Sibaji Raha

Bose Institute

Detection of Heavy Cosmic Ray Particle (Z>20) at Hanle, Ladakh, at an altitude of

4.5 km a.m.s.l. using SSNTD

Research GoalsWe are looking for strangelets. They are nuggets of Strange QuarkMatter (SQM) which according to certain theoretical modelsshould be present in cosmic ray flux at high mountain altitudes,albeit with a very low flux.

An ideal way to look for such rare events in cosmic rays at highaltitudes is through the deployment of large area passive (SSNTD)detector arrays.

Advantages of SSNTDs(a) They do not require power for their operation. Hence suitablefor deployment at remote locations(b) Existence of natural thresholds of registration provides aneasy way of reducing the background.(c)They offer good charge and energy resolution.

Choice of SSNTDsA commercially available polymer identified as PolyethyleneTerephthalate (PET) was found to have much higher (Z/β >120) detection threshold compared to other widely usedSSNTDs like CR-39 (Z/β > 6-20), Lexan (Z/β > 57) etc. andhence is the detector of choice for our purpose.

Systematic studies were carried out to characterize andcalibrate PET and also the standard CR-39 detectors(Intercast, Italy make) using heavy ion beams fromaccelerators at IUAC, India; GSI, Germany and REX-ISOLDE, CERN.

Also pilot studies were carried out in which PET as well asCR-39 detectors were given open air exposures at varioushigh altitude locations like Darjeeling (2200m), Ooty (2200m) and Hanle (4500 m) in India.

Calibration curves

PET CR-39

Detectors exposed to cosmic rays

Darjeeling Ooty

Kolkata Hanle

Exposure at Hanle (4500 m)A stack of three A4 size PET detector films (thickness100µm) were mounted on Perspex stands. On top ofthe PET films small (5 cm x 5 cm) pieces of standardCR-39 detectors (thickness 700 µm) were placed.The detectors were given open air exposures for 320days at Hanle (32º 42’ N, 79º 42’ E) in the Ladakhregion of the Himalayas.Charged particle flux recorded on CR-39 at Hanle is5.8 x 10-4/(cm2.s.sr). These are mostly due to cosmicray protons and alphas and also some radon alphas.

Particle tracks on CR-39 after 2hr etching

One unusually large heavy ion track observed at Hanle

One heavy ion track observed onthe bottom surface of one CR-39plate and also on the top surfaceof the PET film placedunderneath

The ion appears to have passed through the CR-39 plate,entered the PET film and then stopped after ~32 µmFrom the track parameter measurements and from the useof the calibration curves we have found Z=25 ± 2From range measurements we have determined the energyof the ion as it impinges on top of the CR-39 plate to be(1.7± 0.2) GeVThe ion was incident on the detector at an angle of ~ 60º

Heavy ion track images

Bottom Surface of CR-39 Continuation of the track on PET

Major Axis distribution of tracks on CR-39 after 2.6 hr etching

VT/VB distribution of tracks on CR-39

Identification of heavy ion from Range vs dE/dx curve on PET

Unusual event

This one heavy ion track was observed after searching an area~ 50 cm2.

This gives the upper limit of the heavy ion flux density as 7.2× 10-10/(cm2.s)

The expected flux density of such primary cosmic ray heavyions at the altitude of Hanle is ~10-20/(cm2.s)

This is a very intriguing observation and further data analysisis in progress.

We are planning to set up large area passive detector arrays atplaces like Hanle (alt. 4500 m) and Sandakphu (alt. 3600 m)in the Himalayan region to begin a systematic search for rareevents in cosmic rays.

Thank You

Thank You