In several field of human activity (medicine, industry, control systems, high-energy physics experiments, etc.) faster and more efficient scintillators are required to improve the characteristics of the employed apparatuses. The suppression of the slow components is one of the more important features to be searched, in order to speed up the operations and consequently to reduce the dose to the patient, to the operators, compatibly with light yield maximization and reduction of high radiation damage of the crystal. The technique developed at IFAC-CNR of Firenze for photoluminescence decay kinetics measurements in the uv/visible range is particularly suitable for simultaneous observation of fast (up to 1 ns) and slow (down to 1 s) decay components, with an intensity variation of several orders of magnitude. Decay-kinetics measurements can be carried out on solid samples which show uv/visible photoluminescence by excitation with the lines of an excimer laser (XeCl at 308 nm and KrF at 249 nm) or nitrogen laser (at 337 nm). The emission is detected by a fast photomultiplier (PMT) and recorded by a digital sampling oscilloscope. Time-resolved spectra (with a gate of 20 ns) or steady state spectra can be obtained through an OMA (Optical Multichannel Analyser) in the range 200-800 nm. Emission band shapes are interpreted on the basis of the Jahn-Teller effect and pseudo-Jahn-Teller effect acting on the emitting levels.Many of the above activities are carried out in cooperation with the Physics Institute of Academy of Sciences of Prague - Czech Republic ( M. Nikl), the Physics Institute of Tartu – Estonia (S. Zazubovich), Dipartimento di Scienza dei Materiali – Università di Milano-Bicocca (A. Vedda), ENEA – Casaccia (S.Baccaro) and Dipartimento di Fisica – Università di Roma 3 (F.Somma).

TIME-RESOLVED FLUORESCENCE SPECTROSCOPY ON SOLIDS

M. Bacci, P. Fabeni, D. Mugnai, G.P. Pazzi, A. Ranfagni, C. Susini

Investigated materials:Crystals:

Lead Tungstate (PbWO4)

Perovskites (YAlO3:Ce3+, LuxY1-xAlO3:Ce3+)

Garnets (Y3Al5O12 :Ce3+, Lu3Al5O12 :Ce3+)

to be employed as scintillators in:high-energy physics (CERN),medicine (PET), industry (quality control) and security (airport).

Doped alkali halides (KCl:Tl- type)Quantum Dots in alkali halides (e.g. CsPbBr3 in CsBr:Pb2+)

Amorphous materials:Phosphate glasses (e.g. 50% NaPO3 /40% GdPO4 /10% CePO4)

to be employed as scintillators.

10-6

10-5

0.0001

0.001

0.01

0.1

1

10

10 100 1000 104 105 106

Photo-luminescence decay of YAP 7765/8 (50 ppm Ce),

exc. = 337 nm (N2-laser),emiss.= 480 nm.

laser

decay

2exp.fit

emis

sion

inte

nsity

time (ns)

RT

1 = 16.5 ns

2 = 2450 ns

10-5

0.0001

0.001

0.01

0.1

1

0 200 400 600 800 1000 1200 1400

Decay kinetics of KCl:Tl (100 ppm), excitation by pulsed Xe-lamp (50 W) at 252 nm,

475 nm emission at 50K. Deconvolution by the sum of three exponentials:

interpretation of the anomalous slow-components decay by the tunneling effect in a Jahn-Teller model.

50K50K-fit3lamp

emis

sion

inte

nsity

(a. u

.)

time s)

s) A4.6 0.84185 0.02810940 0.00043

JAHN-TELLER EFFECT

If a non-linear molecule (or polyatomic ion) has a degenerate electronic level (apart from Kramers degeneracy) it is unstable with respect to displacements of the atoms

A vibronic degeneracy substitutes the original electronic degeneracy

The above theorem plays an important role not only in determining the crystal structure but also in affecting the spectroscopic properties of the involved physical systems.

The structured emission band has been interpreted on the basis of an excited state consisting of two closely lying triplet levels (3T1, 3T2). Ligand field, spin-orbit, Jahn-Teller and pseudo Jahn-Teller interactions were included in the Hamiltonian. The temperature dependence of the band shape (solid line: 4 K; long dashed line: 300 K) is well accounted for by considering a quantum distribution function on the excited states.

INTERPRETATION OF THE EMISSION SPECTRA OF THE SCINTILLATOR PbWO4

Selected references: 1) M. Nikl, K. Nitsch, E. Mihokova, N. Solovieva, J.A. Mares, P. Fabeni, G.P. Pazzi, M. Martini, A.. Vedda , S. Baccaro, Appl. Phys. Letters, 77, 2159-2161 (2000); 2) M. Nikl, P. Bohácek, E. Mihóková, N. Solovieva, A. Vedda, M. Martini, G. P. Pazzi, P. Fabeni, M. Kobayashi, J. Appl.Phys., 91, 2791-2797 (2002); 3) A. Ranfagni, D. Mugnai, P. Fabeni, G.P. Pazzi, Phys.Rev. B 66, 184107/1-5 (2002); 4) M. Bacci, E. Mihokova, and L. S. Schulman, Phys. Rev. B, 66 (2002) 132301.

10-6

10-5

0.0001

0.001

0.01

0.1

1

10-7 10-6 10-5 0.0001

PL decay of phosphate glasses 4/1 (Na52%,Gd45%, Ce3%),308 nm excitation (XeCl-laser), 350 nm emission.

laser

19 K

80 K

200 K

300 K

emis

sion

inte

nsity

(a. u

.)

time (s)

slow componentsincrease

10-5

0.0001

0.001

0.01

0.1

1

10-8 10-7 10-6 10-5

laser

Mo160

Mo160Y80

Mo160Ce40

emis

sion

inte

nsity

(a.u

.)

time (s)

PL decay kinetics of PWO:Mo,Ce or Y-codoped, 308 nm excitation (XeCl-laser), 500 nm emission.

RT

Istituto di Fisica Applicata “Nello Carrara” IFAC – CNR, Firenze

http://www.ifac.cnr.it