Slučajne koincidencije i komptonsko rasijanje kod višedetektorskog spektrometra tipa PRIPJAT

Abstract

U radu se razmatraju brzine brojanja fonskih, dvostrukih i trostrukih, koincidencija kod spektrometra PRIPJAT, sa šest NaI(Tl) detektora, geometrijom mjerenja ~0.7x4p sr i vremenom rezolucije koincidencija 40 ns. Analizovane su i diskutovane dvostruke i trostruke koincidencije u energetskom opsegu (200-3000) keV, u cijelom dijapazonu i u energetskim prozorima koji odgovaraju oblastima fotopikova gama zraka, koji se u fotopiku registruju samo u integralnom i nekoincidentnom režimu rada spektrometra. Razmatrani su zraci energija 662 keV (deekscitacija 137Ba nakon b - -raspada 137Cs), 911 keV (deekscitacija 228Th pri b - -raspadu 228Ac, u nizu 232Th) i 1461 keV (deekscitacija 40Ar pri raspadu 40K). Brzine brojanja u fonskom spektru, kao i brzine brojanja u spektrima 137Cs, 232Th i 40K nakon oduzimanja fona raspoloživim kombinacijama parova (uglovi ~90° i ~180°) i trojki detektora koje mogu registrovati dvostruke i trostruke koincidencije, pokazale su da glavni doprinos fonskim dvostrukim koincidencijama daju fotoni rasijani iz kristala u kristal.

The counting rates of the background double and triple coincidences at the PRIPYAT spectrometer, with six NaI(Tl) detectors, registration geometry ~0.7x4 sr, and time resolution for coincidences 40 ns, are considered in the work. Double and triple coincidences in the energy range (200-3000) keV, in the whole region and in the energy windows corresponding to the photo-peaks of gamma rays, which are registered in the photo-peaks in the spectrometer integral and non-coincidence mode of counting only, have been analyzed and discussed. Considered rays were ones with the energy 662 keV (de-excitation of 137Ba, after b - -decay of 137Cs), 911 keV (de-excitation of 228Th, b - -decay of 228Ac, in the 232Th series) and 1461 keV (de-excitation of 40Ar, decay of 40K). Counting rates in the background spectrum, as well as in the spectra of 137Cs, 232Th and 40K after background subtraction, obtained by the detector duplets (angles ~90° and ~180°) and triplets of detectors capable to register double and three-fold gamma coincidences, showed that the main contribution to the background double coincidences was given by the photons scattered from crystal to crystal.