Creation and Evolution of Excited States in α Particle Tracks in Anthracene Crystals

Abstract

Abstract The kinematics of excited states in anthracene crystals bombarded by 5 MeV x particles is studied. The elementary processes which account for the transitions from the primary excited states to the lowest singlet S1, and triplet T 1, excited states is described. The equations governing the evolution of the S 1 and T 1 excitons in the x particle track are then solved, and the scintillation decay curve is calculated. This calculGed result is in good agreement with all available experimental results. The experimental part of this work are scintillation decay curves measurements. The scintillation decay was measured between 0.5 nsec and 40 μsec. The influence of the initial very fast singlet excitons quenching by triplet excitons can be seen in the beginning of the scintillation. The delayed component isdescribed by the triplet exciton kinematics. The magnetic field effect on the scintillation was investigated. This effect is attributed to an effect on the T 1 - T 1 annihilation and an effect on the triplet excitons quenching by radicals which are formed in the x particle track. The problem of creation and evolution of excited states in organic crystals bombarded by x particles is studied by analysing the radioluminescence: the intensity of the emitted light as a function of time. temperature. external magnetic field. and direction of the x particle. An anthracene crystal bombarded by ionizing particles exhibits the same eniission spectrum as the fluorescence spectrum which is attributed to the radiative transition from the first excited singlet state S 1 to the ground state S o(S 1 → S 0 + hv). The formation of the S 1 excitons is a result of a sequence of elementary processes which we review in this paper. In a first stage (10−15 sec) the primary x particle and the secondary electrons are slowing down in the organic material and create excited states in a broad energy range at about 20 eV (excitons. plasmons. charge carriers). In a second stage, from 10−15 sec to 10−12 sec the absorbed energy is dissipated in the crystal and transformed into singlet (S 1) and triplet (T 1) excitons and phonons by non radiative transitions: electronic and vibrational relaxation. autoionization and charge carriers recombinations. In the third stage (10−12 sec to 10−5 sec) occurs the emission of light which can be studied experimentally in the range 10−9 sec to 10−5 sec. During this stage, also take place the diffusion ofthe excitons and bimolecular annihilations. The light emission of the singlet excitons resulting from the two first stages is the “prompt component” of the scintillation; its decay is not exponential, and extends over tens of nanoseconds; the light arising from singlet excitons created by annihilation of two triplets (T 1 + T 1 → S 1 + S 0) is the “delayed component” and lasts microseconds. In Section I we develop a kinetic study of the scintillation. as complete as possible, in which we use experimental and theoretical published results about the involved elementary processes. Afterwards we present our experiments (Section II) which are measurements of the intensity of a scintillation as a function of time between I nsec and 40 μsec in various conditions of temperature and external magnetic field. These experimental results are discussed in Section III. The light intensity of the scintillation is calculated between 0 and 40 μsec and compared to our experimental results and other available data. The magnetic field effect have proven to be of great importance in understanding thedetailsofthe triplet exciton interactionsand thecreation of paramagnetic defects in α particle tracks.