P. Dorenbos

High efficiency of lutetium silicate scintillators, Ce-doped LPS, and LYSO crystals

We have introduced a new scintillator: cerium doped lutetium pyrosilicate Lu/sub 2/Si/sub 2/O/sub 7/ (Ce: LPS). A high light output (average value: 26,300 ph/MeV), a relatively good energy resolution (10%) and a fast decay time (38 ns) without afterglow make this new scintillator very attractive. We compare its properties to those of another recently developed cerium doped lutetium based silicate, Ce: Lu/sub 2(1-x)/Y/sub 2x/SiO/sub 5/ (LYSO).

Scintillation Properties of Praseodymium Activated

Scintillation properties of LuAG:Pr grown by Furukawa Co. Ltd., Japan, have been studied. The best crystals display light outputs up to 19000 ph/MeV and an energy resolution of 4.6% at 662 keV. The scintillation yield is found to be a function of size and temperature of the sample; it can be enhanced by 40% upon heating to 450 K. Radioluminescence spectra show both d- f and f-f transitions of Pr3+ ions; the contribution of the latter increases with temperature. The scintillation decays are complex, with a fast decay constant of 20 ns. The presence of 176Lu induces high background activity.

{\rm Lu}_{3}{\rm Al}_{5}{\rm O} _{12}

A study of the spectroscopic and scintillation properties of Ce doped YAlO3 single crystals is presented. The relation between the method of crystal growth, in vacuum or by the Czochralski method, and the above properties was studied specifically. Vacuum grown crystals show a scintillation light yield of about 14000 photons per megaelectronvolt of absorbed x-ray energy. It is concluded that cation vacancies present in the Czochralski grown crystals cause a decrease of the light yield. The influence of colour centres such as F and F- centres on the spectroscopic and scintillation properties will be discussed.

Single Crystals

Commercially available LaBr3:5% Ce3+ scintillators show with photomultiplier tube readout about 2.7% energy resolution for the detection of 662u2009keV γ-rays. Here we will show that by co-doping LaBr3:Ce3+ with Sr2+ or Ca2+ the resolution is improved to 2.0%. Such an improvement is attributed to a strong reduction of the scintillation light losses that are due to radiationless recombination of free electrons and holes during the earliest stages (1–10u2009ps) inside the high free charge carrier density parts of the ionization track.

Spectroscopy and scintillation properties of cerium doped YAlO3 single crystals

The scintillation properties of RbGd/sub 2/Br/sub 7/ crystals, doped with Ce/sup 3+/ concentrations of 0.02, 0.11, 0.88, 2.05, 4.1, and 9.8%, are studied under X-ray and /spl gamma/-quanta excitations. For the RbGd/sub 2/Br/sub 7/ sample doped with 9.8% Ce, the authors measured a light yield of 56000/spl plusmn/6000 photons per MeV of absorbed /spl gamma/-ray energy with a main decay time of 43/spl plusmn/1 ns, using a Hamamatsu R1791 photomultiplier (PMT), a /sup 137/Cs radioactive source, and a shaping time of 10 /spl mu/s. A time resolution of 790/spl plusmn/10 ps was measured for the RbGd/sub 2/Br/sub 7/:9.8% Ce compound, using BaF/sub 2/ as second scintillator, two XP2020Q PMTs, a /sup 22/Na source, and an energy threshold set at E/spl ges/511 keV. With the R1791 PMT, an energy resolution of 4.1% (FWHM over peak position) for the 662-keV full absorption peak has been observed for two crystals of 7/spl times/4/spl times/2 mm/sup 3/ and 15/spl times/5/spl times/1 mm/sup 3/ with 4.1 and 9.8% Ce content, respectively. Moreover, the nonproportional responses of three RbGd/sub 2/Br/sub 7/:Ce compounds with different concentrations (0.11, 2.05, and 9.8%) were studied revealing an almost-constant light output response from 17.4 keV to 1 MeV. These properties are compared to three other well-known scintillators: NaI:Tl, CsI:Tl, and Lu/sub 2/SiO/sub 5/:Ce.

Improvement of γ-ray energy resolution of LaBr3:Ce3+ scintillation detectors by Sr2+ and Ca2+ co-doping

We have characterized the gamma and X-ray scintillation properties of Ce/sup 3+/ doped Lu/sub 2/Si/sub 2/O/sub 7/, a lutetium pyro-silicate (LPS) material. The rare earth ions are located in an octahedral site with C/sub 2/ symmetry. The compound exhibits chemical stability, transparency in a wide optical range and congruent melting, which allows growing of single crystals of good optical quality. The melting temperature is about 2000/spl deg/C, slightly lower than that of Lu/sub 2/SiO/sub 5/ (LSO). Two maxima at 305 nm and 355 nm are observed in the optical absorption spectrum. The emission spectrum of Ce/sup 3+/ in LPS shows a broad band peaking at 380 nm. For a nominal Ce/sup 3+/ atomic concentration of /spl sim/0.01, under gamma-ray excitation we observe light yields in the range 13000-23000 photons/MeV, depending on the preparation atmosphere. The actual cerium concentration inside the crystal still needs to be determined. The scintillation decay time is around 30 ns and, in the first studies, neither a long component nor afterglow was observed.

Scintillation properties of RbGd/sub 2/Br/sub 7/:Ce. Advantages and limitations

In this work we study the persistent luminescence properties of europium-doped alkaline earth silicon oxynitrides (CaSi2O2N2, SrSi2O2N2 and BaSi2O2N2). All compounds show afterglow emission, with an emission spectrum which is similar to the steady state photoluminescence. The afterglow decay time for BaSi2O2N2:Eu and SrSi2O2N2:Eu is about 50 and 100 minutes respectively, while for CaSi2O2N2:Eu the afterglow intensity is very low. Although the persistent luminescence can be induced by ultraviolet light (250-300 nm) in all three phosphors, only for BaSi2O2N2:Eu low energy radiation (350-500 nm) allows filling of the traps responsible for the afterglow.

A novel inorganic scintillator: Lu/sub 2/Si/sub 2/O/sub 7/:Ce/sup 3+/ (LPS)

The electron and photon response of an aluminum canned LaCl/sub 3/:10% Ce/sup 3+/ crystal were measured using the Compton coincidence technique (CCT). The LaCl/sub 3/:10% Ce/sup 3+/ electron response increases from 7 keV to 30 keV by about 10%. Above 30 keV, the electron response levels, i.e., it is flat within 5%. The Monte Carlo N particle code (MCNP4C) was used in the photon response calculation. The calculated theoretical photon response is in good agreement with the measured photon response. An energy resolution (full width at half maximum over the peak position) of 4.2/spl plusmn/0.5% was observed for the 662 keV full absorption peak. The energy resolution as function of photon energy exhibits a linear relationship with the inverse square root of the energy. The step like curves of NaI(Tl) with a semi plateau in the energy range between 100 and 500 keV have not been observed for LaCl/sub 3/:10% Ce/sup 3+/.

Persistent luminescence in MSi2O2N2:Eu phosphors

The cross-luminescence (CRL) resulting from radiative electronic transitions from the mainly anion-related valence band to the uppermost cation core band has been studied in a number of complex halides containing CsCl, RbF, KF, and BaF2. The energy of the CRL photons is determined by the energy difference of the two bands, and the shapes of the spectra reflect the grouping of the molecular orbitals in the clusters involving cations with a hole in the core shell and nearest-neighbour halide ions. For crystals with more than one type of cation, the spectrum reveals information about the active cation which contains the core hole and also about the other cations. Correlations have been established between the local symmetry of the clusters in different crystals and the number and position of the subbands in the CRL spectra.

Nonproportionality and energy resolution of a LaCl/sub 3/:10% Ce/sup 3+/ scintillation crystal

A universal model for lanthanide (Ln) materials is presented that describes a systematic and material independent variation of the electronic structure over the Ln series (La, Ce, Pr, ..., Lu). The model is derived from experimental data on 4f and 5d energies of Ln ions as impurities in luminescent materials but, as will be shown, can fruitfully be applied to stoichiometric Ln materials as well. The validity and usefulness of the model is demonstrated by application to the Ln sulfides and the well-known Ln oxides LnO, Ln 2 O 3 , and LnO 2 for which the model correctly predicts insulating, semiconducting, or metallic behavior, nature, and magnitude of band gap energies and chemical stability of Ln materials as well as valence and valence changes of Ln ions. The model may serve as a reliable tool to accelerate design of a broad range of Ln materials with deliberately chosen properties.