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Preparation of isotopically pure superfluid 4He suitable for constructing a high density neutron source
Volume 64A, number 2
PHYSICS LETTERS
12 December 1977
PREPARATION OF ISOTOPICALLY PURE SUPERFLUID4 He SUITABLE FOR CONSTRUCTING A HIGH DENSITY NEUTRON SOURCE* R.J. SCOTT’ and P.V.E. McCLENTOCK Department of Physics, University of Lancaster, Lancaster, UK Received 4 October 1977 We have developed an isotopic purification technique by which the 411e/3He ratio may be raised beyond 2 X l0’~. This purity is sufficient for an experimental realisation of the neutron bottle recently proposed by Golub and Pendlebury.
In a recent letter [1], Golub and Pendlebury have pointed out that superfluid 4He is perhaps the only material potentially suited to forming the internal medium of a container for Ultra-Cold Neutron (UCN) gas at very high densities. The feasibility of using helium in such a device depends crucially, however,impurities on en3He isotopic suring that thesmall proportion of is sufficiently that neutron absorbtion by 3He does not seriously affect the storage time: assuming that wall losses can be minimised to the extent that ~ decay becomes the dominant loss mechanism, then a 4He/3He ratio greater than 2 X 1010 will be required. The main purpose of the present paper is to report that we have developed a purification technique by means of which this stringent requirement can cornfortably be met, The 4He/3He ratio (~=R 43)in naturally occurring well helium isofabout 6 X 106. straightforward “filtration” superfluid 4HeBy through a high quality superleak, the purity has been raised [2] to R 43magni= 9 but this still falls almost an order of 2.5 Xshort i0 of the purity required for efficient neutron tude storage. It is believed that higher purities may, in fact, have been achieved by using a heat flush technique [3]; but an element of uncertainty has remained because the 3He content of the product had fallen below the minimum level detectable by mass spectrometry (R 43 = 2.5 X 109). Our new isotopic separation apparatus, whose construction and operation will be described in detail elsewhere [4] ,has been designed spe*
Work supported by the Science Research Council under GR/A/0388.3.
Deceased.
cifically to circumvent this particular problem. Not only does it purify the 4He, but it also enables the measured lower bound on R 43 to be raised by several orders of magnitude. In common with its prototype [31,the apparatus uses away a windfrom of normal fluid to isotopicpure impurities a superleak: thecarry isotopically superfluid component, flowing in the opposite direction, passes through the superleak and fills up a receiving vessel. In contrast to the prototype, however, the present apparatus incorporates a needle value to prevent the superleak from being contaminated by helium of natural isotopic purity while the main helium bath was still above the lambda temperature. Once the 2~ receiving vessel was full, a secondary heat flush was used to drive any 3He atoms (which might by some unknown means have entered the product), via a3 copot. nical heat exchanger novel design, into a 2cm The sample of heliumoffor subsequent isotopic analysis wasThe taken fromofthis action the small cone pot. concentrator is illustrated diagrammatically in fig. 1. A heat flux Q applied to the copper receiving vessel a causes a thermal counterfiow of the normal and superfluid components to take place between it and the copper cone b whose outer surface is cooled by the pumped Hell bath c. To a first approximation it may be assumed that there are no temperature gradients in either of the helium baths or within the copper of the cone; but there will, however, be Kapitza teniperature discontinuities between the copper and the helium on both the inside and the outside of the cone. It is straightforward to demonstrate that, under 0n’ these conditions, the axial normal fluid velocity 205
Volume 64A, number 2
PHYSICS LETTERS
~ -
=
I I
-
>-
-
—~-:~--
~-
-1
-“i
:1 - -
~
,~~
-.
a Fig. 1. The cone concentrator (diagrammatic): a — copper pot containing 21 of isotopically purified 4He; b copper cone; c — pumped bath of Hell of natural isotopic purity; d — 2 cm3 sample pot; e — needle valve. The arrows indicate the direction of normal fluid flow within the purified 411e when a heat flux Q is applied to the thickwalled copper pot a.
averaged over the cross section, remains constant along the length of the cone. It was hoped, therefore, that the wind of normal fluid would carry any 3He atoms up into the small copper pot d from which the sample for analysis was to be taken. A needle value e was provided to isolate the sample in d once this process wasjudged to be complete. To avoid the possibility of 3He atoms remaining “plated” onto the inside walls of the cone, held in position by the component of Vn normal to the surface, the device was operated under highly turbulent conditions usually with On> 10cm s~1. In practice, after the main receiving vessel had been filled with helium purified by means of the primary heat flush, the secondary flush was operated for 4 hours at the level corresponding to 0.2 W cm2 in the cone; and finally for 1 hour at 1W cm2. Analysis” of helium taken from d indicated a 3He content below the minimum detectable level, i.e. R 43> 2.5 X ~ A calibration experiment was then performed to determine the factor by which the cone concentrator increases R43. Exactly the same procedures were fol~ We are grateful to the Bureau of Mines, U.S. Department of the Interior, Amarillo, TX, USA for analysing our samples by mass spectrometry.
206
12 December 1977
lowed as previously, except that vessel a was filled with helium which had been “doped” deliberately with a small proportion of 3 He. After operating the secondary flush in the same way as before, samples were taken both from a and from d. Analysis*’ showed: 3 and for a, R > 2.5 X l0~; factor for d, R43 3.2 X7.8i0X i05. hence, a43concentration of at=least Making the reasonable assumption that the same cnncentration factor had been achieved during the purification run we may conclude that, for our 4He product,R 43 >2 X 1015. This comfortably fulfils the isotopic purity criterion for an experimental rcalisation of the neutron bottle proposed by Golub and Pendlebury [1]. The purity is arising also more cient to eliminate difficulties fromthan the suffitendency 3He atoms to condense [5] on the negative ion in of so that it will now be possible to extend to lower 4He, temperatures the experiments on the breakdown of superfluidity [6] in liquid 4He which provided the original motivation for developing the purification tecunique. We may note that the results of the present work are consistent with the expectation [3] that it should actually be impossible for any 3He atoms to reach the receiving vessel by diffusing against the primary heat flush. This being so, we suggest that future purification machines can with reasonable confidence be constructed to a simpler design, omitting the secondary heat flush, cone heat exchanger and associated components, and relying solely on the primary heat flush for 3He removal. Finally, we comment that the superfluid 4He collected in a during the purification procedure is arguably the purest macroscopic sample of any single isotopic species yet to have been prepared: the usual non-helium impurities found in liquid 4He, such as colloidal air, should have all been removed along with the 3He by the primary heat flush and superleak. Indeed, it seems unlikely that elemental material of greater purity can occur in bulk anywhere.
We gratefully acknowledge the valuable technical assistance of N. Bewley, G. Caley, I.G. Marsden, I. Miller and A. Muirhead.
References Ill R. Golub and J.M. Pendlebury, Phys. Lett. 62A (1977) 337.
Volume 64A, number 2
PHYSICS LETTERS
[21 P.P. Fatouros, DO. Edwards, F.M. Gasparini and S.Y. Shen, Cryogenics 15 (1975) 147. [3] M. Atkins and P.V.E. MeCintock, Cryogenics 16 (1976) 733. [4] P.V.E. McClintock, to be published.
12 December 1977
[5] D.R. Allum and P.V.E. McClintock, J. Phys. C9 (1976) L273. [6] D.R. Alium, P.V.E. McClintock, A. Phillips and R.M. Bowley, Phil. Trans. Roy. Soc. London A284 (1977) 179.
207
PHYSICS LETTERS
12 December 1977
PREPARATION OF ISOTOPICALLY PURE SUPERFLUID4 He SUITABLE FOR CONSTRUCTING A HIGH DENSITY NEUTRON SOURCE* R.J. SCOTT’ and P.V.E. McCLENTOCK Department of Physics, University of Lancaster, Lancaster, UK Received 4 October 1977 We have developed an isotopic purification technique by which the 411e/3He ratio may be raised beyond 2 X l0’~. This purity is sufficient for an experimental realisation of the neutron bottle recently proposed by Golub and Pendlebury.
In a recent letter [1], Golub and Pendlebury have pointed out that superfluid 4He is perhaps the only material potentially suited to forming the internal medium of a container for Ultra-Cold Neutron (UCN) gas at very high densities. The feasibility of using helium in such a device depends crucially, however,impurities on en3He isotopic suring that thesmall proportion of is sufficiently that neutron absorbtion by 3He does not seriously affect the storage time: assuming that wall losses can be minimised to the extent that ~ decay becomes the dominant loss mechanism, then a 4He/3He ratio greater than 2 X 1010 will be required. The main purpose of the present paper is to report that we have developed a purification technique by means of which this stringent requirement can cornfortably be met, The 4He/3He ratio (~=R 43)in naturally occurring well helium isofabout 6 X 106. straightforward “filtration” superfluid 4HeBy through a high quality superleak, the purity has been raised [2] to R 43magni= 9 but this still falls almost an order of 2.5 Xshort i0 of the purity required for efficient neutron tude storage. It is believed that higher purities may, in fact, have been achieved by using a heat flush technique [3]; but an element of uncertainty has remained because the 3He content of the product had fallen below the minimum level detectable by mass spectrometry (R 43 = 2.5 X 109). Our new isotopic separation apparatus, whose construction and operation will be described in detail elsewhere [4] ,has been designed spe*
Work supported by the Science Research Council under GR/A/0388.3.
Deceased.
cifically to circumvent this particular problem. Not only does it purify the 4He, but it also enables the measured lower bound on R 43 to be raised by several orders of magnitude. In common with its prototype [31,the apparatus uses away a windfrom of normal fluid to isotopicpure impurities a superleak: thecarry isotopically superfluid component, flowing in the opposite direction, passes through the superleak and fills up a receiving vessel. In contrast to the prototype, however, the present apparatus incorporates a needle value to prevent the superleak from being contaminated by helium of natural isotopic purity while the main helium bath was still above the lambda temperature. Once the 2~ receiving vessel was full, a secondary heat flush was used to drive any 3He atoms (which might by some unknown means have entered the product), via a3 copot. nical heat exchanger novel design, into a 2cm The sample of heliumoffor subsequent isotopic analysis wasThe taken fromofthis action the small cone pot. concentrator is illustrated diagrammatically in fig. 1. A heat flux Q applied to the copper receiving vessel a causes a thermal counterfiow of the normal and superfluid components to take place between it and the copper cone b whose outer surface is cooled by the pumped Hell bath c. To a first approximation it may be assumed that there are no temperature gradients in either of the helium baths or within the copper of the cone; but there will, however, be Kapitza teniperature discontinuities between the copper and the helium on both the inside and the outside of the cone. It is straightforward to demonstrate that, under 0n’ these conditions, the axial normal fluid velocity 205
Volume 64A, number 2
PHYSICS LETTERS
~ -
=
I I
-
>-
-
—~-:~--
~-
-1
-“i
:1 - -
~
,~~
-.
a Fig. 1. The cone concentrator (diagrammatic): a — copper pot containing 21 of isotopically purified 4He; b copper cone; c — pumped bath of Hell of natural isotopic purity; d — 2 cm3 sample pot; e — needle valve. The arrows indicate the direction of normal fluid flow within the purified 411e when a heat flux Q is applied to the thickwalled copper pot a.
averaged over the cross section, remains constant along the length of the cone. It was hoped, therefore, that the wind of normal fluid would carry any 3He atoms up into the small copper pot d from which the sample for analysis was to be taken. A needle value e was provided to isolate the sample in d once this process wasjudged to be complete. To avoid the possibility of 3He atoms remaining “plated” onto the inside walls of the cone, held in position by the component of Vn normal to the surface, the device was operated under highly turbulent conditions usually with On> 10cm s~1. In practice, after the main receiving vessel had been filled with helium purified by means of the primary heat flush, the secondary flush was operated for 4 hours at the level corresponding to 0.2 W cm2 in the cone; and finally for 1 hour at 1W cm2. Analysis” of helium taken from d indicated a 3He content below the minimum detectable level, i.e. R 43> 2.5 X ~ A calibration experiment was then performed to determine the factor by which the cone concentrator increases R43. Exactly the same procedures were fol~ We are grateful to the Bureau of Mines, U.S. Department of the Interior, Amarillo, TX, USA for analysing our samples by mass spectrometry.
206
12 December 1977
lowed as previously, except that vessel a was filled with helium which had been “doped” deliberately with a small proportion of 3 He. After operating the secondary flush in the same way as before, samples were taken both from a and from d. Analysis*’ showed: 3 and for a, R > 2.5 X l0~; factor for d, R43 3.2 X7.8i0X i05. hence, a43concentration of at=least Making the reasonable assumption that the same cnncentration factor had been achieved during the purification run we may conclude that, for our 4He product,R 43 >2 X 1015. This comfortably fulfils the isotopic purity criterion for an experimental rcalisation of the neutron bottle proposed by Golub and Pendlebury [1]. The purity is arising also more cient to eliminate difficulties fromthan the suffitendency 3He atoms to condense [5] on the negative ion in of so that it will now be possible to extend to lower 4He, temperatures the experiments on the breakdown of superfluidity [6] in liquid 4He which provided the original motivation for developing the purification tecunique. We may note that the results of the present work are consistent with the expectation [3] that it should actually be impossible for any 3He atoms to reach the receiving vessel by diffusing against the primary heat flush. This being so, we suggest that future purification machines can with reasonable confidence be constructed to a simpler design, omitting the secondary heat flush, cone heat exchanger and associated components, and relying solely on the primary heat flush for 3He removal. Finally, we comment that the superfluid 4He collected in a during the purification procedure is arguably the purest macroscopic sample of any single isotopic species yet to have been prepared: the usual non-helium impurities found in liquid 4He, such as colloidal air, should have all been removed along with the 3He by the primary heat flush and superleak. Indeed, it seems unlikely that elemental material of greater purity can occur in bulk anywhere.
We gratefully acknowledge the valuable technical assistance of N. Bewley, G. Caley, I.G. Marsden, I. Miller and A. Muirhead.
References Ill R. Golub and J.M. Pendlebury, Phys. Lett. 62A (1977) 337.
Volume 64A, number 2
PHYSICS LETTERS
[21 P.P. Fatouros, DO. Edwards, F.M. Gasparini and S.Y. Shen, Cryogenics 15 (1975) 147. [3] M. Atkins and P.V.E. MeCintock, Cryogenics 16 (1976) 733. [4] P.V.E. McClintock, to be published.
12 December 1977
[5] D.R. Allum and P.V.E. McClintock, J. Phys. C9 (1976) L273. [6] D.R. Alium, P.V.E. McClintock, A. Phillips and R.M. Bowley, Phil. Trans. Roy. Soc. London A284 (1977) 179.
207