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P R O C E E D I N G S S U P P L E M E N T S

ELSEVIER Nuclear Physics B (Prec. Suppl.) 48 (1996) 463-465

Status of the Lake BAIKAL Telescope

CH. SPIERING

for the B A I K A L Collaboration

A first large deep underwater detector for muons and neutrinos, NT-200, is currently under construction in Lake Baikal. Part of the detector consisting of 36 optical modules (NT-36) has been operated over nearly 2 years in 1993 and 1994. In March 1995, a 72-PMT version was deployed. We describe the construction and performance of the detector,and review the main results obtained so far.

1. D e t e c t o r a n d P e r f o r m a n c e

The Baikal Neutrino Telescope [1] is being deployed in the Siberian Lake Baikal, at a depth of 1.1 km, see fig.1.

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the frame carrying the detector, each 21.5 m in length, are at a height of 250 m above the b o t t o m of the lake. Two underwater electri- cal cables and one optical cable connect the detector site with the shore station. Detector components are deployed form the ice cover in later winter.

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Figure 1: Installations at the Baikal site (sta- tus 1995); 1-3: shore cables, 4-6: string stations for shore cables, 7: string with the telescope, 8: environmental string, 9-14: ultrasonic emitters.

In April 1993 we put into operat ion the s ta t ionary detector NT-36 ( 36 P M T s at 3 strings), since April 1994 a modified version of NT-36 was taking data. An array carrying 72 P M T s has been deployed in March 1995 (see fig.2). These arrays are steps towards the Neutrino Telescope NT-200 which will con- sist of 192 optical modules. The 7 arms of

Figure 2: Schematical view of NT-72, deployed in 1995. It consists of 36 PMT-pairs at 4 short and 1 long string. Open circles indicate positions of the distant calibration laser (see text).

The optical modules are grouped in pairs along the strings, directed alternatively up- ward and downward. The two P M T s of a pair are connected in coincidence and define a channel. The local coincidences are manda- tory for the suppression of the background from bioluminescence and P M T noise: the typical 1 p.e. counting rate is 100 kHz for a single tube, but only 200 Hz for a channel.

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464 Ch. Spiering /Nuclear Physics B (Proc. Suppl.) 48 (1996) 463-465

A muon-trigger is formed by the require- ment of > 3(4) fired channels within a time window of 500 ns. A second system (monopole trigger) searches for t ime pat- terns characteristic for slowly moving, bright particles like nuclearities or GUT magnetic monopoles catalyzing proton decays.

The t ime calibration of the array is per- formed with the help of a stationary nitrogen laser positioned just above the array. The spatial position of the components of the ar- ray is monitored by a hydroacoustical system. A special "environmental" string carries de- vices to measure the optical parameters of the water, temperature, pressure, sound velocity and water currents.

NT-36 was operated from April 13th, 1993, up to March 1994. During 240 days of data taking, 7.107 events for the basic trigger 3/1 (> 3 hits at _> 1 string) have been taken, 107 of them fulfilling the trigger condition 6/3 (i.e. >_ 6 hits at 3 strings), suitable for unambiguous track reconstruction. A mod- ified array, NT-36 ~, came into operation on April 3th, 1994. During 242 days operating time, 9.7.107 events for trigger 3/1 have been taken. In Spring 1995, a 72 PMT array was deployed. Due to extraordinary high temper- atures the deployment period was only haft as long as normally. Therefore neither all failed modules of N T - 3 6 ~ could be repaired nor all problems with the new modules could be cured. Due to that , only 26 of the 36 chan- nels were operational just after deployment.

After more than 2 years of operation the following conclusions with respect to the de- tector performance can be drawn:

- Seasonal variations of the water lumines- cence (reaching sometimes an order of magni- tude) do not influence the muon trigger rate. - Upward pointing modules loose after 150 days ~50 % of their sensitivity due to sedi-

mentation, downward modules do not show any remarkable change of their sensitivity. - Displacements of the whole array due to wa- ter currents are small (< 2m) and monitored by our ultrasonic system. The relative posi- tions of the optical modules do not change by more thazl 20 cm. - All mechanical components of the system worked reliably, particularly none of the mod- ules did leak over 2 years. On the other hand, some of the electronical components do not satisfy the standards requested for long-time remote operation.

2. S e l e c t e d R e s u l t s

First physical results based on the data taken with 1993 array have been presented at di~erent occasions (see [2] and refs. therein). Some of the results are given in the following:

a) Muon Spectra Muon angular distributions are well de-

scribed by MC expectations. Converting the measured angular dependence into a depth dependence of the vertical flux, good agree- ment with other published values is observed (fig.3).

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Figure 3: Vertical muon flux vs. water depth.

Ch. Spiering /Nuclear Physics B (Proc. Suppl.) 48 (1996) 463-465 465

b) Search for Neutrino Candidates

With NT-36, a signal-to-noise ratio of 1:50 has been achieved over the full lower hemi- sphere. Here, the signal is given by upward muons from interactions of atmospheric neu- trinos, and the noise are downward muons faking upward muons. In selected angular regions, however, S/N is much better. E.g., searching for muons just around the opposite zenith (as expected from neutralino annihila- tion in the center of the Earth) already with NT-36, the S/N-ratio approaches unity. At the time when this manuscript was prepared, a first possible candidate for a nearly vertical upward muon was found in the 1994 data. A fully functioning 72 PMT array would yield S / N ~ 0.5-1 over most of the lower hemi- sphere, and a bigger array like NT-200 would do even better.

c) Search for Magnetic GUT Monopoles

From monopole induced proton decays one would expect Cherenkov light signals gener- ated by the decay products.

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Figure 4: Upper limits (90% CL) on the flux of magnetic GUT monopoles as a function of their velocity/3, for different catalysis cross sections o'o.

For certain regions of the parameter space in /3 (monopole velocity) and ac (catalysis cross section), GUT monopoles would cause sequential hits in individual channels in time windows of 10 -4 - 10 -3 sec. Having searched for such enhanced counting rates we deduce upper limits for the flux of monopoles cat- alyzing the decay of free protons with cross section a¢ = O.17. Cro . 13 -2 . E.g., with a o = 10 - 2 9 c m 2 and ~ = 10 -4 one gets a flux limit of 10 - is cm -2 sec -1 sr -1, just at the Parker limit and of the same order of magni- tude as limits obtained by our collaboration with dedicated setups in 1984-89 (rigA).

d) Laser Experiment 1995 Just after deployment of NT-72, a calibra-

tion experiment with a distant laser was per- formed. Fig. 1 indicates the different posi- tions of the laser. An isotropic light source was used simulating distant electromagnetic showers. From the time and amplitude pat- tern of the hit channels light attenuation and scattering lengths, )% and )~s, could be de- termined. In agreement with measurements performed with standard methods over many years, we found Aa = (20.0+0.3)m at 475nm and As ~ 15-20m.

Future Plans In 1996, the present array will be upgraded

quantitatively (up to ~ 96 PMTs) and qual- itatively.

References

[1] I.A.Belolaptikov et al., Nucl.Phys.B (Proc.Suppl.),19 (1991)375; I.A.Sokalsky, Ch.Spiering (eds.), The Baikal Neutrino Telescope NT-200, BAIKAL 92-03;

[2] I.A.Belolaptikov et al., Proc. 24rd ICRC, (Rome 1995),Vol.2, pages 536, 742, 770, 789, 841, 1001, 1043.