We have been systematically carrying out experimental and simulation studies of the polarization properties of atmospheric ˇCerenkov events (ACE) produced by very high energy (VHE) and ultrahigh energy (UHE) γ-ray and cosmic

ray proton progenitors. We present here an interim report on the work, based on some recent simulation investigations

Bhat, C.L.

Bhattacharyya, S.

Dumora, D.

Gokhale, V.N.

Procureur, J.

Rannot, R.C.

Tickoo, A.K.

English

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IL NUOVO CIMENTO Vol. 24 C, N. 4-5 Luglio-Ottobre 2001Polarization state of atmospheric Cˇerenkov events —guidancefrom simulation studies(∗)V. N. Gokhale(2), S. Bhattacharyya(2), R. C. Rannot(2), D. Dumora(1)J. Procureur(1), A. K. Tickoo(2) and C. L. Bhat(2)(1) Centre d’Etude Nucle´aire de Bordeaux-Gradignan, Universite´ de Bordeaux 1rue du Solarium, 33175 Gradignan Cedex, France(2) Bhabha Atomic Research Centre, Nuclear Research Laboratory - Mumbai 400085, India(ricevuto il 6 Novembre 2000; approvato il 12 Febbraio 2001)Summary. — We have been systematically carrying out experimental and simu-lation studies of the polarization properties of atmospheric Cˇerenkov events (ACE)produced by very high energy (VHE) and ultrahigh energy (UHE) γ-ray and cosmicray proton progenitors. We present here an interim report on the work, based onsome recent simulation investigations.PACS 96.40 – Cosmic rays.PACS 96.40.De – Composition, energy spectra, and interactions.PACS 96.40.Pq – Extensive air showers.PACS 01.30.Cc – Conference proceedings.1. – IntroductionIt is expected from elementary considerations of the development of extensive airshowers (EAS) in the atmosphere that the accompanying atmospheric Cˇerenkov events(ACE) should be significantly linearly polarized, with polarization vector (−→p ) orientedtowards the impact point of the EAS core on the ground. If so, as was pointed out firstby [1], simultaneous measurements of −→p of an ACE at 2 or more locations on the groundcan provide an independent method for locating EAS cores and hence estimating, amongother things, the primary energy. Equally interestingly, the two “information-bits” asso-ciated with the −→p , viz., degree and angle of polarization, may have a useful diagonisticpotential and may help to classify the progenitor type—an important requirement forVHE and UHE γ-ray astronomy and cosmic ray mass composition studies through theatmospheric Cˇerenkov detection technique.(∗) Paper presented at the Chacaltaya Meeting on Cosmic Ray Physics, La Paz, Bolivia,July 23-27, 2000.c© Societa` Italiana di Fisica 675676 V. N. GOKHALE, S. BHATTACHARYYA, R. C. RANNOT, ETC.Core Distance (m)0 20 40 60 80 100 120 140 160 180 200Degree of Polarization0.00.10.20.30.40.50.60.70.8 1 TeV gamma 2 TeV protonFig. 1. – 〈p〉 as a function of core distance for 1 TeV gamma and 2 TeV proton.Previous work on the potentially important subject has primarily concentrated on thedegree of polarization information through estimation or actual measurement of relativeradial and transverse ACE flux components. Thus, first experimental work by Galbraithand Jelley [2] led them to the qualitative conclusion that the Cˇerenkov photon flux withelectric vectors (polarization directions) along the direction of an EAS core is compara-tively more than that along the perpendicular direction, implying a non-negligible degreeof linear polarization, at least for the overall event population. More quantitative sim-ulation and experimental measurements on degree of polarization of ACE produced byprotons and iron primaries of ∼ 1017 eV energies have been done by [3]. They concludedthat both the types of ACE are significantly polarized, with a peak degree of polariza-tion ∼ 30% at ∼ 125 m from the associated EAS core, beyond which ACE becomesless polarized, reaching essentially zero degree of polarization at 600 m core distance.More importantly, they concluded rather pessimistically about the possible progenitorclassification role of the degree of polarization by underlying the fact that this parameterchanges only by 5% for each 100 g cm−2 change in the depth of cascade maximum. Morerecently [4] and [5] have discussed the results of their simulation studies of polarizationstate of ACE produced by γ-ray and proton primaries in the TeV energy region. Whileagain concluding that both the event types are significantly linearly polarized upto a coredistance of ∼ 125 m or so, they have opined that the differences in the magnitude of thisPOLARIZATION STATE OF ATMOSPHERIC CˇERENKOV EVENTS ETC. 677Core Distance (m)20 40 60 80 100 120 140 160 180 200  ∆θ (deg)-30-20-100102030 1 TeV gamma2 TeV protonFig. 2. – ∆θ as a function of core distance for 1 TeV gamma and 2 TeV proton.parameter between the two event types seem to be too small to significantly supplementor enhance the event characterization capability of a typical Cˇerenkov imaging telescope.In the present work, we would like to report some new results, based on our ongoingstudies using VHE and UHE γ-ray photon and cosmic ray proton particle beams.2. – Data base generationThe CORSIKA air-shower simulation code, suitably modified for the present exercise,has been used for generating data-bases in the VHE and UHE regions, viz., 1 TeV (2 TeV)for γ-rays (protons) in the former case and 200 TeV (400 TeV) for γ-rays (protons) inthe latter case. In this specially adapted CORSIKA version, in addition to registeringthe direction cosines of the propagation vector of each Cˇerenkov photon reaching ourobservation level, we also log the arrival direction of each Cˇerenkov light-producing sec-ondary electron and muon. These two directions together enable to infer the electric (orpolarization) vector orientation of a given Cˇerenkov photon. The azimuthal angle of theprojection of this vector on the observation plane with respect to, say, the geographicalN-S direction is taken as the effective polarization vector orientation. The data-baseshave been generated for our observatory location at Mt. Abu in Rajasthan, Western In-dia (24.62◦N, 72.75◦E, 1275 m a.s.l.). Both the types of primary particles are made to beincident on the atmosphere along a representative zenith angle of 20◦. Cˇerenkov photons678 V. N. GOKHALE, S. BHATTACHARYYA, R. C. RANNOT, ETC.Core  distance  (m)0 20 40 60 80 100 120 140 160Degree   of   Polarization0.100.150.200.250.300.350.400.45gammaprotonFig. 3. – 〈p〉 as a function of core distance for 200 TeV gamma and 400 TeV proton.produced in the wavelength range λ ∼ 300–600 nm and escaping atmospheric absorptionto reach the ground level are recorded along with all related information, viz., coordi-nates of production point, direction cosines of the Cˇerenkov photon and or the parentelectron or muon, the coordinates of the impact point on the ground, the wavelengh ofthe detected photon and the Cˇerenkov photon bunch size, are used in the simulations.In the VHE region the Cˇerenkov photons are sampled with the help of a linear detectorarray, comprising 7 × 2 m × 2 m light collectors with the nearest-neighbour spacing of4 m. In the UHE region, on the other hand, the ACE is sampled with the help of asquare array of 144×1m×1m detectors, covering an overall area of 220 m ×220 m withan inter-element spacing of 20 m. In both these cases, the shower core is made to becoincident with the centre of the middle detector.The ACE photons collected by each detector for a given event are divided into 3 equalparts which are assumed to go through 3 ideal linear polarizers (no attenuation) whoseaxes have been pre-fixed to be shifted by 120◦ with respect to one another. Each polarizerwill thus transmit photons as per ∼ cos2 χ functional form, where χ is the angle betweenthe photon electric vector direction (projected) and the polarizer axes. If Ii(i = 1–3)are the transmitted fluxes in the 3 cases, the degree (p) and angle (θ) of polarization arePOLARIZATION STATE OF ATMOSPHERIC CˇERENKOV EVENTS ETC. 679Core  distance  (m)0 20 40 60 80 100 120 140 160 180∆θ  (deg)-10-8-6-4-20246810gammaprotonFig. 4. – ∆θ as a function of core distance for 200 TeV gamma and 400 TeV proton.given by [6]p =2√I1(I1 − I2) + I2(I2 − I3) + I3(I3 − I1)I1 + I2 + I3,(1)θ =12tan−1√3(I3 − I2)(2I1 − I2 − I3) .(2)3. – Results and discussionFigure 1 gives plots of average degree of linear polarization (〈p〉) as a function of thecore distance R in the VHE case, i.e. for γ-rays of 1 TeV energy and protons of 2 TeVenergy. For γ-rays, 〈p〉 is found to increase more or less monotonically from ∼ 10%at R ∼ 15 m to a peak value of ∼ 50% at R ∼ 130 m and then decrease to ∼ 30%at R ∼ 180 m. In case of VHE protons, on the contrary, 〈p〉 is found to have a weakdependence on R. These results are in good agreement with those obtained by Hillas [4]for comparable primary energies. With regard to the angle of polarization information,it is convenient to define the parameter ∆θ as the error in predicting the core directionthrough measurements of the polarization vector, −→p—it is the difference in the direction680 V. N. GOKHALE, S. BHATTACHARYYA, R. C. RANNOT, ETC.Core Distance R (meters)0 20 40 60 80 100 120 140 160 180∆R / R0.10.20.30.40.50.60.70.80.9gammaprotonFig. 5. – Distribution of ∆R/R as a function of R for 200 TeV gamma and 400 TeV proton.of −→p as given by eq. (2) for a given detector element and the actual core direction, i.e. theline joining the centre of the element with the array centre, the latter being the knownposition of the shower core in the present simulation exercise.Figure 2 gives a plot of ∆θ as a function of the EAS core-distance (R), as predictedby measurements made for the 7 element linear array used by us in the VHE case. As isevident from the figure, ∆θ is corrrectly predicted to be ∼ 0◦ for both γ-ray and protonprogenitors, as would be expected from the above-referred consideration (see sect. 1)that the polarization vector is oriented along the EAS core, which, in the present case,is known to be coincident with the centre of the linear detector array.The corresponding behaviours of p and ∆θ as a function of R for UHE γ-ray andproton primaries are shown in figs. 3 and 4, respectively. Again, 〈p〉 is found to increasesystematically for both particle species for R ∼ 30–100 m range, between 16–32% forγ-rays and between 16-25% for protons. For larger R, 〈p〉 is seen to decrease, reaching avalue of ∼ 15% at R ∼ 150 m. Figure 4 gives the resulting plots of ∆θ as a function ofR for, both, γ-ray and proton progenitors. For R ∼ 60–130 m, the most “active” core-distance range where most of the ACE are expected to be recorded in actual practice,∆θ is found to be reasonably small for both primary species. ∆θ is found to increase forsmaller and larger R values lying outside the above referred core-distance interval.How is this behaviour of ∆θ expected to reflect on the corresponding error in thePOLARIZATION STATE OF ATMOSPHERIC CˇERENKOV EVENTS ETC. 681location of EAS core through the ACE polarization technique? To estimate this error,represented by the quantity ∆R/R, we proceed as follows: We take all the array elementswhich are equidistant from the array centre—the number of such equidistant detectorelements varies from 4–128. We determine the point of intersection of all the polarizationvectors which are associated with each set of equidistant detector-elements. As each suchpoint of intersection is a possible core position, as follows from the hypothesis that thepolarization vectors of a majority of Cˇerenkov photons in an ACE point towards the EAScore, the error ∆R/R in the core position is readily obtained by noting the separation ofeach such point of intersection from the array centre. Figure 5 gives plots of ∆R/R as afunction of the core-position for UHE γ-ray and proton events.A comparison of this figure with the corresponding plots of fig. 4 shows that ∆R/Rclosely mirrors the behaviour of ∆θ which is quite understandable. The important thingto note is that for the most important R range ∼ 60–130 m, the error ∆R/R in deter-mining the shower core through polarization technique is expected to be ≤ 20% for bothgamma and proton primaries, which is quite reasonable and compares favourably withthat expected from the conventional Cˇerenkov wavefront fitting or lateral distributionfunction techniques.We are presently extending these investigations to intermediate energy interval of tensof TeV and are including other cosmic ray nuclei, like neon and iron nuclei in the eventprogenitor family. Results of our experimental studies on the polarization properties ofACE in the UHE region have been discussed in [7].4. – ConclusionsSimulation investigations carried out here confirm the work of other authors that ACEproduced by VHE and UHE particle-beams (γ-ray and protons) are significantly linearlypolarized, the mean degree of linear polarization for γ-ray events being systematicallyhigher than for proton events over the core distance range R ∼ 60–130 m. It is also shownthat angle of polarization information can be effectively used to locate the shower-corefor, both, γ-ray and proton events, the mean error ∆R/R in the core-location being≤ 20% over the core distance range 60–130 m.REFERENCES[1] Bhat C. L. et al., Bull. Astron. Soc. India, 27 (1999) 343.[2] Galbraith W. and Jelley J. V., J. Atmos. Terr. Phys., 6 (1955) 250.[3] McComb T. J. L. et al., Proceedings of the 16th ICRC (Kyoto), vol. 9 (1979) p. 53.[4] Hillas A. M., Space Sci. Rev., 75 (1996) 17.[5] de la Calle I. et al., Proc. GeV-TeV Gamma Ray Astrophysics Workshop, AIP Conf.Proc., 515 (1996) 515.[6] Sen A. K. et al., ICARUS, 86 (1990) 248.[7] Tickoo A. K. et al., Proceedings of the 26th ICRC (Salt Lake City) OG 4.3.19 (1999).

Polarization state of atmospheric Cerenkov events-guidance from simulation studies

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Polarization state of atmospheric Cerenkov events-guidance from simulation studies

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