we did not find any helium in the boron ester.

In the present experiments an artificially produced element has been spectroscopically identified and quantitatively measured for the first time. The reaction by which the helium is formed has already been known from physical experiments; but it is not without interest that the

* Ellis and Henderson, ‘ Nature,’ vol. 133, p. 530 (1934); also Fermi, Amaldi, D’Agostino, Rasetti, and Segre, ‘ Proc. Roy. Soc.,’ A, vol. 146, p. 483 (1934).

t Jaeckel, ‘ Z. phys. Chem.,’ vol. 91, p. 493 (1934). The result can hardly claim to give more than the order of magnitude as three quantities entering the calculation are given only with one significant figure.

+ Chadwick and Goldhaber, ‘ Proc. Roy. Soc.,’ A, vol. 151, p. 479 (1935).

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422 Artificially Produced Helium

methods of chemistry are sensitive enough to detect the products of artificial transmutation and to permit an entirely independent quantitative study.* It may be that there are atomic processes going on with so little energy that radio-physical or radio-chemical methods cannot be applied at all, yet where a chemical procedure, similar to the one described, may be able to discover the newly formed elements. Experiments on these lines are in progress.

The senior author wishes to express his thanks to the Imperial College for laboratory facilities; to Imperial Chemical Industries, Ltd., for their assistance; to Professor F. L. Hopwood, director of the Radium Department, St. Bartholomew’s Hospital, London, Professor S. Russ, director of the Radium Department, Middlesex Hospital, London, and Professor Stefan Meyer, director of the Institute for Radium Research in Vienna, for kindly supplying the radon-beryllium tubes.

Summary

By a micro-method it was possible to collect, in a pure state, the helium produced by neutron bombardment from boron according to the following reaction:

510B + fn - 24He + 37Li.

The neutrons from a radioactive source equivalent to the decay of only 140 me. radon gave sufficient helium for a spectroscopic identification. With stronger radioactive sources, of the order of 2 curie radon, enough helium was obtained for its quantitative determination; the neutrons from 2-04 curie radon, mixed with beryllium, produced during its decay 2-4 x 10-7 cc. helium. From this figure it can be deduced that 1 me. sec. of a radon beryllium source emits at least 6-7 x 103 neutrons.

* See also ‘ Nature,’ vol. 137, p. 560 (1936).



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