An experiment to study cosmic ray protons and nuclei in the 1011–1014 eV energy range (the ‘TUS’ project)

Adv Space Res Vol 26, No 11, pp 1871-1874.2000 0 2001 COSPAR Pubhshed by Elsewer Science Ltd All rlehts reserved Pnnted In &eat Brltaln 0273-l 177/00 $20 00 + 0 00 PII SO273-1177(99)01241-7

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AN EXPERIMENT TO STUDY COSMIC RAY PROTONS AND NUCLEI IN THE 1O11-1O14 EV ENERGY RANGE (THE ‘TUS’ PROJECT) I A Avetisyan’ , Yu P Gordeev2, A N Kvashnm’ , N S Konovalova’ , 0 Yu Nechaev2, M I Panasyuk2, V I Rubtsov’ , Yu I Stozhkov’ , E D Tolstaya2, B M Yakovleg

‘Lebedev Physrcs In&t&e, Russian Academy of Scrences, 117924, Lenmsku prospect 53, Moscow, Russra ‘Skobeltsyn Institute of Nuclear Physics Moscow State Unwersliy, 119899, Moscow, Russia

ABSTRACT An experiment to study cosmtc ray protons and nuclei m the 10’ ‘-1 014eV energy range is discussed The mstrument measures the number of particles m a shower, generated by the primary particle m a lead absorber (-14 cm thick) The mstrument consists of two Cherenkov charge detectors, and two plastic scmtillators viewed by 4 photo multipliers under a lead absorber (see Fig 1) The geometry factor of the m&unrent v&h account for the probabrhty of particle recording is 0 015 m2 sr Computer stmulatlons of the mstrument operation were made to make sure that the mstrument is capable of measuring u-regularmes m the proton spectrum, which presumably exist at E-l 0i2 eV 0 2001 COSPAR Published by Elsewer Science Ltd All rights reserved INTRODUCTION The problem of the existence of an irregularity m the proton spectrum at energies -1012 first arose as the result of the experiment on the ‘Proton’ satellite (Gngorov et al , 1970) The result of this experiment was that the spectrum of protons has the power index y - 1 = 16 at E I lOI and y-1=22-23

at E 2

1012 eV No uregularmes

were discovered

m the spectra of other

components and this gave evidence m favour of a fundamental difference between the spectrum of protons and other nuclei The attempts to confirm this result by means of passive detectors - X-ray emulsion chambers (the JACEE, RUNJOB, MSU and others experiments) did not give an unambtguous result and the problem still remams unsolved (see e g JACEE, 1997) However, it 1s qmte clear that the experimental solution of tins problem will be of great importance for the development of the theoretical concepts of cosmic ray origin (Swordy, 1993) The ‘TUS’ instrument described below installed onboard a satellite, durmg a 6 month exposure is capable of making statistically reliable measurements of the energy spectra and chemical composmon of cosmic rays (CR) m the range of energies 10”-1014 eV The moderate mass and dimensions of the mstrument (-250 kg, 800x600x500 mm - mstde a contamer) permit to install it on board a satellite of the ‘Cosmos’ series

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CONCEPT OF THE ‘TUS’ INSTRUMENT The instrument measures the charge of the mcident particle and its energy release The design is based on the mstruments which were used to measure the hardron spectra at mountam altitudes (These mstruments measured the number of particles m the shower, generated m lead by a high energy hardron and their results were later fully confirmed by X-ray emulsion chamber measurements) A schematic drawmg of the Instrument is given m Fig 1 The mstrument consists of two Cherenkov counters (CC) used to detect the charge of the particles, plastic scmullators (Pl and P2) and lead absorbers (Pbl, Pb2, Pb3)

30

cln

--

!

Fig 1 A schematic drawmg of the ‘TUS’ mstrument. When the charged particle passes through the CC its charge Z is determmed accordmg to the mtensity of the Cherenkov light, generated m a plexiglass disk which has the drameter of 160 mm and thickness of 50 mm One flat surface of the disk 1s m optical contact vvlth the photocathode of the photo multipher (PM-49), the other one is coated m black pamt to suppress backscatter It should be mentioned, that the design of the Cherenkov detectors is based on the expenence acqmred m the ‘Sokol’ experiment, the charge resolution and backscatter suppression capabihties of these detectors are discussed in detail m (Grigorov, 1990) In order to achieve umform sensitivity over the whole PM-49 photo cathode, the sensmvtty was decreased m those locations, where it exceeded the average value (using black pamt) The instrument employs two identical plastic scintillators with 340 mm diameter and 20 mm thickness The top and bottom surfaces of the scmtillators have been pohshed, and the side surfaces covered m black pamt (except the PM-84 positions) to ehmmate reflection After the nuclear mteraction of the primary particle m Pbl an electron-photon cascade develops The flash of l&t m

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The ‘TUS’ Project

the scmtlllator mduced by the electron cascade due to total mner reflectlon m the plastic scmtlllator reaches the photo cathodes of the 4 PM-84 tubes located on the side surface of the scmtlllator at 90” angles respectively to each other The amplitudes of the hght pulses m the 4 photo multlphers permit to establish the coordmates of the electron-photon cascade axis, the same 1s done for the second scmtlllator Thus, it 1s possible to define the direction of the cascade axis and the coordmates of the pnmary particle at the charge detector level The geometry factor of the instrument was calculated accordmg to the analytical formula given m (Kurnosova et al , 1980) r=t

[(S, +S, +7cL2)-

(S, +S, +zL~)~ -4S,S,

1

cm2ster

(1)

For a telescope conslstmg of two circular detectors wth area S, and S, , which are located L cm apart , and for an lsotroplc dlstnbutlon of the mcldent CR In our case the area of one charge detector S, = 200 cm2, the absorber area S, = 900 cm2 and the distance between them 1s L = 30cm For one charge detector

I-, z 150 cm2 ster, and for the ‘TUS’ instrument,

which has two such

detectors, r g 300 cm2 ster With account for the interaction probability of protons m lead which 1s W, =0 5, the geometry factor of the instrument TW =0 015 m2 ster COMPUTER SIMULATIONS

OF THE ‘TUS’ INSTRUMENT

OPERATION

Computer slmulatlons were made to define the accuracy of the ‘TUS’ instrument measurements The GEANT code was used for slmulatlon of lsotroplcally mcldent showers at random posltlons The proton showers started at various depths according to an exponentially dropping survival factor The coordinates of the shower and the number of particles were used to calculate the amphtudes m the eight PM-84 photo multlphers reading the plastic scmtlllators Then the ‘reverse’ problem was solved for the these simulated showers, urlth account for mstrumental errors and the fluctuations m the number of photoelectrons Comparmg the mltlal coordmates of the simulated showers with results of the ‘reverse’ problem solution we obtamed the accuracy of the cascade direction and coordmates measurement

1 E+2

lE+l

1 E+O IE-1

1E+O

1E+l

lE+2

E (Tev)

Fig 2 Incident proton spectrum mth a ‘knee’ (1) and the proton spectrum measured Instrument (2) The units along the vertical axis are arbitrary

by the ‘TUS’

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Thus, rt was determmed, that for pnmary partrcle energies of the order of lOi* eV the instrument will permit to measure the cascade axis drrection with the accuracy of -0 7” and the coordmates of the incident particle at charge detector level v&h the accuracy of -0 2 cm At present the instrument 1s undergoing on-ground cahbratrons, usmg cosmrc ray muons The showers are of the order of several tens of partrcles, and the error m the coordinates at the scmtrllator level are of the order of 1 cm The most crucial question for us was whether the TUS mstrument would be able to record rrregularrtres m the proton spectrum. Simulations show, that rf the mcrdent proton spectrum has a ‘knee’ it wrll be reliably measured by the ‘TUS’ instrument Thrs 1s illustrated in Fig 2 CONCLUSIONS An mstrument wluch 1s based on a prevrously used, and therefore, well tested techmque and has relatively small mass (-250 kg) and dimensions is proposed. The geometry factor of the mstrument wrth account for the proton measurement efficiency 1s 0 015 m2 sr It is planned to launch the ‘TUS’ instrument on a satellite of the ‘Cosmos’ senes Below we make an estimate of the number of the number of protons collected by the instrument during a 6-month exposure The number of particles wrth energy exceeding E, N(> E) recorded by an instrument during exposure time T IS given by the expression N(> E) = I(> E)IYT (2) Where I(> E) IS the integral particle spectrum Assummg, that the spectral mdex for protons remams constant upto E = 100 TeV, usmg the data for I(> E) according to (Ryan et al , 1972), and taking the exposure time T to be 6 months (4380 hours) , the number of protons collected by the ‘TUS’ instrument wrll be -1 5104 events for E > lOI , -3 7~10~events with E > 1013 and - 9 events wrth E > 1014 REFERENCES Gngorov, N , V Nesterov, I Rappoport, et al. Yudernaya Fzzzka , 11, 157-172 (1970) Grrgarov, N , Yadernayu Fzzzku ,51, 157-172 (1990) JACEE collaboration, Proc XXVZCRC, Durban l-4, (1997) Kumosova, L.V., et al , Trudy FIAN, 1980, vol 122, p.49 Ryan, M J , Ormes, J F , Balasubrahmanyan, V K ,Phys Rev Lett ,1972, v 28, p 985 Swordy, S , Proc XXIII ZCRC, Calgary, Invited, Rapporteur and Highlight Papers (1993)