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Observation of the storage of ultra-cold neutrons in liquid helium
Physica 107B(1981)587-588 North-Holland Publishing Company
JF 1
OB.SERVATiON OF THE STORAGE OF ULTRA-COLD NEUTRONS IN bIQUID HELIUM
C.Jewell §, B.Heckel §, P.Ageron §, R.Golub §§, ~.Mampe?P.V.E.McClinteck §§§ §Institut Laue-Langevin 38042 Grenoble,FRANCE.
§§Physics Dept. Tech. Univ. Munch~n 8046 Garchlng, DBR.
§§§Physics Dept. Univ. of Lancaster Lancaster,ENGLAND
We report the first observations of the storage of Ultra-Cold neutrons (UCN) in liquid Helium at temperatures approaching IK. Vhile the experimental results are such as to preclude a detailed comparison with theory we have demonstrated that UCN are actually stored in liquid Helium and our results are consistent, with theoretical expectations. Introduction Ultra-Cold neutrons (UCN)~are neutrons with energies so low (~10"ev,lO"K) that they suffer total reflection from many materials at any angle of incidence and hence can be stored in material bottles for significant periods of time,(l,2) behaving in many ways like a very dilute gas. UCN have been produced by extraction of the low energy tail of the Maxwell distribution from nuclear reactors, and by slowing down faster neutrons by reflection from moving mirrors (3) and crystals (4). UCN have also been produced by downscatterlng I mev neutrons in liquid Helium (5). This latter method offers the possibility of producing UCN densities significantly greater than can be expected on the basis of Liouville's theorem (6,7), which applies to all other types of UCN source, if the interaction rates of the UCN with the liquid H~lium are close to those predicted theoretically (8). We report here the results of an experiment in ~hlch we measured the storage time of UCN in a vessel containing liquid Helium at temperatures slightly higher than iK.
transmission of thls UC~ transport system so we cannot make a direct comparison between the production rate observed here and theory (7) as we did earlier, (5). The total neutron (capture) flux (normallsed at 2200 m/see) was measured to be 3xI0 q neutrons/cm~/sec over a beam area of 3x5 cm* . The steady state counting rate of the detector with the neutron beam on (shutter open) and the UCN valve open, is a measure of the production rate modified by the system transmission snd detection efficiency. In most of the runs this was measured to be between 3 and 3.5 counts per sec. The helium was cooled by pumping on the cryostat with a Roots pump through a 5 cm ID approxlmately 5 m long pumping line which passed through a liquid Nitrogen cooled trap. The temperature was measured by means of a Carbon resistor. Measurements were made by exposing the UCN container to a beam containing neutrons with energies of 1 mev.After a suitable equilibration time (20 sees)the beam was shut off by closing the beam shutter.After a variable delay the UCN valve was opened and the total number of neutrons reaching the d e t e c t o r was recorded.
Description of experiment.
Discussion of results.
The UCN were produced in a cylindrical shaped liquid Helium filled chamber of 316 Stainless Steel, having a ID of 4.4 cm and height 28 cm installed in the tail of a cryostat. The chamber was fitted with a cold UCN valve which permitted the UCN to be held in the Helium filled chamber for variable periods of time after the incident beam was shut off by means of a beam shutter. After exitting the Helium filled storage chamber, the UCN rise about 90 cm. They then enter a stainless steel horizontal transport pipe (6.7 cm I/l, 2 m long) which transports them over a concrete radiation shield. The UCN then pass through a 90 ° bend which directs them downward to a He 3 containing UCN detector which is located 1 meter below the horizontal transport pipe. We have not measured the
Fig. I shows the results of two typical runs. Each plotted point represents the average of 9 (Run A) or 12 (Run B) individual measurements. In these runs the background was negligible. We have neglected the uncertainties in the time of operation of the valves (~ I/4 see). Although we possess a Helium purifier constructed by two of us ~C.J. and P.E.V. McC) which removes the He" from the liquid by means of the heat flush technique (9), runs carried out using "purified" Helium ~ always gave storage times somewhat less than those achieved when the cryostat was filled with "impure" liquid Helium. This is because the transfer from the purifier to the experimental cryostat required the cryostat to be moved and this resulted in some additional contamination. Thus we
0378.4363/81/0000-0000/$02.50
© North-Holland Publishing Company
587
588
do not really know the He ~ concentration in our experimental cryostat. However, measurements (I0) have shown that one expects He ~ /He" ratios on the order of 1 in i0 ~ after substantial pumping, although both larger and smaller values were reported. We assume this value in suOsequent analysis.
!
TABLE I. ~wail-s 35
I
Run A B
T-K I 1.19-1.20 1.12-1.17
20 L
19-3o
35
~ 3o
T=ug-I.Z0K
'10
=,
-
0
RUN B
T =1.12-1.17K -~=9.1".3sec 2; 2 I
& I
6 I
8 I
10 |
12 I
|~total s I |~theory" i~expss | 7.8-8.4 I 7.75-I
I ~.7-1o.6 19.1 ~
No/PT" .65Z. 15
.4o±.1
P ~ (6) where P is the production rate times system efficiency measured as described above. The ratio of these quantitP s is seen to be less than I because in the measurement of P we are sensitive to neutrons with higher energies than in the storage experiment. In addition, the energy dependence of the system transmission is sensitive to contamination of the low temperature psrt of the UCN transpQrt system by condensed gases. If the He 3 content was greater than we assumed above, the storage time due to He ~ interactions would be increased somewhat. On the other hand, if there were no He 3 present, the He 4 storage time deduced from the data would be I0 seconds (Run A) and 12 seconds (Run B), which values are within a factor of 2-3 of the theory. Thus we have demonstrated that UCN are, indeed, stored in liquid Helium for times which are consistent with theoretical predictions, although a detailed comp~rlson of theory with experiment will have to wait for a larger apparatus which can operate at lower temperatures (--0.6K). Such an apparatus is now under construction. We would like to thank A.Beynet,D.Brochier, A.RambQud and S. Pujol for technical assistance. References.
RUN A
I
would decrease the storage time due to this process, thus increasing the storage time deduced for the He ~ scattering. In ?able I we list the contributions to the expected storage time and compare this with the experimental results. The detected number of UCN extrapolated to zerc delay time (No) should be given by
1/, I
16 I
Time spent by UCN in Helium chamber- sec. Fig. I. Number of UCN reaching the detector as a function of residence time in the Helium filled chamber. In addition to absorption by He 5 and upscattering (8) by Helium ~ (this latter is the major interest of this work) UCN can also be lost as the result of wall interactions and leakage through cracks and around the valve. (We neglect this latter effect). For pure 316 stainless steel we calculate a storage time of 35 seconds due to the absorption of the elements in the walls. If the walls were covered with a contaminant, this
l.R.Golub and J.M.Pendlebury, Rep. Prog in Phys 42, 439 (1979). 2.A.St'eyerl,Springer Tracts in Mod. Phys. 80, 57, (1977). ~.A.Steyerl, NIM/25,461 (1975) 4.T.Brun et al, Phys.Letts.75A.223 (1980) 5.P.Ageron et al, Phys. Letts.66A,469,(1978 6.R.Oolub and J.M.Pendlebury,Phys Letts .Golub and J.M.Pendlebury, Phys.Letts. 62A,337 (1977). ~q-R.Golub,Phys.Letts.72A,387(1979) 9.P.V.E. McClintock, Cryogenics 18,201(1978 I0. P.C.Tully, U.S. Bureau of Mines, Report of Investigations 805~,~;ashington D.C. (1975).
JF 1
OB.SERVATiON OF THE STORAGE OF ULTRA-COLD NEUTRONS IN bIQUID HELIUM
C.Jewell §, B.Heckel §, P.Ageron §, R.Golub §§, ~.Mampe?P.V.E.McClinteck §§§ §Institut Laue-Langevin 38042 Grenoble,FRANCE.
§§Physics Dept. Tech. Univ. Munch~n 8046 Garchlng, DBR.
§§§Physics Dept. Univ. of Lancaster Lancaster,ENGLAND
We report the first observations of the storage of Ultra-Cold neutrons (UCN) in liquid Helium at temperatures approaching IK. Vhile the experimental results are such as to preclude a detailed comparison with theory we have demonstrated that UCN are actually stored in liquid Helium and our results are consistent, with theoretical expectations. Introduction Ultra-Cold neutrons (UCN)~are neutrons with energies so low (~10"ev,lO"K) that they suffer total reflection from many materials at any angle of incidence and hence can be stored in material bottles for significant periods of time,(l,2) behaving in many ways like a very dilute gas. UCN have been produced by extraction of the low energy tail of the Maxwell distribution from nuclear reactors, and by slowing down faster neutrons by reflection from moving mirrors (3) and crystals (4). UCN have also been produced by downscatterlng I mev neutrons in liquid Helium (5). This latter method offers the possibility of producing UCN densities significantly greater than can be expected on the basis of Liouville's theorem (6,7), which applies to all other types of UCN source, if the interaction rates of the UCN with the liquid H~lium are close to those predicted theoretically (8). We report here the results of an experiment in ~hlch we measured the storage time of UCN in a vessel containing liquid Helium at temperatures slightly higher than iK.
transmission of thls UC~ transport system so we cannot make a direct comparison between the production rate observed here and theory (7) as we did earlier, (5). The total neutron (capture) flux (normallsed at 2200 m/see) was measured to be 3xI0 q neutrons/cm~/sec over a beam area of 3x5 cm* . The steady state counting rate of the detector with the neutron beam on (shutter open) and the UCN valve open, is a measure of the production rate modified by the system transmission snd detection efficiency. In most of the runs this was measured to be between 3 and 3.5 counts per sec. The helium was cooled by pumping on the cryostat with a Roots pump through a 5 cm ID approxlmately 5 m long pumping line which passed through a liquid Nitrogen cooled trap. The temperature was measured by means of a Carbon resistor. Measurements were made by exposing the UCN container to a beam containing neutrons with energies of 1 mev.After a suitable equilibration time (20 sees)the beam was shut off by closing the beam shutter.After a variable delay the UCN valve was opened and the total number of neutrons reaching the d e t e c t o r was recorded.
Description of experiment.
Discussion of results.
The UCN were produced in a cylindrical shaped liquid Helium filled chamber of 316 Stainless Steel, having a ID of 4.4 cm and height 28 cm installed in the tail of a cryostat. The chamber was fitted with a cold UCN valve which permitted the UCN to be held in the Helium filled chamber for variable periods of time after the incident beam was shut off by means of a beam shutter. After exitting the Helium filled storage chamber, the UCN rise about 90 cm. They then enter a stainless steel horizontal transport pipe (6.7 cm I/l, 2 m long) which transports them over a concrete radiation shield. The UCN then pass through a 90 ° bend which directs them downward to a He 3 containing UCN detector which is located 1 meter below the horizontal transport pipe. We have not measured the
Fig. I shows the results of two typical runs. Each plotted point represents the average of 9 (Run A) or 12 (Run B) individual measurements. In these runs the background was negligible. We have neglected the uncertainties in the time of operation of the valves (~ I/4 see). Although we possess a Helium purifier constructed by two of us ~C.J. and P.E.V. McC) which removes the He" from the liquid by means of the heat flush technique (9), runs carried out using "purified" Helium ~ always gave storage times somewhat less than those achieved when the cryostat was filled with "impure" liquid Helium. This is because the transfer from the purifier to the experimental cryostat required the cryostat to be moved and this resulted in some additional contamination. Thus we
0378.4363/81/0000-0000/$02.50
© North-Holland Publishing Company
587
588
do not really know the He ~ concentration in our experimental cryostat. However, measurements (I0) have shown that one expects He ~ /He" ratios on the order of 1 in i0 ~ after substantial pumping, although both larger and smaller values were reported. We assume this value in suOsequent analysis.
!
TABLE I. ~wail-s 35
I
Run A B
T-K I 1.19-1.20 1.12-1.17
20 L
19-3o
35
~ 3o
T=ug-I.Z0K
'10
=,
-
0
RUN B
T =1.12-1.17K -~=9.1".3sec 2; 2 I
& I
6 I
8 I
10 |
12 I
|~total s I |~theory" i~expss | 7.8-8.4 I 7.75-I
I ~.7-1o.6 19.1 ~
No/PT" .65Z. 15
.4o±.1
P ~ (6) where P is the production rate times system efficiency measured as described above. The ratio of these quantitP s is seen to be less than I because in the measurement of P we are sensitive to neutrons with higher energies than in the storage experiment. In addition, the energy dependence of the system transmission is sensitive to contamination of the low temperature psrt of the UCN transpQrt system by condensed gases. If the He 3 content was greater than we assumed above, the storage time due to He ~ interactions would be increased somewhat. On the other hand, if there were no He 3 present, the He 4 storage time deduced from the data would be I0 seconds (Run A) and 12 seconds (Run B), which values are within a factor of 2-3 of the theory. Thus we have demonstrated that UCN are, indeed, stored in liquid Helium for times which are consistent with theoretical predictions, although a detailed comp~rlson of theory with experiment will have to wait for a larger apparatus which can operate at lower temperatures (--0.6K). Such an apparatus is now under construction. We would like to thank A.Beynet,D.Brochier, A.RambQud and S. Pujol for technical assistance. References.
RUN A
I
would decrease the storage time due to this process, thus increasing the storage time deduced for the He ~ scattering. In ?able I we list the contributions to the expected storage time and compare this with the experimental results. The detected number of UCN extrapolated to zerc delay time (No) should be given by
1/, I
16 I
Time spent by UCN in Helium chamber- sec. Fig. I. Number of UCN reaching the detector as a function of residence time in the Helium filled chamber. In addition to absorption by He 5 and upscattering (8) by Helium ~ (this latter is the major interest of this work) UCN can also be lost as the result of wall interactions and leakage through cracks and around the valve. (We neglect this latter effect). For pure 316 stainless steel we calculate a storage time of 35 seconds due to the absorption of the elements in the walls. If the walls were covered with a contaminant, this
l.R.Golub and J.M.Pendlebury, Rep. Prog in Phys 42, 439 (1979). 2.A.St'eyerl,Springer Tracts in Mod. Phys. 80, 57, (1977). ~.A.Steyerl, NIM/25,461 (1975) 4.T.Brun et al, Phys.Letts.75A.223 (1980) 5.P.Ageron et al, Phys. Letts.66A,469,(1978 6.R.Oolub and J.M.Pendlebury,Phys Letts .Golub and J.M.Pendlebury, Phys.Letts. 62A,337 (1977). ~q-R.Golub,Phys.Letts.72A,387(1979) 9.P.V.E. McClintock, Cryogenics 18,201(1978 I0. P.C.Tully, U.S. Bureau of Mines, Report of Investigations 805~,~;ashington D.C. (1975).