Cosmic Ray Research activities at the Physics Department,

Gauhati University

Kalyanee BoruahProfessor in Physics, GU

8th. Winter Workshop and School on Astroparticle Physics (WAPP 2013), 17-19 Dec, 2013

Centre for Astroparticle Physics and Space Science, Bose Institute, Mayapuri, Darjeeling

Plan of my talk

1.Review of work by GUCR group and important findings

2.Mini-array concept & early work3.Radio-emission from EAS4.Application of CORSIKA Simulation5.New proposal to study atmospheric effects.

Review of Cosmic rays Research by GU Cosmic Ray (GUCR) Group (1970-present)

• First Stage : 1970-82 : EAS array with GM trays (up to primary

energy 1016eV), Cerenkov detector (Photomultiplier Tube & parabolic mirror) & radio antenna. Pulses were recorded by photographic method. LF radio-emission (30,40 & 60 MHz) detected using wide band half wave dipole antenna & HF (80,110 & 220MHz) using Yagi antenna. Analytical & MonteCarlo simulation of cerenkov emission

Important Findings (1970-1982)

Measured Cerenkov pulse height and EAS rate spectrum in agreement with Monte Carlo simulation with pure proton composition at about 10^16eV primary energy.

Correlation study between pairs of (LF,MF) radio frequencies showed positive correlation when both frequencies are above or below the theoretical cutoff frequency (75MHz), while negative correlation between higher and lower frequency pairs, showing different emission mechanisms.

Radio field strength increases at low frequency

Review of Cosmic rays Research… Second stage : 1982-94

: Conventional EAS array with plastic scintillators (up to Ep= 1017eV) with DST & ASTEC funding. More optical cerenkov and radio antenna with receivers installed.

Microprocessors used for controlling and automatic data recording.

Fortran programs developed for simulating EAS in detail using Monte Carlo technique.

Theoretical study of radio and cerenkov emission.

Measurement of lateral distribution of Cerenkov emission indicated proton enhancement above 10^16eV primary energy.

Rise of field strength of radioemission with lowering of radio frequency to VLF (Very Low Frequency) region. This could not be explained by any of the existing theories, but by a new method called Transition Radiation phenomenon that occurs when a charged particle crosses a boundary between two media of different dielectric properties.

Important Findings (1982-1994)

Third stage : 1994- present• 1994-2004 :DAE-BRNS project of UHE

cosmic ray detection by miniarray of eight scintillation detectors covering 2 sq m area. Microprocessor based & computer controlled data acquisition system. (published in NIM & AstroparticlePhysics.)

• Theoretical simulation of hadronic interaction, Higgs production (published in PRD) .

• Fabrication of RPC detectors for cosmic ray detection(ISRO Project & ICTP-TWAS grant).

Review of Cosmic rays Research…

• Study of 30kHz radio-emission using loop antenna with miniarray.

• Study of low cost, efficient RPC design. • Simulation using CORSIKA code for study of

charm production, model dependence, mass composition, LF radioemission, gamma hadron discrimination & neutrino production.

• Work on digital signal processing for analysing radio pulses recorded in association with EAS.

• Design of FPGA based trigger, monitoring & control system for particle detector array in collaboration with Dept. of Instrumentation, GU.

Review of Cosmic rays Research… 2004-13 :

• GU miniarray could detect UHE cosmic rays of primary energy 1017-1018 eV.

• Efforts have been made to detect radio emission associated with UHE cosmic ray air showers as detected by the miniarray detector, using loop antenna, placed close to the miniarray. However, when triggered by miniarray pulse, no coincidence was observed. On the other hand when the miniarray channel was decoupled, radio-radio coincidence could be observed.

Important Findings (1994-present)

The new findings may be explained by a model based on mechanism of transition radiation, which shows that the radio antenna picks up signal emitted by excess charged particles after striking ground. Calculations based on CORSIKA simulation shows that this effect is detectable near the core of an EAS, where particle density is large. Mini array being effective at a large distance >300m from the core requires a distance and a time delay(~10μs) . Their acceptance area are not overlapping. Therefore, the mini-array and the loop antenna cannot be placed close to each other.

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Mini array Concept – Arrival time spread (thickness of shower front in ns) increases with core distance

• A mini-array is a low cost and an unconventional particle detector array capable of detecting UHE cosmic ray air shower using Linsley’s effect, i.e, increase of shower disk thickness with core distance.• Gauhati University mini-array (operated during 1994 to 2007) consisted of eight closely spaced plastic scintillators, and could detect Cosmic rays of primary energy 1017–1018 eV.

Mini-array method

BLOCK DIAGRAM OF THE EXPERIMENTAL SETUP

•Linsley derived the empirical formula, using experimental data of Volcano Ranch Array σ(r) = Brβ . . . . , (1) where B = 0.0158 and β = 1.5

• Capdevielle et al. derived the same from their simulation with CORSIKA (near vertical shower ) σ(r) = B(1+r/c)β . . . . , (2) where B = 2.6, c = 25 and β = 1.4

Thickness of shower front ‘σ’ in ns

• LDF as used for earlier miniarray, for large shower and large core distances is given by ρ(r) = CNr-n. Where, C= 853, N= shower size & n= 3.8.• GU miniarray detector system is designed specially to measure both charged particle density and arrival time spread at the detector level. Core distance ‘r’ and shower size ‘N’ are derived as secondary parameters.

Lateral Distribution Function

The effective area of mini array A(N) is an annular ring with inner radius rmin

Determined by minimum time spread σ1

& outer radius rmax determined by density threshold ρ1 as selected

A(N) = π( rmax2 - rmin

2)

Effective area of the miniarray

Radio-emission : Historical development

Theory

1960- Askaryan predicted radio Cerenkov from –ve charge excess.

1966- Kahn & Lerche developd geomagnetic charge separation model of dipole & transverse current through the atmosphere.

Experiment

1965- Jelley detected 44MHz radio pulse associated with EAS => Intensive research VLF(few kHz) to VHF (hundreds of MHz).

1967- Allan found polarization depends on geomagnetic field.

1970 - Experimental work ceased due to technical problem, man-made interference & advent of alternative techniques.

Correlation study between pairs of (LF,MF) radio frequencies showed positive correlation when both frequencies are above or below the theoretical cutoff frequency (75MHz), while negative correlation between higher and lower frequency pairs, showing different emission mechanisms.

Radio field strength increases at low frequency

1970-84 : Work done by GUCR Group

Later development• 1985 – Nishimura proposed Transition Radiation (TR)

mechanism to explain high field strength at low frequency (LF)

• 2001- Askaryan type charge excess mechanism plays a major role in dense media such as ice & used to detect neutrino induced shower (RICE)

• 2003- Falcke & Gorham proposed coherent geosynchrotron radiation from highly relativistic electron positron pairs gyrating in earth’s magnetic field.

• 2004- Huege & Falcke: analytic calculation using synchrotron theory from individual particle is applied to air showers. Detailed Monte Carlo simulation is used to study dependence on shower parameters.

Present understanding• UHECRs produce particle showers in atmosphere• Shower front is ~2-3 m thick ~ wavelength at 100 MHz• e± emit synchrotron in geomagnetic field ~ 0.3G (10-

100MHz)-Geosynchrotron emission. • Emission from all e± (Ne) add up coherently• Radio power grows quadratically with Ne.

• The mechanisms for the highest and the lowest frequencies are found to be very different.

• VHF emission is well explained by geo synchrotron mechanism, but VLF (<1MHz) emission is yet unclear, may be explained by Transition radiation mechanism.

Geomagnetic charge Segmentation

LF radio emissionKahn & Lerche’s Model.

transition radiation emission from a charge e

Transition Radiation• The existence of Transition radiation was

first suggested by Frank & Ginzburg(1946)• emitted when a uniformly moving charged

particle traverses the boundary separating two media of different dielectric properties.

• Later, Garibian deduced wave solutions in the radiation zone, a method used by Dooher (1971) to calculate Transition radiation from magnetic monopoles.

• We extend and apply TR theory to develop a prototype model for radioemission following Dooher’s approach.

Theoretical Model:• This method involves solving Maxwell’s

equations and resolving field vectors into Fourier components with respect to time as suggested by Fermi [1940]. The magnetic field component of the Transition Radiation field is effective in producing induced current in the loop antenna.

• A FORTRAN program is written to calculate the arrival time of the transition radiation at the position of the loop antenna, from different elements of the shower front after striking ground, and the corresponding induced field strength , using charge excess derived from CORSIKA simulation..

Geometrical Model

SIMULATION• The excess charge distribution at the ground level

are estimated using CORSIKA simulation code.• The particle output file from CORSIKA is first

decoded with available FORTRAN code and the decoded output is further processed with a C++ program to get the excess of e- over e+.

• The whole ground area (assumed plane) is divided into elements of area 10m × 10m. Negative charge excess and their average arrival time are recorded for each element using a Fortran program.

• Another program is written to evaluate the inducing electric field at the loop antenna due to k th element on the ground and the corresponding arrival time, to get the pulse profile. This information is transformed to the frequency domain by using DFFT.

(a) Radio pulse profiles and (b) dependence of peak field strength on primary energy at 300 m lateral distance from the shower centre.

(a) (b)

Comparison with REAS 3 and experimental observation due to Hough et al. at 1017 eV (left) and Comparison with earlier GUCR model at 1018 eV. 

Radio emission Result

• We have used a simple geometrical model for production of TR from cosmic ray EAS using charge excess distribution as calculated from CORSIKA Simulation. The model helps to establish the observed higher field strength at lower frequency.

• Also information about primary energy and mass composition may be obtained from measurement of radio frequency and field strength.

• Loop antenna for detection of LF radio-emission associated with giant EAS may be operated along with miniarray suitable for UHE giant EAS detection by suitably adjusting the time delay due to TR.

Photographic view of the Experimental Setup.

Application of CORSIKA Simulation

1.Mass Composition2.Study of model dependence

3.Gamma hadron discrimination

4.Neutrino shower

CORSIKA SIMULATION• Success of an air shower experiment in respect of design and measurement depends on accurate theoretical simulation work. • Standard EAS simulation code CORSIKA is a detailed Monte Carlo program to study the EAS in the atmosphere initiated by photons, protons, nuclei and many other particles. It recognizes more than 50 elementary particles & gives type, energy, momentum, location, direction and arrival times of all secondary particles that are created in an air shower and pass selected observation level.

Lateral density distribution of cherenkov photons for proton & Helium primary of energy 10^17 eV.

Mass Composition study

Lateral density distribution of cherenkov photons for Oxygen & Iron primary of energy 10^17 eV.

Mass Composition study

Parameterization for lateral density distribution at 1017eV

Mass Composition study

Study of model dependence

Study of model dependence

Study of model dependence

Gamma hadron discrimination

Gamma hadron discrimination

Gamma hadron discrimination

Gamma hadron discrimination

Application of Principal Component Analysis (PCA) method for Gamma-hadron separation

Gamma hadron discrimination

Neutrino shower

Neutrino shower

Neutrino shower

Neutrino shower

Neutrino shower

Neutrino shower

New Proposal : Simulation and Design

of the water Cerenkov array

Scientific backgroundBased on our success in detecting High Energy

cosmic ray showers using various detectors, and expertise in Cerenkov and Scintillation detectors operating with microprocessor and computer based high speed data acquisition system, we now propose to set up a small air shower array consisting of water Cerenkov detectors at the roof top of Physics building of Gauhati University. Water Cerenkov method was used by us to calibrate our air Cerenkov detectors.

Motivation

The atmosphere acts as a part of the detector for recording an EAS. However, for showers arriving detector level after crossing the maximum development, absorption effect of the atmosphere is dominant. The shower maximum for about 100 TeV is near 500 gr/cm2 (~5200m above sea level) and most of EAS arrays in this energy range are at heights below the shower maximum height, specially for higher zenith angle EAS events. Therefore, for ground based observation of cosmic gamma source, we need to consider the atmospheric absorption effect.

The mechanism of the development of EAS events in the atmosphere affects directly the characteristics of the secondary particles. So it is very important to investigate theoretically the characteristics of the secondary particles, such as longitudinal and lateral distribution, arrival time distribution, zenith angle distribution etc. and to recognize the factors affecting the observable parameters.

The whole EAS phenomenon may be simulated using Monte Carlo Technique and different characteristics may be studied more accurately.

• Here, we have applied results of CORSIKA simulation code to design the proposed detector array in terms of detector size, position and separation.

Longitudinal Distribution of electrons

Lateral Distribution of electrons

The theory of lateral Distribution of the shower particles was developed by Nishimura, Kamata & Greisen and is known as NKG formula

Shower sizes at ground level ‘Ne’ are derived from CORSIKA Simulation, for different primary energy and mass compositions and densities at different core distances calculated. The threshold condition is determined when the core strikes the centre of the array.

We aim to study some factors related with atmosphere by using a small array of Water Cerenkov Detectors (WCD) and by comparison of measurements with simulation results. A minimum of four particle detectors are necessary for measurement of air shower parameters. Preliminary calculation using CORSIKA shows that an array of four WCDs of area 1sq m [radius 56cm] each, positioned at the corners of a square of side 5m may be used to detect primary particle of threshold energy 100TeV. A change in the cross sectional area of WCDs or a change in the length of the side would change the energy threshold.

Detector Design

<--------------------------5m ------------------------>

The array is proposed to be located on the roof of the Physics Department of Gauhati University. The signal output from the PMTs are to be discriminated for suitable trigger and connected to Time to Amplitude Converter (TAC). The time lag of three of the WCDs with reference to the fourth one are to be recorded. These data are to be used for measuring zenith angle distribution which may be correlated with atmospheric thickness and pressure.

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