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Measurement of the ultra cold neutron production rate in an external liquid helium source
Volume 66A, number 6
PHYSICS LETTERS
26 June 1978
MEASUREMENT OF THE ULTRA COLD NEUTRON PRODUCTION RATE IN AN EXTERNAL LIQUID HELIUM SOURCE P. AGERON and W. MAMPE Institut Laue-Langevin, 38042 Grenoble, France
and R. GOLUB and J.M. PENDELBURY University of Sussex, Falmer Brighton BN1 9QH Sussex, UK Received 22 April 1978
Ultra Cold Neutrons have been produced by down scattering of cold neutrons (~. = 10 A) on liquid helium. The measured production rate is in agreement with the calculated value.
An Ultra Cold Neutron (UCN) source has been proposed [1] where UCN are produced by down scattering of 10 A neutrons in liquid helium at about 1 K. The helium is in a closed container, the walls of which are transparent to the incident neutrons from a cold neutron guide but totally reflecting for UCN at any angle of incidence (neutron bottle). The number p of UCN produced in one cm 3 of liquid helium, with a differential energy transfer cross section: ( d ~ / d E ) ( E ~ EUCN), placed in a cold neutron beam with a differential flux: (dC)/dE)(E), is: Elim p =j 0
d~ de d E u c N J d E d--~(E) ~ - ~ ( E - - ' E u c N ) ,
(1)
where Eli m = 0.0023 K is the maximum energy of UCN which can be stored in a stainless steel bottle. As shown in ref. [1]:
f d E ~-~. de (E) ~dZ ( E + E u c N )
(2) d¢ dE
] / /EUCN -E S" k "
(E o) 2No V--E-0-° (o),
where E 0 = 12 K, k 0 = 0.7 A -1 are the energy and the momentum transfer of the only existing state o f liquid helium with which a neutron can exchange all its ener-
gy. S(ko) = f dES(E, k) = 0.1 for k 0 = 0.7 h -1 and T ~. 1 K (S(E, k) scattering law of liquid helium), o = 1.1 × 10 -24 cm 2, bound helium atom cross section, N = 0.0218 × 1024 cm -3, atom density of liquid helium. At the exit of the cold neutron guides of the Higll Flux Reactor at Grenoble [2] : (d~b/dE)(12 K) --- 2.2 X 107 cm - 2 s -1 K -1 for a total flux (normalized at o = 2200 m/s) of~b = 4.5 X 109 cm - 2 s -1. In these conditions p = 2 UCN cm -3 s -1 . The steady state density of UCN, 19, in the bottle is the product of the production rate p and the mean life time T o f UCN in the bottle [3] : p = p × T.
(3)
The life time T depends not only on the/3 decay of the neutron, but also on the losses by absorption and up scattering in the collisions of UCN either with the wall of the bottle or with the liquid helium atoms. If these losses could be made small compared to the/3 decay (magnetic or ideal material bottles, very pure helium-4 (less than 10 -11 helium-3) at sufficiently low temperature), UCN densities up to about 1000 cm -3 could be reached which is much higher than the present densities up to now obtained by any existing UCN source (less than 1 c m - 3 ) . The aim of the experiment here reported,'was to check whether there is a production of UCN in liquid 469
Volume 66A, number 6 e'
PHYSICS LETTERS
26 June 1978
e
~.~
b
\
'
¢
lm
I
I
Fig. 1. Experimental set up. (a) cryostat, (b) electropolished stainless steel liner 1D = 4.5 cm, (c) incident cold neutron beam 13 × 5 cm 2, (d) electropolished stainless steel guide 1D = 6.7 cm, stainless steel piston in (e) closed, (e') open, position, (f) four elbows f'dter, (g) detector. helium at about 1 K according to eqs. (1) and (2). For this purpose we did not attempt to store the UCN, which were produced, but only to measure the rate of continuous flow of UCN from the liquid helium.
The experimental set up (fig. 1). The tail of a stan-
T
I Helium level 50 _i [era]
A
Helium Temperott re [K] counting rote [s-1
ii
4o
I'll
\
\
20 I.
~
\\\
I
\
\
\ I
I
15
18
I 2
24
\
1Time [hi
3
6
9
Fig. 2. Counting rate, (A) open position, (B) closed position (C) liquid helium level above the bottom of the cryostat, (D) helium temperature, as a function of time. 470
dard cryostat (a) internally lined with an electropolished stainless steel sheet (b) is placed in a cold neutron beam (c). The cryostat had been filled with commercial liquid helium which was pumped by a ROOTS type vacuum pump down to a temperature of 1.15 K. The upper part of the cryostat was connected, at right angle, to a neutron guide tube (d) where a piston allowed the introduction (closed position) or the removal (open position) of a 0.5 mm thick stainless steel sheet (e) which is totally reflecting for UCN and partly transparent to higher energy neutrons. Furthermore four 90 ° elbows ( 0 fffltered the UCN on their way to the detector (g).
The experimental remits (fig. 2). During the time from the filling to the complete emptying of the cryostat, the counting rates both with the piston in "open" (curve A) or "closed" (curve B) position, were recorded as well as the level of the liquid helium above the bottom of the cryostat (curve C) and the pressure, thus the saturation temperature of the helium (curve D). The counting rate raised when the pressure of helium decreased below 1 0 0 - 5 0 Torr (helium temperature around 2.5 K), then it remained stable (4.8 c s -1 piston open, 1.3 piston closed) when the pressure decreased slowly from 1.3 to 0.45 Torr (temperature from 1.3 to 1.15 K) and finally fell down to back-
Volume 66A, number 6
PHYSICS LETTERS
ground level (0.5 c s - 1 ) when the helium level came down below the upper level o f the incident beam. A flux map of the beam has been made at the position of the liquid helium, by gold foil activation, which has allowed to determine the mean flux (normalized at v = 2 2 0 0 m s - 1 ) to be ~' = 3.5 × 108 cm - 2 s -1 over a volume o f helium V = 207 cm 3. The transmission t~ of the line between the cryostat and the detector has been measured using the UCN source of ILL [4]. A bottle was filled by this source during 5 s and, after a storage time of 2 s, emptied into a detector during 13 s, either directly or through the line. A UCN transmission factor of 0.17 has been measured.
Discussion. The expected counting rate of such a set up can be calculated by using eqs. (1) and (2) and the experimental values for the incident flux and the transmission o f the line, with, in addition: fD = 0.8 estimated efficiency of the detector, t w = 0.9 calculated transmission o f the walls of the cryostat for incident 12 K neutrons. The losses by up scattering on the gas are calculated to be negligible and the losses by absorption in helium3 may be neglected because the counting rate was practically independent o f the height of liquid helium in the path:
26 June 1978
Elim
c= f
deucN
q~' d~b
(eo) 2vo
o ! / ~EUCN X V--~O S(ko)t~twf D = 4.4 c S-1. Considering the uncertainties in the transmission factors the experimental value o f 3.5 c s -1 is in reasonable agreement with the calculated one. The successful operation o f this type of UCN source requires, in addition, that the contribution to the storage time of the UCN up scattering in the liquid helium be sufficiently small as discussed in ref. [1]. An experiment to check this is now in preparation.
References [1] R. Golub and J.M. Pendelbury, Phys. Lett. 62A (1977) 337. [2] Neutron beam facilities at the HFR available for users, Institut Laue-Langevin Scientific Secretary Office (1977). [3] R. Golub and J.M. Pendelbury, Phys. Lett. 53A (1975) 133. [4] P. Ageron, M. Hetzelt, W. Mampe, R. Golub, J.M. Pendelbury, K. Smith and J. Robson, Intern. Syrup. on Net]tron inelastic scattering IAEA-SM-219/58 (Vienna, 1977).
47!
PHYSICS LETTERS
26 June 1978
MEASUREMENT OF THE ULTRA COLD NEUTRON PRODUCTION RATE IN AN EXTERNAL LIQUID HELIUM SOURCE P. AGERON and W. MAMPE Institut Laue-Langevin, 38042 Grenoble, France
and R. GOLUB and J.M. PENDELBURY University of Sussex, Falmer Brighton BN1 9QH Sussex, UK Received 22 April 1978
Ultra Cold Neutrons have been produced by down scattering of cold neutrons (~. = 10 A) on liquid helium. The measured production rate is in agreement with the calculated value.
An Ultra Cold Neutron (UCN) source has been proposed [1] where UCN are produced by down scattering of 10 A neutrons in liquid helium at about 1 K. The helium is in a closed container, the walls of which are transparent to the incident neutrons from a cold neutron guide but totally reflecting for UCN at any angle of incidence (neutron bottle). The number p of UCN produced in one cm 3 of liquid helium, with a differential energy transfer cross section: ( d ~ / d E ) ( E ~ EUCN), placed in a cold neutron beam with a differential flux: (dC)/dE)(E), is: Elim p =j 0
d~ de d E u c N J d E d--~(E) ~ - ~ ( E - - ' E u c N ) ,
(1)
where Eli m = 0.0023 K is the maximum energy of UCN which can be stored in a stainless steel bottle. As shown in ref. [1]:
f d E ~-~. de (E) ~dZ ( E + E u c N )
(2) d¢ dE
] / /EUCN -E S" k "
(E o) 2No V--E-0-° (o),
where E 0 = 12 K, k 0 = 0.7 A -1 are the energy and the momentum transfer of the only existing state o f liquid helium with which a neutron can exchange all its ener-
gy. S(ko) = f dES(E, k) = 0.1 for k 0 = 0.7 h -1 and T ~. 1 K (S(E, k) scattering law of liquid helium), o = 1.1 × 10 -24 cm 2, bound helium atom cross section, N = 0.0218 × 1024 cm -3, atom density of liquid helium. At the exit of the cold neutron guides of the Higll Flux Reactor at Grenoble [2] : (d~b/dE)(12 K) --- 2.2 X 107 cm - 2 s -1 K -1 for a total flux (normalized at o = 2200 m/s) of~b = 4.5 X 109 cm - 2 s -1. In these conditions p = 2 UCN cm -3 s -1 . The steady state density of UCN, 19, in the bottle is the product of the production rate p and the mean life time T o f UCN in the bottle [3] : p = p × T.
(3)
The life time T depends not only on the/3 decay of the neutron, but also on the losses by absorption and up scattering in the collisions of UCN either with the wall of the bottle or with the liquid helium atoms. If these losses could be made small compared to the/3 decay (magnetic or ideal material bottles, very pure helium-4 (less than 10 -11 helium-3) at sufficiently low temperature), UCN densities up to about 1000 cm -3 could be reached which is much higher than the present densities up to now obtained by any existing UCN source (less than 1 c m - 3 ) . The aim of the experiment here reported,'was to check whether there is a production of UCN in liquid 469
Volume 66A, number 6 e'
PHYSICS LETTERS
26 June 1978
e
~.~
b
\
'
¢
lm
I
I
Fig. 1. Experimental set up. (a) cryostat, (b) electropolished stainless steel liner 1D = 4.5 cm, (c) incident cold neutron beam 13 × 5 cm 2, (d) electropolished stainless steel guide 1D = 6.7 cm, stainless steel piston in (e) closed, (e') open, position, (f) four elbows f'dter, (g) detector. helium at about 1 K according to eqs. (1) and (2). For this purpose we did not attempt to store the UCN, which were produced, but only to measure the rate of continuous flow of UCN from the liquid helium.
The experimental set up (fig. 1). The tail of a stan-
T
I Helium level 50 _i [era]
A
Helium Temperott re [K] counting rote [s-1
ii
4o
I'll
\
\
20 I.
~
\\\
I
\
\
\ I
I
15
18
I 2
24
\
1Time [hi
3
6
9
Fig. 2. Counting rate, (A) open position, (B) closed position (C) liquid helium level above the bottom of the cryostat, (D) helium temperature, as a function of time. 470
dard cryostat (a) internally lined with an electropolished stainless steel sheet (b) is placed in a cold neutron beam (c). The cryostat had been filled with commercial liquid helium which was pumped by a ROOTS type vacuum pump down to a temperature of 1.15 K. The upper part of the cryostat was connected, at right angle, to a neutron guide tube (d) where a piston allowed the introduction (closed position) or the removal (open position) of a 0.5 mm thick stainless steel sheet (e) which is totally reflecting for UCN and partly transparent to higher energy neutrons. Furthermore four 90 ° elbows ( 0 fffltered the UCN on their way to the detector (g).
The experimental remits (fig. 2). During the time from the filling to the complete emptying of the cryostat, the counting rates both with the piston in "open" (curve A) or "closed" (curve B) position, were recorded as well as the level of the liquid helium above the bottom of the cryostat (curve C) and the pressure, thus the saturation temperature of the helium (curve D). The counting rate raised when the pressure of helium decreased below 1 0 0 - 5 0 Torr (helium temperature around 2.5 K), then it remained stable (4.8 c s -1 piston open, 1.3 piston closed) when the pressure decreased slowly from 1.3 to 0.45 Torr (temperature from 1.3 to 1.15 K) and finally fell down to back-
Volume 66A, number 6
PHYSICS LETTERS
ground level (0.5 c s - 1 ) when the helium level came down below the upper level o f the incident beam. A flux map of the beam has been made at the position of the liquid helium, by gold foil activation, which has allowed to determine the mean flux (normalized at v = 2 2 0 0 m s - 1 ) to be ~' = 3.5 × 108 cm - 2 s -1 over a volume o f helium V = 207 cm 3. The transmission t~ of the line between the cryostat and the detector has been measured using the UCN source of ILL [4]. A bottle was filled by this source during 5 s and, after a storage time of 2 s, emptied into a detector during 13 s, either directly or through the line. A UCN transmission factor of 0.17 has been measured.
Discussion. The expected counting rate of such a set up can be calculated by using eqs. (1) and (2) and the experimental values for the incident flux and the transmission o f the line, with, in addition: fD = 0.8 estimated efficiency of the detector, t w = 0.9 calculated transmission o f the walls of the cryostat for incident 12 K neutrons. The losses by up scattering on the gas are calculated to be negligible and the losses by absorption in helium3 may be neglected because the counting rate was practically independent o f the height of liquid helium in the path:
26 June 1978
Elim
c= f
deucN
q~' d~b
(eo) 2vo
o ! / ~EUCN X V--~O S(ko)t~twf D = 4.4 c S-1. Considering the uncertainties in the transmission factors the experimental value o f 3.5 c s -1 is in reasonable agreement with the calculated one. The successful operation o f this type of UCN source requires, in addition, that the contribution to the storage time of the UCN up scattering in the liquid helium be sufficiently small as discussed in ref. [1]. An experiment to check this is now in preparation.
References [1] R. Golub and J.M. Pendelbury, Phys. Lett. 62A (1977) 337. [2] Neutron beam facilities at the HFR available for users, Institut Laue-Langevin Scientific Secretary Office (1977). [3] R. Golub and J.M. Pendelbury, Phys. Lett. 53A (1975) 133. [4] P. Ageron, M. Hetzelt, W. Mampe, R. Golub, J.M. Pendelbury, K. Smith and J. Robson, Intern. Syrup. on Net]tron inelastic scattering IAEA-SM-219/58 (Vienna, 1977).
47!