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Free neutron oscillations
Nuclear Instruments and Methods in Physics Research A284 (1989) 9-12 North-Holland, Amsterdam
FREE NEUTRON OSCILLATIONS Gabriele PUGLIERIN Istituto Nazionale di Fisica Nucleare, Sezione di Padova, Padova, Italy
The status of the free neutron oscillation search is described and the new experiment at the ILL reactor in Grenoble aiming at investigating the neutron-antineutron oscillation time up to 10 8 s is discussed. 1. Introduction The study of the free neutron oscillations is the most direct way to look for a A B = 2 baryon number violating interaction [1], as foreseen for instance by many grand unification theories. Experiments can be performed with very high sensitivity, comparable to or even better than the present limits [2] derived from the experiments searching for matter instability deep underground. Free neutron oscillations can only be observed when the intensity of the external fields is strongly reduced. If i1 E is the energy difference between the neutron and antineutron due to these fields, the free neutron oscillation condition requires that AE - t << 1,
where t is the observation time for the oscillation. In fact, the general expression for the probability to grow an antineutron component from an initially pure neutron state: Pn(t)
Sm 2 DEZ+Sm2sin2(VAE2+8m, t),
(2)
under condition (1) reduced to the field free oscillation case (3) n( t ) = ( sm ' t ) 2= ( t /Tnn) 2, where 8m is the AB = 2 mixing mass for neutron to antineutron and rn ~i is the neutron-antineutron oscillation time . The experiments can be performed by propagating a neutron beam under vacuum, over a long distance, in a region shielded from external magnetic fields and looking for an antineutron component at the end. According to expression (3), in order to get the maximum sensitivity, the intensity of the neutron beam has to be very high, the drift length very long, the neutron velocity very small and the antineutron detection efficiency very high . Up to now two experiments have been completed, one at the ILL reactor in Grenoble using a cold neutron P
0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
beam from a beam guide [3], and a second one in Pavia with a thermal neutron beam [4]. The present experimental limit T.I, > 10 6 s [3] is expected to be improved by two orders of magnitude with the new experiment under way at the Institute Laue-Langevin at Grenoble, which will be described in detail in the following. A project aiming at higher sensitivity is planned at the Moscow Meson Factory later in the 1990s [5]. 2. The new Grenoble experiment This experiment is performed by a Heidelberg, ILL, Padova and Pavia collaboration. It takes advantage of the installation of the second neutron cold source at the ILL reactor. Neutrons are extracted from the reactor at the temperature of liquid deuterium and transported by a totally reflecting neutron guide 12 x 6 cm2 in cross section down to the experimental hall . The guide, 60 m long, is bent in such a way as to eliminate all fast neutrons and gammas coming directly from the reactor. The resulting beam is a clean and cold neutron beam, whose total intensity is 2 x 10' 1 n/s; its wavelength profile is shown in fig. 1. The experimental set-up is schematically shown in fig. 2. At the exit of the bent guide, the neutrons enter a 95 m long drift vessel, where a vacuum < 10 -5 Torr can be kept . The first part of the vessel is a 80 m long tube of 1.2 m diameter. In the first part of this tube neutrons are propagated through a special neutron guide, whose reflecting walls make an angle of 3 mrad with the beam axis . The guide is made of 32 pieces, each 1 m long . It acts as a collimator since at each reflection on the walls the neutron divergency is reduced and the neutron trajectory is made more parallel to the beam axis . Thus neutrons can be freely propagated over a long distance inside the tube. A passive magnetic shield, made of a 75 m long and 1 m diameter [t-metal tube, is installed inside the first part of the drift vessel and also surrounds the guide. I. NEW PHENOMENA
10
G Pugherin / Free neutron oscillations
v
U C
v)
r
E . t9 E
Fig 1 Neutron energy spectrum of the new cold neutron beam at the ILL reactor in Grenoble .
Together with the active shield placed outside, it reduces the magnetic field all along the propagation path to the level B < 20 nT as required by the free neutron oscillation condition. The effective oscillation time, evaluated from the beam energy spectrum and the divergency distributions starting for each neutron from the last reflection on the guide till the exit of the shielded region, is (t2)'/2=0.1 s. The antineutron component present in the beam is detected by letting the beam pass through a 200 gm thick carbon foil, enough to annihilate the antineutron component and almost transparent to the neutron beam .
The beam is subsequently absorbed into a 6 LiF beam dump at a 10 m distance from the target . The target is placed inside the second part of the drift vessel which consists of an AI tube, of 1 .4 m diameter and 5 m long, followed by a 8 m long inox tube of the same diameter . The axis of the second part of the drift vessel is 2 cm lower than the axis of the first part to take into account the gravity effect on neutrons . For the same reason the axis of the beam at the entrance of the drift vessel is 7 cm higher than the axis of the drift tube . With such an arrangement the target, which has a diameter of 1.1 m, contains more than 99% of the beam . Antineutron annihilations on the target are detected by searching for the annihilation products, on the average 5 pions, in the detector placed around the beam tube and with the target in the middle. The detector, fig. 3, is a tracking device organized in four quadrants. Each quadrant consists of 10 planes of limited streamer tubes (LST) supported by an Al honeycombe structure, (p) = 0.3 g cm -3 , so that passing tracks are well measured and the event vertex on the target can be reconstructed with a precision which has been estimated by Monte Carlo calculation ±4 cm on the average for each coordinate. Each streamer tube has a cross-section of 0.9 x 0.9 cm2 and is 5 m long . Two layers of the 2 cm thick scintillator counters are placed respectively before and after the LST planes . They provide the first level trigger and can distinguish by time of flight the particle direction . Immediately behind the second layer of the scintillators there are 6 + 6 planes of LST interspaced with, respectively, 1 and 2 mm thick Pb plates supported by two 2.5 mm thick Al plates . This structure, - 4 radiation lengths, allows the detection of y's from iT ° 's and provides a measurement of their direction as well as an estimate of the total energy of the event, expected to be - 2 MN , while the total momentum should be - 0. A veto system for the cosmic rays is realized with scintillator counters placed all around the detector and
H53 neutron
beam dump
beam
Fig. 2. The experimental set-up at the ILL in Grenoble for neutron-antineutron oscillation study.
G. Pugherin / Free neutron oscillations
02~0 4300 Oi4B
Fig. 3 . Cross-sectional view of the detector . separated from it by 10 cm of lead in order to avoid the possibility of an annihilation product autovetoing the event. Scintillators are also placed below the platform supporting the detector and are used as the veto of neutral cosmic rays interacting inside the detector . All the scintillator counters are equipped with two phototubes, whose signals are processed by specially designed mean timer, in order to keep to a minimum the signal width and the dead time of the experiment . The trigger of the experiment can be done in many different ways : from the simple coincidence between the inner and outer scintillators layer of one quadrant, up to several majority conditions for LST planes firing at the same time in the inner as well in the outer part of the detector, each trigger corresponding to different levels of antineutron detection efficiency and background rejection. It has been evaluated by Monte Carlo calculation that by requiring as a trigger the coincidence of the two
scintillator layers of a quadrant with at least 4 out of S planes of LST firing at the same time, in the same quadrant, the antineutron detection efficiency is 0.77. At the moment this trigger is not activated since the level of radiation around the target is still rather high, however a trigger with the coincidence of the four layers of scintillator counters of two quadrants has been installed and provides a 45% antineutron annihilation detection efficiency . Data have been collected with this trigger for a time largely sufficient for investigating the oscillation up to Tnn > 10 7 S. The trigger rate is 1 .5 events s -1 , mostly due to passing cosmic rays. These are eliminated by software in the off-line analysis. Having lined the inner wall of the drift tube around the target with a 3 mm thick layer of 6 LiF all along the detector length, the noise in the detector from the scattered neutrons on the target and from the beam in general is such that the recorded I . NEW PHENOMENA
12
G. Pughenn / Free neutron oscillations RUN
#
EVENT # 8
513 #
18331
Hits Cluster .
267 100
#Hits/#Cluster # Scintillatorr
2
7
7
IL
2
Sumt
8 .37(nsec)
OL OL
4 5
Sumt Sumt
14 .75 ( nsec) 16 .67(nsec)
IU IU
5 6
Sumt sumt
-4 .22(nsec) 6 .00(nsec)
sumt sumt
10 .12(nsec) 7 .17(nsec)
ID 4 OD 5
Fig. 4. Display of one recorded event with the beam on . The three projections are shown and the corrected crossing time for the scintillator counters is reported (I, O for inner, outer sector ; L, R, U, D for left, right, up, down). events are very clear, and selection criteria can easily be applied in the off-line analysis . Fig. 4 shows one event
present conditions thus result can be reached within - 400 days of running time.
the target . These events are essentially due to the neutral cosmic rays interacting in the detector. They arrive
References
as it appears when the beam is on . What is left after the computer selection is a sample of events to be scanned to search for annihilations on
at a rate of 0.2 Hz and are rejected since they either do not have a vertex on the target or do not satisfy the energy and momentum requirements, or do not satisfy the time of flight analysis. The real background is due to those cosmic ray
events interacting on the target. It can be estimated from the number of events with the vertex on the drift tube wall. This evaluation is under way and results should come quite soon, so that in a short time the neutron-antineutron oscillation will be probed first at the level
Tnn > 10 7 s. Improvement of the apparatus is in progress in view of the final goal : Tnn >_ 10 8 s; however already under the
R.N. Mohapatra, these Proceedings (Int . Workshop on Fundamental Physics with Slow Neutrons, Grenoble, France, 1989) Nucl . Instr. and Meth. A284 (1989) 1. P.K. Kabir and J.V. Noble, University of Virginia, Preprint, 1989 ; P.K. Kabir, Phys . Rev. Lett . 51 (1983) 231. [2] J.M. Richard, presented at this Workshop (Int. Workshop on Fundamental Physics with Slow Neutrons, Grenoble, France, 1989). [3] G. Fidecaro et al ., Phys . Lett . 156B (1985) 122. [4] G. Bressi et al., Z. Phys . (1989) in press. [5] V.A . Kuz'min, Proc. Int. Coil . Matter non Conservation, Frascati 1983, eds. E. Bellotti and S. Stipcich ; V. Lobashev, private communication.
FREE NEUTRON OSCILLATIONS Gabriele PUGLIERIN Istituto Nazionale di Fisica Nucleare, Sezione di Padova, Padova, Italy
The status of the free neutron oscillation search is described and the new experiment at the ILL reactor in Grenoble aiming at investigating the neutron-antineutron oscillation time up to 10 8 s is discussed. 1. Introduction The study of the free neutron oscillations is the most direct way to look for a A B = 2 baryon number violating interaction [1], as foreseen for instance by many grand unification theories. Experiments can be performed with very high sensitivity, comparable to or even better than the present limits [2] derived from the experiments searching for matter instability deep underground. Free neutron oscillations can only be observed when the intensity of the external fields is strongly reduced. If i1 E is the energy difference between the neutron and antineutron due to these fields, the free neutron oscillation condition requires that AE - t << 1,
where t is the observation time for the oscillation. In fact, the general expression for the probability to grow an antineutron component from an initially pure neutron state: Pn(t)
Sm 2 DEZ+Sm2sin2(VAE2+8m, t),
(2)
under condition (1) reduced to the field free oscillation case (3) n( t ) = ( sm ' t ) 2= ( t /Tnn) 2, where 8m is the AB = 2 mixing mass for neutron to antineutron and rn ~i is the neutron-antineutron oscillation time . The experiments can be performed by propagating a neutron beam under vacuum, over a long distance, in a region shielded from external magnetic fields and looking for an antineutron component at the end. According to expression (3), in order to get the maximum sensitivity, the intensity of the neutron beam has to be very high, the drift length very long, the neutron velocity very small and the antineutron detection efficiency very high . Up to now two experiments have been completed, one at the ILL reactor in Grenoble using a cold neutron P
0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
beam from a beam guide [3], and a second one in Pavia with a thermal neutron beam [4]. The present experimental limit T.I, > 10 6 s [3] is expected to be improved by two orders of magnitude with the new experiment under way at the Institute Laue-Langevin at Grenoble, which will be described in detail in the following. A project aiming at higher sensitivity is planned at the Moscow Meson Factory later in the 1990s [5]. 2. The new Grenoble experiment This experiment is performed by a Heidelberg, ILL, Padova and Pavia collaboration. It takes advantage of the installation of the second neutron cold source at the ILL reactor. Neutrons are extracted from the reactor at the temperature of liquid deuterium and transported by a totally reflecting neutron guide 12 x 6 cm2 in cross section down to the experimental hall . The guide, 60 m long, is bent in such a way as to eliminate all fast neutrons and gammas coming directly from the reactor. The resulting beam is a clean and cold neutron beam, whose total intensity is 2 x 10' 1 n/s; its wavelength profile is shown in fig. 1. The experimental set-up is schematically shown in fig. 2. At the exit of the bent guide, the neutrons enter a 95 m long drift vessel, where a vacuum < 10 -5 Torr can be kept . The first part of the vessel is a 80 m long tube of 1.2 m diameter. In the first part of this tube neutrons are propagated through a special neutron guide, whose reflecting walls make an angle of 3 mrad with the beam axis . The guide is made of 32 pieces, each 1 m long . It acts as a collimator since at each reflection on the walls the neutron divergency is reduced and the neutron trajectory is made more parallel to the beam axis . Thus neutrons can be freely propagated over a long distance inside the tube. A passive magnetic shield, made of a 75 m long and 1 m diameter [t-metal tube, is installed inside the first part of the drift vessel and also surrounds the guide. I. NEW PHENOMENA
10
G Pugherin / Free neutron oscillations
v
U C
v)
r
E . t9 E
Fig 1 Neutron energy spectrum of the new cold neutron beam at the ILL reactor in Grenoble .
Together with the active shield placed outside, it reduces the magnetic field all along the propagation path to the level B < 20 nT as required by the free neutron oscillation condition. The effective oscillation time, evaluated from the beam energy spectrum and the divergency distributions starting for each neutron from the last reflection on the guide till the exit of the shielded region, is (t2)'/2=0.1 s. The antineutron component present in the beam is detected by letting the beam pass through a 200 gm thick carbon foil, enough to annihilate the antineutron component and almost transparent to the neutron beam .
The beam is subsequently absorbed into a 6 LiF beam dump at a 10 m distance from the target . The target is placed inside the second part of the drift vessel which consists of an AI tube, of 1 .4 m diameter and 5 m long, followed by a 8 m long inox tube of the same diameter . The axis of the second part of the drift vessel is 2 cm lower than the axis of the first part to take into account the gravity effect on neutrons . For the same reason the axis of the beam at the entrance of the drift vessel is 7 cm higher than the axis of the drift tube . With such an arrangement the target, which has a diameter of 1.1 m, contains more than 99% of the beam . Antineutron annihilations on the target are detected by searching for the annihilation products, on the average 5 pions, in the detector placed around the beam tube and with the target in the middle. The detector, fig. 3, is a tracking device organized in four quadrants. Each quadrant consists of 10 planes of limited streamer tubes (LST) supported by an Al honeycombe structure, (p) = 0.3 g cm -3 , so that passing tracks are well measured and the event vertex on the target can be reconstructed with a precision which has been estimated by Monte Carlo calculation ±4 cm on the average for each coordinate. Each streamer tube has a cross-section of 0.9 x 0.9 cm2 and is 5 m long . Two layers of the 2 cm thick scintillator counters are placed respectively before and after the LST planes . They provide the first level trigger and can distinguish by time of flight the particle direction . Immediately behind the second layer of the scintillators there are 6 + 6 planes of LST interspaced with, respectively, 1 and 2 mm thick Pb plates supported by two 2.5 mm thick Al plates . This structure, - 4 radiation lengths, allows the detection of y's from iT ° 's and provides a measurement of their direction as well as an estimate of the total energy of the event, expected to be - 2 MN , while the total momentum should be - 0. A veto system for the cosmic rays is realized with scintillator counters placed all around the detector and
H53 neutron
beam dump
beam
Fig. 2. The experimental set-up at the ILL in Grenoble for neutron-antineutron oscillation study.
G. Pugherin / Free neutron oscillations
02~0 4300 Oi4B
Fig. 3 . Cross-sectional view of the detector . separated from it by 10 cm of lead in order to avoid the possibility of an annihilation product autovetoing the event. Scintillators are also placed below the platform supporting the detector and are used as the veto of neutral cosmic rays interacting inside the detector . All the scintillator counters are equipped with two phototubes, whose signals are processed by specially designed mean timer, in order to keep to a minimum the signal width and the dead time of the experiment . The trigger of the experiment can be done in many different ways : from the simple coincidence between the inner and outer scintillators layer of one quadrant, up to several majority conditions for LST planes firing at the same time in the inner as well in the outer part of the detector, each trigger corresponding to different levels of antineutron detection efficiency and background rejection. It has been evaluated by Monte Carlo calculation that by requiring as a trigger the coincidence of the two
scintillator layers of a quadrant with at least 4 out of S planes of LST firing at the same time, in the same quadrant, the antineutron detection efficiency is 0.77. At the moment this trigger is not activated since the level of radiation around the target is still rather high, however a trigger with the coincidence of the four layers of scintillator counters of two quadrants has been installed and provides a 45% antineutron annihilation detection efficiency . Data have been collected with this trigger for a time largely sufficient for investigating the oscillation up to Tnn > 10 7 S. The trigger rate is 1 .5 events s -1 , mostly due to passing cosmic rays. These are eliminated by software in the off-line analysis. Having lined the inner wall of the drift tube around the target with a 3 mm thick layer of 6 LiF all along the detector length, the noise in the detector from the scattered neutrons on the target and from the beam in general is such that the recorded I . NEW PHENOMENA
12
G. Pughenn / Free neutron oscillations RUN
#
EVENT # 8
513 #
18331
Hits Cluster .
267 100
#Hits/#Cluster # Scintillatorr
2
7
7
IL
2
Sumt
8 .37(nsec)
OL OL
4 5
Sumt Sumt
14 .75 ( nsec) 16 .67(nsec)
IU IU
5 6
Sumt sumt
-4 .22(nsec) 6 .00(nsec)
sumt sumt
10 .12(nsec) 7 .17(nsec)
ID 4 OD 5
Fig. 4. Display of one recorded event with the beam on . The three projections are shown and the corrected crossing time for the scintillator counters is reported (I, O for inner, outer sector ; L, R, U, D for left, right, up, down). events are very clear, and selection criteria can easily be applied in the off-line analysis . Fig. 4 shows one event
present conditions thus result can be reached within - 400 days of running time.
the target . These events are essentially due to the neutral cosmic rays interacting in the detector. They arrive
References
as it appears when the beam is on . What is left after the computer selection is a sample of events to be scanned to search for annihilations on
at a rate of 0.2 Hz and are rejected since they either do not have a vertex on the target or do not satisfy the energy and momentum requirements, or do not satisfy the time of flight analysis. The real background is due to those cosmic ray
events interacting on the target. It can be estimated from the number of events with the vertex on the drift tube wall. This evaluation is under way and results should come quite soon, so that in a short time the neutron-antineutron oscillation will be probed first at the level
Tnn > 10 7 s. Improvement of the apparatus is in progress in view of the final goal : Tnn >_ 10 8 s; however already under the
R.N. Mohapatra, these Proceedings (Int . Workshop on Fundamental Physics with Slow Neutrons, Grenoble, France, 1989) Nucl . Instr. and Meth. A284 (1989) 1. P.K. Kabir and J.V. Noble, University of Virginia, Preprint, 1989 ; P.K. Kabir, Phys . Rev. Lett . 51 (1983) 231. [2] J.M. Richard, presented at this Workshop (Int. Workshop on Fundamental Physics with Slow Neutrons, Grenoble, France, 1989). [3] G. Fidecaro et al ., Phys . Lett . 156B (1985) 122. [4] G. Bressi et al., Z. Phys . (1989) in press. [5] V.A . Kuz'min, Proc. Int. Coil . Matter non Conservation, Frascati 1983, eds. E. Bellotti and S. Stipcich ; V. Lobashev, private communication.