2420 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 55, NO. 4, AUGUST 2008
Scintillation Properties of Praseodymium Activated
Lu
3
Al
5
O
12
Single Crystals
Winicjusz Drozdowski, Pieter Dorenbos, Johan T. M. de Haas, Renata Drozdowska, Alan Owens, Kei Kamada,
Kousuke Tsutsumi, Yoshiyuki Usuki, Takayuki Yanagida, and Akira Yoshikawa
Abstract—Scintillation properties of LuAG:Pr grown by
Fu-rukawa Co. Ltd., Japan, have been studied. The best crystals display light outputs up to 19000 ph/MeV and an energy resolu-tion of 4.6% at 662 keV. The scintillaresolu-tion 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 - and - transitions ofPr3+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.
Index Terms—Energy resolution, LuAG, praseodymium,
scintil-lation yield, scintillator.
I. INTRODUCTION
R
ARE earth activated wide band gap oxide crystals can be very useful as detectors of ionizing radiation in nuclear and high-energy physics, space exploration, nuclear medicine, and industry. Parity-allowed ultraviolet transitions characterized by a high oscillator strength together with the absence of abundant levels make trivalent cerium one of the most promising activators. Therefore diverse -doped materials such as e.g., perovskites (LuAP) and(YAP), garnets (LuAG) and (YAG),
oxyorthosilicates (LSO) and (YSO), and pyrosilicates (LPS) and (YPS), have been intensively studied in recent years [1]–[11]. In the meantime, the same materials activated with others ions such as or
have also received some attention.
Scintillation properties of -doped lutetium aluminum garnet crystals have been thoroughly investigated recently [12]–[18]. With a density of 6.7 , an energy resolution of 5.2%, and a scintillation decay time of 20 ns, LuAG:Pr is
Manuscript received January 7, 2008; revised March 8, 2008. Current version published September 19, 2008. This work was supported by the European Space Agency, Noordwijk, The Netherlands.
W. Drozdowski is with Faculty of Applied Sciences, Delft University of Technology, 2629 JB Delft, The Netherlands, on leave from the Institute of Physics, Nicolaus Copernicus University, 87-100 Torun, Poland (e-mail: w.a.drozdowski@tudelft.nl; wind@fizyka.umk.pl).
P. Dorenbos, J. T. M. de Haas, and R. Drozdowska are with Faculty of Ap-plied Sciences, Delft University of Technology, 2629 JB Delft, The Netherlands. A. Owens is with the European Space Agency, ESTEC SCI-A, 2201 AZ No-ordwijk, The Netherlands.
K. Kamada, K. Tsutsumi, and Y. Usuki are with Materials Research Labora-tory, Furukawa Company Ltd., Tsukuba, Ibaraki 305-0856, Japan.
T. Yanagida and A. Yoshikawa are with the Institute of Multidisciplinary Re-search for Advanced Materials, Tokohu University, Aoba-ku, Sendai, Miyagi 980-8577, Japan.
Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TNS.2008.2000845
TABLE I
PHYSICALPROPERTIES OF THESTUDIEDSAMPLES
regarded as an attractive material for a number of applications. In this paper we present a standard characterization and some results of more advanced, previously unreported studies on :Pr. Our research comprises temperature-depen-dent measurements of pulse height spectra, scintillation time profiles, and radioluminescence spectra between 78 and 600 K. We also investigate the proportionality and size dependence of scintillation yield, and the intrinsic activity of LuAG:Pr.
II. MATERIALS ANDEXPERIMENT
Single crystals of :Pr were grown by the Czochralski method by Furukawa Co. Ltd., Japan. The concentration of ions in these crystals was determined to fall between 0.22 and 0.24 mol%. Details about the growth technology can be found in [14]. The studied cuboid samples are listed in Table I.
Room temperature pulse height spectra were collected under 662 keV gamma excitation from a source. For the mea-surement of yield proportionality we also employed a , a , and a variable-energy X-ray source. Intrinsic activity spectra were recorded inside a lead castle.
To determine the absolute light output and energy resolution of :Pr, we used the procedures and assump-tions described in [19]. Two detectors, a Hamamatsu R1791 photomulitplier tube (PMT) and an Advanced Photonix SD630-70-72-510 silicon avalanche photodiode (APD), were physically the same, whilst a Hamamatsu S3590-18 silicon PIN photodiode had been replaced with another one of identical type. The studies of proportionality, intrinsic activity, and size dependence of yield, were performed with the PMT. For optimization of observed yield and resolution, the shaping time was set to 3 . The effective quantum efficiency and the reflectivity of the PMT for emission were estimated as 27% and 21%, respectively.
Temperature-dependent pulse height spectra and scintillation time profiles were taken with a setup described by Bizarri [20]. The crystals were kept in clean vacuum inside a Janis cryostat
Fig. 1. A Cs pulse height spectrum of LuAG:Pr (crystal:P02), recorded with a photomultiplier tube. The data symbols come from the experiment; the solid curve is a gaussian fit.
TABLE II
PHOTOELECTRONYIELD ANDENERGYRESOLUTION OF THESTUDIEDLUAG:PR SAMPLES(INCASE OF THEB01ANDB02 BLOCKSALLTHREEMEASUREMENT
GEOMETRIES AREINCLUDED)
and excited by a source. The scintillation decay mea-surement was based on the delayed coincidence single photon counting method [21].
A typical setup consisting of a Philips PW2253 X-ray tube with a Cu anode, operated at 35 kV and 25 mA, an Acton Research Corporation VM504 monochromator, a Hamamatsu R943-02 photomultiplier, a Janis cryostat, and a LakeShore 331 programmable temperature controller, was used to record X-ray excited emission spectra at various temperatures.
III. RESULTS
A. Scintillation Yield and Energy Resolution
An example of a pulse height spectrum of :Pr, measured with a PMT, is shown in Fig. 1, whereas the values of photoelectron yield and energy resolution (at 662 keV) derived from all recorded spectra are summarized in Table II. One can easily notice that the polished crystals (P01, P02, C01) display higher yields and better resolutions than the unpolished crystals (B01, B02). In case of the latter two, which are cuboidal blocks, the yield is also dependent on the measurement geometry, i.e., on the way of placing the sample on the PMT window. We note that the energy resolution of the polished samples, 4.6–4.8%, is better than the value of 5.2% reported so far [18].
Fig. 2. Photoelectron yield of LuAG:Pr as a function of height (crystals: P02, C01, B02). The data symbols are the values from Table II; the solid curves are fits based on (1).
TABLE III
PHOTOELECTRON ORELECTRON-HOLEPAIRYIELD ANDABSOLUTE SCINTILLATIONYIELD OFLUAG:PR(CRYSTAL: P02)
The decrease of light output with increasing crystal height is a known effect, already characterized quantitatively for :Ce [10], [11], and recently also for :5%Ce [22] and [23]. A simple two-ray (“2R”) model proposed by Wojtowicz et al. [10] introduces a loss parameter , describing the scintillation light loss inside the material caused by optical absorption and photon scattering. The square data symbols in Fig. 2 correspond to the photoelectron yields in Table II. The solid curves result from fitting the “2R” equation:
(1) throughout two sets of data, separated according to crystal polishing. Thereby the following sets of fit parameters have
been obtained: ,
(polished samples), and ,
(unpolished sample). These values indicate that scintillation light loss due to self-absorp-tion and scattering in LuAG:Pr is reasonably small. In this sense :Pr is slightly worse than :5%Ce, ( [22]), but much better than e.g., :Ce (LuAP:Ce, [10]). This feature is of importance for maintaining high yield with increasing crystal size.
Table III contains the values of scintillation light output of a thin plate of LuAG:Pr, determined on the basis of pulse height measurements performed with a PMT, a silicon PIN photodiode, and a silicon APD. The lower value obtained with the PIN pho-todiode may be due to a difference in thickness of silicon dead layers between the detector employed presently and the one used in [19]. Based on the “2R” model we note that the yield of a
2422 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 55, NO. 4, AUGUST 2008
Fig. 3. Yield proportionality of LuAG:Pr (crystal: C01), compared to LaBr :5%Ce [14].
Fig. 4. X-ray excited emission spectra of LuAG:Pr (crystal: B01), recorded below and at room temperature.
1 mm high, polished sample is relatively unaffected by internal light losses. Thus we regard the number of 19000 photons per 1 MeV as the intrinsic absolute light output of :Pr.
Fig. 3 shows the photoelectron yield of LuAG:Pr, displayed as a function of excitation energy. The non-proportionality be-tween 17 and 1274 keV is 8%. Compared to :Ce [22], it is a good result, which is an important reason for the splendid energy resolution of :Pr [24].
B. Radioluminescence
X-ray excited emission spectra of :Pr, measured below and above room temperature, are presented in Figs. 4 and 5, respectively. Three spectral regions can be distinguished:
i) host emission of excitonic origin ( ), ii) luminescence ( ), and iii) luminescence ( ). As illus-trated in Fig. 6, the contribution from these emissions to the ra-dioluminescence of LuAG:Pr varies with temperature. The host emission appears only at low temperatures. The
luminescence dominates almost across the entire thermal range, however above 400 K its contribution clearly decreases in favour of the luminescence. The total radioluminescence keeps increasing up to 600 K.
Fig. 5. X-ray excited emission spectra of LuAG:Pr (crystal: B01), recorded above room temperature.
Fig. 6. Contributions from the host,4f5d ! 4f , and 4f ! 4f emission to radioluminescence of LuAG:Pr (crystal: B01), as functions of temperature.
C. Thermal Dependence of Yield
Fig. 7 displays the values of scintillation yield of :Pr as a function of temperature, derived from two different sets of experimental data: pulse height spectra recorded with an electronic shaping time of 3 and radi-oluminescence spectra limited to the spectral range of the luminescence. Based on the pulse height measurements we observe a strong increase of photoelectron yield between 200 and 450 K. The yield at 450 K is 2.4 times higher than at 200 K. A further rise of temperature results in a decrease of yield, which is in agreement with the descending role of the transitions. A similar tendency is repro-duced by the areas under the X-ray excited
emission, but the maximum of yield is located between 300 and 400 K.
D. Scintillation Kinetics
Scintillation time profiles have been recorded in two time ranges: 0–200 ns and 0–5000 ns for monitoring the fast and slow components, respectively. An example of the latter is provided by Fig. 8. The decay curve at 125 K is formed by three compo-nents with time constants of , , and
Fig. 7. Yield of LuAG:Pr (crystal: B01) as a function of temperature, deter-mined by two different techniques. Both curves are normalized to 1.0 at 600 K.
Fig. 8. A scintillation time profile of LuAG:Pr (crystal: B01) at 125 K.
TABLE IV
FASTSCINTILLATIONDECAYTIMECONSTANT INLUAG:PR AS AFUNCTION OFTEMPERATURE
53%. The two longer components of about 150 ns and close to 1 are present at any temperature from 78 to 600 K. At this stage their nature is unknown. Although the 1 component might be linked to transitions, its contribution is much larger than could be expected based on Fig. 6.
The values of the fast decay constant in the scintillation time profiles of LuAG:Pr, summarized in Table IV, are almost con-stant at 20 ns between 78 and 600 K. However, some exceptions take place. The most distinct one, illustrated in Fig. 9, involves a prolongation of the fast decay time from 20 to 31 ns at 500 K.
Fig. 9. Scintillation time profiles of LuAG:Pr (crystal: B01) at 300, 500, and 600 K.
Fig. 10. An intrinsic pulse height spectrum of LuAG:Pr (crystal: C01).
E. Intrinsic Activity
Naturally occurring lutetium consists of two radioisotopes, of which is stable, whereas has a half-life of
years and an abundance of 2.59%. The only decay mode of is decay with emission of electrons up to 596 keV and transformation of into excited . The corresponding de-excitation -rays have energies of 88, 202, 307, and 401 keV, with the two middle ones being dominant [25].
A self-activity spectrum of :Pr is shown in Fig. 10. The presence of results in a relatively high intrinsic count rate of 210 . The complex structure of the spectrum is built by individual -rays and their sums, together with associated -continua, and expectedly with some contribution of Hf , , , and X-rays. Only the peak at 1460 keV is not related to and must be produced by natural radioactivity of from the surrounding environment.
F. Discussion
Observations such as the dependence of yield on temperature (Fig. 7), the presence of two slow components in scintillation time profiles (Fig. 8), and the prolongation of the fast decay
2424 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 55, NO. 4, AUGUST 2008
constant at 500 K (Fig. 9) suggest an important role of ther-mally activated processes of energy migration in the scintilla-tion of LuAG:Pr. Although at this stage it is difficult to deter-mine the scintillation mechanism precisely, it seems that there are at least two channels of energy transfer from the host to the activator ions: i) consecutive capture of charge carriers at and ii) diffusion of excitons. The former is responsible for the fast component of 20 ns in scintillation pulses at any tempera-ture. Since praseodymium ions are likely to act as efficient hole traps, valence band holes are probably trapped first:
followed by captures of conduction band electrons and ter-minating at excited states of :
The latter mechanism, involving creation and migration of excitons, which is supported by the excitonic emission observed in radioluminescence spectra of :Pr (Fig. 4), would explain the presence of slow components in scintillation time profiles.
We note that the suggested consecutive capture channel of energy transfer is likely to introduce processes of charge car-rier trapping. Existence of traps has been already found to in-fluence the scintillation yield and kinetics of :Ce and :Ce [4], [7]. Most probably LuAG:Pr provides a similar case, nevertheless additional measurements would be necessary to prove it.
IV. CONCLUSIONS
The results presented in this paper demonstrate that Lu :Pr is a promising modern scintillator, characterized by very good energy resolution of 4.6%, low yield non-pro-portionality of 8% between 17 and 1274 keV, fast scintillation response of 20 ns, and very low loss parameter of 0.12 . The light output of 19000 ph/MeV is probably not a strong point, but we have shown that it can be increased by 40% by heating the sample to 450 K. A serious and permanent drawback of LuAG:Pr is its high intrinsic count rate, reducing the suitability for such tasks as gamma detection in low level remote sensing applications.
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