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Perspectives of Experimental Neutrino Physics in India

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Perspectives of Experimental Neutrino Physics in India Proc Indian Natn Sci Acad, 70, A, No.1, January 2004, pp.11–25 c Printed in India. PERSPECTIVES OF EXPERIMENTAL NEUTRINO PHYSICS IN INDIA V S NARASIMHAM Tata Institute of Fundamental Research Mumbai 400 005 (India) (Received 10 February 2003; Accepted 30 June 2003) A description of the early work on experimental observation and study of cosmic ray neutrinos in India is sketched here. This includes experiments to measure the variation of intensity of cosmic ray muons down to great depths underground, the first detectors for neutrino detection, and the consolidation of neutrino data using large scale proton decay experiments. Also included is a brief description of special (Kolar) events recorded in the KGF detectors, that defy explanation on the basis of normal muon or neutrino interactions. Results from the neutrino experiments conducted in other laboratories around the same time period as well as those recorded in all the first-generation proton decay experiments will be reviewed briefly. Key Words: Cosmic Rays; Kolar Gold Fields; Underground Experiments; Neutrino Detectors; Atmospheric Neutrinos; Kolar Events; Proton Decay 1 Introduction It has long been recognized that the secondary cosmic radiation is a rich source of neutrinos with Primary Cosmic Radiation (PCR), the only source of a wide range of energies. This was based on the high energy elementary particles until the advent of understanding that cosmic ray secondaries comprise accelerators in the early 1960s, was the subject of in- high energy pions and muons and their decays in the tense study at a number of laboratories around the Earth’s atmosphere provide a copious source of neu- world including those in India, and in particular at trinos. The first suggestion that cosmic ray neutri- the Tata Institute of Fundamental Research, Mum- nos could be detected by experiments operated deep bai (TIFR). The detection media used were nuclear underground was made in 1960 by Markov and his emulsions flown at balloon altitudes (at Hyderabad) collaborators. An estimate of fluxes was made by and triggered cloud chambers operated at mountain Greisen1, and detailed calculations on fluxes and an- altitudes (at Ooty), to study the composition of pri- gular distributions were first made by Zatsepin and mary cosmic rays and secondary hadron interactions Kuzmin2 with pions and muons as the source of neu- respectively. This was closely followed by extensive trinos. Some of these authors talked of installations air shower (EAS) studies using a network of scintil- of area 300 m2 or 10 m3 in volume but felt that some lation detectors arranged in an array at both sea-level idea of cross-section was necessary and one should and at mountain altitudes. await the results of the then planned accelerator ex- This led to the study of hadron collisions at ultra periments. high energies which are not available from accelerator It was only from the accelerator experiment of beams, by a detailed simulation and comparison with Danby et al.3 at Brookhaven, that one became aware of the EAS data on electrons, muons and hadrons. A dif- the distinction between the muon and electron neutri- ferent path was taken in the study of cosmic ray muons nos and of the large cross-section for ν-N interactions in the early 1950s. The Kolar Gold Fields (KGF) pro- at a few GeV. Detailed estimates of fluxes of muon and vided an ideal environment to study muons at different electron neutrinos and anti-neutrinos from all possible depths underground and thereby allowed the study of decay modes of kaons, in addition to those from pions their energy spectrum and angular distributions up to and muons, were made by Osborne et al.4 and Cowsik very high energies, much beyond what was possible et al.5 with magnet spectrometers operated at sea level. At the Kolar Gold Mines, a detailed mapping of 12 V S NARASIMHAM Fig. 1 The scintillator-lead-GM counter telescope used at great depths in KGF mines. A coincidence of pulses from PMTs viewing the two scintillators and the sandwiched GM counter array provided the signal with negligible background. Table I Summary of muon measurements by MNR experiment in KGF mines. Depth from Depth from the Time of Number of Counting surface in top of the observation: counts rate in meters atmosphere (mwe) Hrs. Mins. observed counts/hr 270 810 60 - 20 10,152 168.3 ¡ 1.7 800 1812 100 - 28 1,029 10.23 ¡ 0.32 1130 3410 211 - 45 142 0.67 ¡ 0.056 1415 4280 944 - 06 127 0.132 ¡ 0.012 3 ¢ £ 2110 6380 2992 - 40 18 (6.0 ¡ 1.4) 10 3 ¥ ¢ £ 2760 8400 2880 - 00 none ¤ 3 47 10 cosmic ray muons as a function of depth and angle periments conducted in KGF, India, and in the ERPM was undertaken since 1960 in a series of experiments mines in South Africa during 1965. by Miyake, Narasimham and Ramanamurty up to ¦ 3 km below ground. It was clear that the muon fluxes 2 Cosmic Ray Muons attenuate rapidly with depth and are so low at the great depth of 8400 hg/cm2, that no count was recorded for A brief description of the muon experiments at KGF 2 months in detectors of area 3 sq. meters. This paved will be useful to understand how the neutrino experi- the way for a realistic estimate of the muon back- ments evolved in a natural manner. ground for a possible neutrino detector using the ad- The KGF mines are situated at 870 m above sea ¨ vantage of great depths for unambiguous detection of level at a latitude of 12 § 54 N near Bangalore, South 6 events from neutrino interactions (see Menon et al. ). India; it has a flat surface topography ( © 40m) over a This was indeed the starting point for the neutrino ex- 3 km 3 km area around the main shaft of the Cham- EXPERIMENTAL NEUTRINO PHYSICS IN INDIA 13 pion reef mine. The mines have an extended network expressed as I h θ I h 0 sec θ exp n sec θ 2 of tunnels underground permitting observations at a 1 . At the depth of 7000 hg/cm , the exponent 2 series of depths down to 3 km, i.e., 8400 hg/cm of n 9 0 5, a fact that is crucial in the identification Kolar rock. The Kolar rock has special characteristics of neutrino events to be discussed in the next section. in terms of density (3.02 g/cm3), and chemical com- 2 position i.e. Z A 0 495 and Z A 6 4 when 3 Neutrino Experiments at KGF compared to the so-called standard rock (0.5, 5.5 re- spectively). The first experiments on depth variation The no-count observation of the experiment of of muon fluxes at KGF were conducted by Sreekantan Miyake et al.10 at the depth of 8400 hg/cm2 led to 7 2 2 et al. up to depths of 900 hg/cm . At KGF, 1 hg/cm the conclusion that the atmospheric muon intensity is equivalent to approximately 30 cm of rock thick- has been attenuated to such an extent that one could ness. search for rare processes like the interaction of high A new series of TIFR experiments was energy neutrinos of the cosmic ray beam. However, started in 1961 by Miyake, Narasimham and with the measured cross-section up to 10 GeV and Ramanamurty8(MNR) covering depths ranging from the estimated fluxes of all neutrinos, it was clear that 2 816 to 8400 hg/cm . They employed a scintillation at least a few kilotons of target would be required to 2 counter-lead telescope of area 1.62 m in a 4-fold record a few neutrino interactions per year. On the coincidence mode of photomultipliers (PMT’s) and other hand, one could detect the muons produced in added another similar unit (with an extra layer of GM the charged current neutrino interactions in the sur- counters) to increase the rate of collection of muons at rounding rock by using modest detector arrays of large 2 depths of 6380 and 8400 g/cm . A picture of the de- area. The target material available in such a case de- tector is shown in Fig. 1. Table I provides some details pends upon the range of the muon inside the rock, of the data collected in these experiments (Miyake et which is proportional to the muon (neutrino) energy. al.10). The counting rates were converted to actual Furthermore, one could suppress the residual atmo- fluxes by taking account of the aperture and the angu- spheric muon background by utilizing the widely dif- lar distribution of muons at each depth of observation. ferent features of the angular distributions. While at- The angular distribution was known only for the very mospheric muons have a steep distribution with a peak shallow depths at that time and one had to infer it for in the vertical direction, the neutrino-induced muons great depths from the Depth vs Intensity relation of have a nearly isotropic distribution but with an energy- muons. dependent excess in the horizontal direction. This fea- The angular distributions of muons were mea- ture was exploited in designing the detector arrays as sured at KGF by a TIFR–Durham University exper- horizontal telescopes, which provided maximum sen- iment (Achar et al.9) using neon flash tubes (glass sitivity for detection of neutrino events. tubes of 1 cm diameter, 2 m long, filled with spec- Very little was known at this time from the accel- troscopic neon and sealed) as visual track detectors; erator experiments about neutrino interactions at en- they had a resolution of approximately 10 in the pro- ergies beyond a few GeV and the success of cosmic jected zenith angle of the tracks.
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