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Status of the Sudbury Neutrino Observatory
ELSEVIER
,am :r_.~:~,:::-1.1[q.1:1 PROCEEDINGS SUPPLEMENTS Nuclear Physics B (Prec. Suppl.) 48 (1996) 378-380
S t a t u s of t h e S u d b u r y
Neutrino
Observatory
M.E Moorhead a for the SNO collaboration aInstitute for Nuclear and Particle Astrophysics, Nuclear Science Division Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, Ca 94720, U.S.A The Sudbury Neutrino Observatory is a 1,000 ton heavy water Cerenkov detector under construction in Sudbury, Canada. Two u reactions on deuterium, one flavour-independent and the other restricted to the electron flavour, will allow model-independent tests of solar v oscillations and, if a supernova occurs in our galaxy, direct searches for v, and v, masses down to 20eV. The status of construction and the methods developed to extract the u signMs are discussed.
1. I n t r o d u c t i o n The Sudbury Neutrino Observatory (SNO) [1] is a 1,000 ton heavy water (D20) Cerenkov detector presently under construction in Sudbury Ontario, Canada. The v reactions which occur in D 2 0 and the extremely low background environment of the detector will allow the following measurements: i) the t'e and ux (flavour independent) fluxes, and their ratio, for SB solar u's, ii) the energy spectrum of SB ~,~'s above 5 MeV, iii) time dependence in the SB flux, iv) detailed studies of the u burst from a galactic supernova, including a search for uj, and Vr masses above 20eV. For the SB solar v measurements, the ue/ux flux ratio and the ue energy spectrum constitute two separate tests of v oscillations which are both independent of solar model flux calculations [2].
~ R o o m
~
~l~9~'t
Cables
~ Norlle
I1[.(12m A©~Idiemeter) IC Vessel
WH Refl ~ I e~s
Figure 1. SNO detector. 2. D e t e c t o r a n d S t a t u s o f C o n s t r u c t i o n The detector is situated two kilometers underground in a dedicated laboratory that has been excavated in the Creighton nickel mine of INCO corporation. This laboratory comprises facilities for changing into clean-room clothing, a lunch room, a car wash for bringing equipment into the clean area, a utility room where the H 2 0 and D 2 0 systems are located, a control room and a 30m x 23m barrel shaped cavity for the detector itself, see Fig. 1. The walls of the cavity have been coated with concrete and low-activity urylon, a water proof radon barrier. Inside this cavity will be located a 12m diameter acrylic vessel for con0920-5632/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved. PII: S0920-5632(96)00276-9
taining the 1,000 tons of D~O. This vessel will be surrounded by 8,000 tons of ultra-pure light water to act as shielding for high energy g a m m a rays coming from the rock and the photomultipliers (PMTs). Immersed in this light water will be a 17m geodesic sphere supporting 9,500 20cm I i a m a m a t s u PMTs, each of which is equipped with a light reflecting concentrator to increase its effective photocathode area by a factor 1.7. All the P M T s and concentrators have been manufactured, tested for acceptance and delivered to the site. About 5,000 have been installed
M.E. Moorhead/Nuclear Physics B (Proc. Suppl.) 48 (1996) 378-380
on the upper half of the geodesic sphere and electrically connected to their respective high voltage cards on the deck above. The front-end electronics cards have been through their final prototype stages and a first batch of production versions is expected in early '96. The H20 system is already operational and the D20 system will be completed by early '96. There will then be a commissioning stage, using light water, where radioactive backgrounds and extraction efficiencies will be measured and compared with those established for earlier prototypes. The acrylic vessel is made of 100 2m x 2m curved panels of 10 cm thickness in the waist region, where the rope support is located, and 5 cm thickness elsewhere. The first equatorial row of panels has been bonded together and work is proceeding on the rest of the upper hemisphere. Once this is finished the bottom halves of both the acrylic vessel and the geodesic P M T support structure will be completed in parallel. Then, the acrylic vessel will be filled with D20 and the cavity with H20. Solar ~ data taking is expected to begin in early '97. 3. D e t e c t i o n o f N e u t r i n o E v e n t s The event rates for solar u's, assuming the full SSM SB flux [3], and for a supernova (SN) at the center of our galaxy are given in Table 1. Apart from the neutral current (NC) reaction, all of the events are detected by the array of 9,500 PMTs via the Cerenkov radiation .emitted by a single electron (or positron) of energy >_ 5 MeV, the detector's threshold. The NC reaction produces a free neutron in the D20 which can he detected, after thermalization, by observing a subsequent neutron capture reaction. There are three capture reactions of interest depending on what additives are placed in the D20: i) Pure D20: In the case of no additive there is a 30% probability of capture on deuterium, which produces a 6.25 MeV gamma. This gamma converts to electrons by Compton scattering and pair production, and the resulting Cerenkov light is detected by the PMTs. Five hundred events a year are expected above the detector's 5 MeV threshold.
379
ii) MgC12: Dissolving 2 tons of MgC12 in the D20 gives an 83°£ chance of neutron capture on 35C1 which produces an 8.5 MeV g a m m a cascade. The higher efficiency and Q-value of this capture (c.f. capture on deuterium in the pure D20 case) increases the number of detected events by a factor of 5 to 2,500 per year, see Fig. 2. iii) ZHe Counters: An array of aHe proportional counters [5] (5 cm diameter tubes of 800m total length) placed vertically in the D20 in a square grid of l m spacing, gives a 420£ chance of neutron capture on 3He. The energy and rise-time of the signals are used to separate n-capture (2,000 per year) from internal alpha and beta backgrounds, see Fig. 3. The dominant background for all these NC detection methods will probably come from photodisintegration of deuterium which produces free neutrons that are indistinguishable from NC neutrons. Thus, the detector components have been carefully selected for extremely low levels of thorium and uranium chain contamination so that the photodisintegration rate is small compared with the NC rate. This small residual photodisintegration rate must be measured, in order to subtract its contribution to the neutron capture signal. Several methods have been developed for this purpose: i) radiochemical extraction and counting of 22STh, 226Ra, 224Ra, 222Rn and 212Pb, ii) analysis of low energy signals seen by the P M T array, iii) delayed coincidences between signals seen by the P M T array, and iv) prompt and delayed coincidences between signals seen by the PMTs and signals in the 3He proportional counters. REFERENCES
1. G.T. Ewan et al., 'Sudbury Neutrino Observatory Proposal', SNO 87-12 (1987). 2. H.H. Chen, Phys. Rev. Lett. 55 (1985) 1534. 3. J.N. Bahcall and M.H. Pinsonneault, Rev. of Mod. Phys. 64 (1994) 885. 4. N. Hata and P. Langacker, Phys. Rev. D 48 (1993) 2937. 5. T.J. Bowles et al., 'Construction of an Array of Neutral-Current Detectors for the Sudbury Neutrino Observatory', SNO internal report.
M.E. Moorhead/Nuclear Physics B (Proc. Suppl.) 48 (1996) 378-380
380
Table 1 Neutrino event rates in SNO (including detection efficiency)
Neutrino reaction
Events/year SSM
Events/SN @10kpc
7000 2500 1000 0 0
80 300 20 70 350
Charged Current (CC): u~ + d ---*p + p + e Neutral Current (NC): ux + d --~ p + n + vx Electron Scattering (ES): u~,= + e - --* u~,= + e Anti-neutrino CC in D20: ~ + d ---* n + n + e + Anti-neutrino CC in H20: ~7~+ p --, n + e +
....
2.5--
i .... L .... I,,,,I,t,,I,,,,I : ~ ! .; .- . i
2.0~
....
} ....
I ....
~',',
;
I,,,,I
°
ompton/Beta
:
i
"~":
•:
.: . . . .
1,5-
: ... :,.. ~: : (" -;~'~,:~:j:
10 3 :..
. ..;:;.:. ~,f:.¢
, ,y., ! ? - . . . : . . ,
. ~ .:. . . . .
~,:
. ~
....
:.Tf.; ~
,
V 0.0-
10 2
I ,
v
0 700_1 ....
600
I''''l
....
200
J.,,,I
....
I
I.'''l
400
'1'
000
Enercdy(keY)
i ....
I
J ....
I ....
-
Boo
1000
i,,,,I,,,,t,,,,
He(n,p)t
: • 500-
10
o
: ii~/Beta
400~ 300=
1
i
0
l
I
2O
i
40
I , , 60
, I , 80
, 3 , , 1 O0
, I , 120
I 1 140 NPMT
Figure 2. Monte-Carlo prediction of the energy spectrum for one year's d a t a with 2 tons of MgCl2 dissolved in the D20. The MSW nonadiabatic solution [4] and the SSM SB flux [3] have been assumed. Dividing the number of P M T hits ( N P M T ) by 10 gives the energy scale in MeV. The signals are as follows: i) low energy exponential = internal radioactivity, ii) gaussian peak at 6 MeV = NC events from n-capture on 35C1, and iii) ' b e t a s p e c t r u m ' up to 14 MeV = CC events (MSW distorted 8B v spectrum).
0 o
200 = 100-
0
200
400
600
Energy (keY)
800
1000
Figure 3. Risetime versus energy plot from aHe proportional counter showing the separation of 3He(n,p)t, alpha, and c o m p t o n / b e t a events. 50% of the 3He(n,p)t window is free of alpha contamination while c o m p t o n / b e t a events are discriminated at essentially the 100% level. 80% of the 3He(n,p)t events fall in a narrow energy window at the 764 keV fllll energy peak.
,am :r_.~:~,:::-1.1[q.1:1 PROCEEDINGS SUPPLEMENTS Nuclear Physics B (Prec. Suppl.) 48 (1996) 378-380
S t a t u s of t h e S u d b u r y
Neutrino
Observatory
M.E Moorhead a for the SNO collaboration aInstitute for Nuclear and Particle Astrophysics, Nuclear Science Division Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, Ca 94720, U.S.A The Sudbury Neutrino Observatory is a 1,000 ton heavy water Cerenkov detector under construction in Sudbury, Canada. Two u reactions on deuterium, one flavour-independent and the other restricted to the electron flavour, will allow model-independent tests of solar v oscillations and, if a supernova occurs in our galaxy, direct searches for v, and v, masses down to 20eV. The status of construction and the methods developed to extract the u signMs are discussed.
1. I n t r o d u c t i o n The Sudbury Neutrino Observatory (SNO) [1] is a 1,000 ton heavy water (D20) Cerenkov detector presently under construction in Sudbury Ontario, Canada. The v reactions which occur in D 2 0 and the extremely low background environment of the detector will allow the following measurements: i) the t'e and ux (flavour independent) fluxes, and their ratio, for SB solar u's, ii) the energy spectrum of SB ~,~'s above 5 MeV, iii) time dependence in the SB flux, iv) detailed studies of the u burst from a galactic supernova, including a search for uj, and Vr masses above 20eV. For the SB solar v measurements, the ue/ux flux ratio and the ue energy spectrum constitute two separate tests of v oscillations which are both independent of solar model flux calculations [2].
~ R o o m
~
~l~9~'t
Cables
~ Norlle
I1[.(12m A©~Idiemeter) IC Vessel
WH Refl ~ I e~s
Figure 1. SNO detector. 2. D e t e c t o r a n d S t a t u s o f C o n s t r u c t i o n The detector is situated two kilometers underground in a dedicated laboratory that has been excavated in the Creighton nickel mine of INCO corporation. This laboratory comprises facilities for changing into clean-room clothing, a lunch room, a car wash for bringing equipment into the clean area, a utility room where the H 2 0 and D 2 0 systems are located, a control room and a 30m x 23m barrel shaped cavity for the detector itself, see Fig. 1. The walls of the cavity have been coated with concrete and low-activity urylon, a water proof radon barrier. Inside this cavity will be located a 12m diameter acrylic vessel for con0920-5632/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved. PII: S0920-5632(96)00276-9
taining the 1,000 tons of D~O. This vessel will be surrounded by 8,000 tons of ultra-pure light water to act as shielding for high energy g a m m a rays coming from the rock and the photomultipliers (PMTs). Immersed in this light water will be a 17m geodesic sphere supporting 9,500 20cm I i a m a m a t s u PMTs, each of which is equipped with a light reflecting concentrator to increase its effective photocathode area by a factor 1.7. All the P M T s and concentrators have been manufactured, tested for acceptance and delivered to the site. About 5,000 have been installed
M.E. Moorhead/Nuclear Physics B (Proc. Suppl.) 48 (1996) 378-380
on the upper half of the geodesic sphere and electrically connected to their respective high voltage cards on the deck above. The front-end electronics cards have been through their final prototype stages and a first batch of production versions is expected in early '96. The H20 system is already operational and the D20 system will be completed by early '96. There will then be a commissioning stage, using light water, where radioactive backgrounds and extraction efficiencies will be measured and compared with those established for earlier prototypes. The acrylic vessel is made of 100 2m x 2m curved panels of 10 cm thickness in the waist region, where the rope support is located, and 5 cm thickness elsewhere. The first equatorial row of panels has been bonded together and work is proceeding on the rest of the upper hemisphere. Once this is finished the bottom halves of both the acrylic vessel and the geodesic P M T support structure will be completed in parallel. Then, the acrylic vessel will be filled with D20 and the cavity with H20. Solar ~ data taking is expected to begin in early '97. 3. D e t e c t i o n o f N e u t r i n o E v e n t s The event rates for solar u's, assuming the full SSM SB flux [3], and for a supernova (SN) at the center of our galaxy are given in Table 1. Apart from the neutral current (NC) reaction, all of the events are detected by the array of 9,500 PMTs via the Cerenkov radiation .emitted by a single electron (or positron) of energy >_ 5 MeV, the detector's threshold. The NC reaction produces a free neutron in the D20 which can he detected, after thermalization, by observing a subsequent neutron capture reaction. There are three capture reactions of interest depending on what additives are placed in the D20: i) Pure D20: In the case of no additive there is a 30% probability of capture on deuterium, which produces a 6.25 MeV gamma. This gamma converts to electrons by Compton scattering and pair production, and the resulting Cerenkov light is detected by the PMTs. Five hundred events a year are expected above the detector's 5 MeV threshold.
379
ii) MgC12: Dissolving 2 tons of MgC12 in the D20 gives an 83°£ chance of neutron capture on 35C1 which produces an 8.5 MeV g a m m a cascade. The higher efficiency and Q-value of this capture (c.f. capture on deuterium in the pure D20 case) increases the number of detected events by a factor of 5 to 2,500 per year, see Fig. 2. iii) ZHe Counters: An array of aHe proportional counters [5] (5 cm diameter tubes of 800m total length) placed vertically in the D20 in a square grid of l m spacing, gives a 420£ chance of neutron capture on 3He. The energy and rise-time of the signals are used to separate n-capture (2,000 per year) from internal alpha and beta backgrounds, see Fig. 3. The dominant background for all these NC detection methods will probably come from photodisintegration of deuterium which produces free neutrons that are indistinguishable from NC neutrons. Thus, the detector components have been carefully selected for extremely low levels of thorium and uranium chain contamination so that the photodisintegration rate is small compared with the NC rate. This small residual photodisintegration rate must be measured, in order to subtract its contribution to the neutron capture signal. Several methods have been developed for this purpose: i) radiochemical extraction and counting of 22STh, 226Ra, 224Ra, 222Rn and 212Pb, ii) analysis of low energy signals seen by the P M T array, iii) delayed coincidences between signals seen by the P M T array, and iv) prompt and delayed coincidences between signals seen by the PMTs and signals in the 3He proportional counters. REFERENCES
1. G.T. Ewan et al., 'Sudbury Neutrino Observatory Proposal', SNO 87-12 (1987). 2. H.H. Chen, Phys. Rev. Lett. 55 (1985) 1534. 3. J.N. Bahcall and M.H. Pinsonneault, Rev. of Mod. Phys. 64 (1994) 885. 4. N. Hata and P. Langacker, Phys. Rev. D 48 (1993) 2937. 5. T.J. Bowles et al., 'Construction of an Array of Neutral-Current Detectors for the Sudbury Neutrino Observatory', SNO internal report.
M.E. Moorhead/Nuclear Physics B (Proc. Suppl.) 48 (1996) 378-380
380
Table 1 Neutrino event rates in SNO (including detection efficiency)
Neutrino reaction
Events/year SSM
Events/SN @10kpc
7000 2500 1000 0 0
80 300 20 70 350
Charged Current (CC): u~ + d ---*p + p + e Neutral Current (NC): ux + d --~ p + n + vx Electron Scattering (ES): u~,= + e - --* u~,= + e Anti-neutrino CC in D20: ~ + d ---* n + n + e + Anti-neutrino CC in H20: ~7~+ p --, n + e +
....
2.5--
i .... L .... I,,,,I,t,,I,,,,I : ~ ! .; .- . i
2.0~
....
} ....
I ....
~',',
;
I,,,,I
°
ompton/Beta
:
i
"~":
•:
.: . . . .
1,5-
: ... :,.. ~: : (" -;~'~,:~:j:
10 3 :..
. ..;:;.:. ~,f:.¢
, ,y., ! ? - . . . : . . ,
. ~ .:. . . . .
~,:
. ~
....
:.Tf.; ~
,
V 0.0-
10 2
I ,
v
0 700_1 ....
600
I''''l
....
200
J.,,,I
....
I
I.'''l
400
'1'
000
Enercdy(keY)
i ....
I
J ....
I ....
-
Boo
1000
i,,,,I,,,,t,,,,
He(n,p)t
: • 500-
10
o
: ii~/Beta
400~ 300=
1
i
0
l
I
2O
i
40
I , , 60
, I , 80
, 3 , , 1 O0
, I , 120
I 1 140 NPMT
Figure 2. Monte-Carlo prediction of the energy spectrum for one year's d a t a with 2 tons of MgCl2 dissolved in the D20. The MSW nonadiabatic solution [4] and the SSM SB flux [3] have been assumed. Dividing the number of P M T hits ( N P M T ) by 10 gives the energy scale in MeV. The signals are as follows: i) low energy exponential = internal radioactivity, ii) gaussian peak at 6 MeV = NC events from n-capture on 35C1, and iii) ' b e t a s p e c t r u m ' up to 14 MeV = CC events (MSW distorted 8B v spectrum).
0 o
200 = 100-
0
200
400
600
Energy (keY)
800
1000
Figure 3. Risetime versus energy plot from aHe proportional counter showing the separation of 3He(n,p)t, alpha, and c o m p t o n / b e t a events. 50% of the 3He(n,p)t window is free of alpha contamination while c o m p t o n / b e t a events are discriminated at essentially the 100% level. 80% of the 3He(n,p)t events fall in a narrow energy window at the 764 keV fllll energy peak.