Novel Ultrafast Lu₂O₃:Yb Ceramics for Future HEP Applications

Abstract

Inorganic scintillators activated by charge transfer luminescence Yb³⁺ are considered promising ultrafast material to break the ps timing barrier for future high energy physics applications. Inorganic scintillators in ceramic form are potentially more cost-effective than crystals because of their lower fabrication temperature and no need for aftergrowth mechanical processing. This paper reports an investigation on Lu₂O₃:Yb and Lu_2xY_2(1−x)O₃:Yb scintillating ceramic samples fabricated by Radiation Monitoring Devices Inc. All samples show X-ray excited luminescence peaked at 370 nm. Ultrafast decay time of 1.1 ns was observed by using a microchannel plate-photomultiplier tube-based test bench at Caltech. Considering its intrinsic high density (9.4 g/cm³), Lu₂O₃:Yb ceramics are promising for future time of fight application for high energy physics experiments.

1. Introduction

Inorganic scintillators are widely used in high energy physics (HEP) experiments to construct total absorption electromagnetic calorimeters, providing the best possible energy resolution and position resolution, as well as good electron and photon identification and reconstruction efficiency. The 2019 Department of Energy (DOE) basic research needs (BRN) report [1] points out that ultrafast, radiation hard and cost-effective scintillators are one of the priority research directions for HEP calorimetry. Ultrafast inorganic scintillators are required by future HEP experiments at both the energy and intensity frontiers to mitigate high event rate and pileup. The Compact Muon Solenoid (CMS) experiment is building a barrel timing layer (BTL) consisting of cerium doped lutetium yttrium oxyorthosilicate (Lu_2(1−x)Y_2xSiO₅:Ce or LYSO) crystals readout by Silicon Photomultipliers (SiPM) for the high luminosity large hadron collider (HL-LHC) [2]. Its timing resolution reaches 30 ps. One of the limiting factors for the timing resolution is the 40 ns decay time of LYSO:Ce crystals, which would also cause pile-up in future high-rate experiments. Ultrafast heavy inorganic scintillators with a few nano-second decay time are important to break the pico-second (ps) timing barrier for time of flight (TOF) and ultrafast calorimetry applications for future HEP experiments. An example of ultrafast inorganic scintillator is BaF₂:Y crystal, which is proposed for the Mu2e-II experiment at Fermilab [2], and also for GHz hard X-ray imaging for future Free-Electron Laser facilities [3,4,5]. We use two figures of merit for such applications: (1) the light yield in the first ns, and (2) the ratio between the light yield in the first ns and its total light yield (U/T).

Charge transfer (CT) luminescence was observed in Yb³⁺ (4f¹³) activated scintillators [6]. It features two emission bands (CT state −> ²F_5/2) and (CT state −> ²F_7/2) with an energy difference of about 10,000 cm⁻¹, and a strong thermal quenching. Depending on the temperature and the composition, ultrafast and fast decay time from sub-nanosecond to tens of nanosecond was observed in Yb³⁺ doped scintillators. Among them, Lu₂O₃:Yb shows a high-density (9.42 g/cm³) and large dE/dx (11.6 MeV/cm), so it is attractive for the HEP community. Its high melting point (2490 °C), however, makes the growth of a single crystal expensive. Ceramics are more cost-effective than single crystals because of the following reasons. (1) Ceramic fabrication does not require melting raw material, so can be conducted with a simpler process at a sintering temperature lower than the melting point for single crystals. (2) Ceramic fabrication allows complex shape with minimum after-growth mechanical processing. It thus has a higher raw material usage and low cost. (3) Activator distribution in ceramics is more homogeneous than in crystals by avoiding segregation.

Cost-effective transparent ceramics have been pursued by industry for decades [7]. Previous studies demonstrate that Lu₂O₃:Yb ceramic plates with optical quality approaching theoretical transmittance can be obtained [8,9,10,11,12]. By fine-tuning the Yb³⁺ doping level, light yield of 500 ph/MeV and decay time of ~1 ns were reported [10,13]. Its light yield in the first ns, however, is still low compared to other candidate ultrafast scintillators. On the other hand, radiation hardness of the scintillators must be investigated for applications in a severe radiation environment, such as the HL-LHC or FCC-hh. Systematic investigation was carried out to understand radiation damage in various inorganic scintillators against γ-rays [14], neutrons [15,16], and protons [16,17,18,19]. The radiation hardness of these rare-earth sesquioxide scintillators needs also be checked.

In this investigation, Lu₂O₃:Yb and Lu_2xY_2(1−x)O₃:Yb ((Lu,Y)₂O₃:Yb) ceramics were fabricated by Radiation Monitoring Devices, Inc, Watertown, MA (RMD). Their optical and scintillation performance were measured at Caltech HEP Crystal Lab. Ultrafast decay time was measured by using a microchannel plate-photomultiplier tube (MCP-PMT)-based test bench. Radiation hardness against γ-rays was also investigated.

2. Materials and Methods

Figure 1 shows seven Lu₂O₃:Yb and Lu_2xY_2(1−x)O₃:Yb ((Lu,Y)₂O₃:Yb) ceramic samples used in this investigation.

Table 1. Dimension and composition of the Lu₂O₃:Yb and (Lu,Y)₂O₃:Yb ceramic samples used in this investigation.

ID	Dimension (mm3)	Composition
RMD-2	Φ9 × 1.5	Lu2O3
RMD-3	Φ9 × 1	Lu2O3
RMD-5	Φ9 × 1.5	(Lu,Y)2O3
RMD-6	Φ9 × 1.5	(Lu,Y)2O3
RMD-7	Φ9 × 2	(Lu,Y)2O3
RMD-8	Φ9 × 1	Lu2O3
RMD-9	Φ9 × 2	(Lu,Y)2O3

lists their detailed information.

Their X-ray excited luminescence spectrum (XEL) was measured by using a HITACHI F-4500 spectrophotometer and an Amptek Eclipse-III X-ray tube. Their transmittance was measured by using a Hitachi U3210 spectrophotometer with 0.2% precision. Their light output (LO) was measured by a Hamamatsu R2059 PMT with a grease coupling for 0.511-MeV γ-rays from a ²²Na source with a coincidence trigger. The corresponding systematic uncertainty of the light output data is 1%. The sample RMD-2 was irradiated by γ-rays from a Cs-137 source at Caltech in two steps to reach a total ionization dose (TID) of 10.1 Mrad. This sample was kept at room temperature and wrapped with Al foil to avoid optical bleaching in the entire process. Both transmittance and light output of the sample were measured before and after irradiation at the Caltech HEP crystal lab.

Figure 2 shows the test-benches used to measure the temporal response for (a) BaF₂ samples to 511 keV γ-rays from a ²²Na source with a coincidence trigger, and (b) Lu₂O₃:Yb ceramic samples to a ²⁴¹Am source. A Photek MCP-PMT240 with a rise time and FWHM of 0.18 and 0.82 ns, respectively, was used to measure the scintillation signal. A 2.5 GHz Agilent MSO 9254A was used to collect and process the signal.

Rise time and decay time of the measured scintillation pulse was obtained by fitting the temporal response of the pulse shape with the following equation [20]:

$$V \left(t\right)=A\left(e^{-\frac{t}{{\tau }_d}}-e^{-\frac{t}{{\tau }_r}}\right)+B,$$

(1)

where V is the measured pulse amplitude, B represents background noise, and τ_r and τ_d are respectively the rise time and decay time. The full width at half maximum (FWHM) of the pulse was calculated by the fitting.

The data presented in this paper are not corrected by the Instrument Response Function.

3. Results and Discussions

Figure 3 shows X-ray excited luminescence spectra measured for the Lu₂O₃:Yb ceramic sample 2 (top) and 3 (middle), and the (Lu,Y)₂O₃:Yb ceramic sample 6 (bottom). All three samples show consistent XEL peaked at ~370 nm. A slight difference was observed for sample 6, which can be attributed to the Y admixture.

Figure 4 shows transmittance spectra measured for the Lu₂O₃:Yb ceramic samples 2 (top) and 3 (middle), and the (Lu,Y)₂O₃:Yb ceramic sample 6 (bottom). The XEL spectra (blue dash lines) are also shown in the figure, as well as the theoretical limit of transmittance (black dots) and the numerical values of the emission weighted longitudinal transmittance (EWLT). The theoretical limit of transmittance is calculated by using the refractive index assuming multiple bounces and no internal absorption. The EWLT value represents the numerical value of transmittance over emission spectrum. The (Lu,Y)₂O₃:Yb sample 6 shows poor transmittance. This is due to the scattering centers in the sample.

Figure 5 shows LO as a function of integrated time measured for the Lu₂O₃:Yb ceramic samples 2 (top) and 3 (middle), and the (Lu,Y)₂O₃:Yb sample 9 (bottom). Taking out the emission-weighted quantum efficiency (EWQE) of 20%, these Lu₂O₃:Yb samples shows light yield of up to 280 ph/MeV with negligible slow component. By using the LO data and the decay time from Figure 6 below the corresponding light yield in the first ns and the U/T ratio are 170 photons/MeV and 61% respectively for the Lu₂O₃:Yb ceramic sample 3, which is very promising. On the other hand, doping with yttrium is found to increase the light output, but also introduce slow light with decay time of ~100 and ~2500 ns.

Figure 6 shows the temporal response measured by a Photek MCP-PMT240 for (a) a BaF₂:Y crystal (top) and a BaF₂ crystal (bottom) and (b) one Lu₂O₃:Yb ceramic sample with an ultrafast decay time of 0.6 and 1.1 ns, respectively. The corresponding rise time and FWHM are 0.2 and 0.9 ns for BaF₂ crystals, and 0.3 and 1.5 ns for Lu₂O₃:Yb. These values can be compared to the MCP-PMT240 response of 0.18 and 0.82 ns, respectively.

Figure 7 shows (a) transmittance and (b) light output as a function of the integral time measured before and after γ-ray irradiation with a total dose of 5.1 Mrad (red) and 10.1 Mrad (blue) for the Lu₂O₃:Yb ceramic sample 2. Radiation damage appears approaching saturation after 5.1 Mrad. Light output and transmittance loss can be attributed to the radiation induced absorption in the Lu₂O₃:Yb sample. Damage recovery and dose rate dependence will be studied for thicker Lu₂O₃:Yb ceramics with better optical quality to reduce the uncertainty in the radiation induced absorption data to facilitate direct comparison with other well-investigated crystal scintillators [14].

Table 2. Scintillation performance of various fast and ultrafast inorganic scintillators.

	BaF2	BaF2:Y	ZnO:Ga	Lu2O3:Yb	YAP:Yb	YAG:Yb	β-Ga2O3	PWO	LYSO:Ce	LuAG:Ce	YAP:Ce	GAGG:Ce	LuYAP:Ce	YSO:Ce
Density (g/cm3)	4.89	4.89	5.67	9.42	5.35	4.56	5.94	8.28	7.4	6.76	5.35	6.5	7.2 6	4.44
Melting points (°C)	1280	1280	1975	2490	1870	1940	1725	1123	2050	2060	1870	1850	1930	2070
X0 (cm)	2.03	2.03	2.51	0.81	2.59	3.53	2.51	0.89	1.14	1.45	2.59	1.63	1.37	3.10
RM (cm)	3.1	3.1	2.28	1.72	2.45	2.76	2.20	2.00	2.07	2.15	2.45	2.20	2.01	2.93
λI (cm)	30.7	30.7	22.2	18.1	23.1	25.2	20.9	20.7	20.9	20.6	23.1	21.5	19.5	27.8
Zeff	51.0	51.0	27.7	67.3	32.8	29.3	27.8	73.6	63.7	58.7	32.8	50.6	57.1	32.8
dE/dX (MeV/cm)	6.52	6.52	8.34	11.6	7.91	7.01	8.82	10.1	9.55	9.22	7.91	8.96	9.82	6.57
λpeak 1 (nm)	300 220	300 220	380	370	350	350	380	425 420	420	520	370	540	385	420
Refractive Index 2	1.50	1.50	2.1	2.0	1.96	1.87	1.97	2.20	1.82	1.84	1.96	1.92	1.94	1.78
Normalized Light Yield 1,3	42 4.8	1.7 4.8	6.6 4	0.95	0.19 4	0.36 4	6.5 0.5	1.6 0.4	100	35 5 48 5	9 32	190	16 15	80
Total Light yield (ph/MeV)	13,000	2000	2000 4	280	57 4	110 4	2100	130	30,000	25,000 5	12,000	58,000	10,000	24,000
Decay time 1 (ns)	600 0.5	600 0.5	<1	1.1 4	1.5	4	148 6	30 10	40	820 50	191 25	570 130	1485 36	75
LY in 1st ns (photons/MeV)	1200	1200	610 4	170	28 4	24 4	43	5.3	740	240	391	400	125	318
LY in 1st ns /Total LY (%)	9.2	60	31	61	49	22	2.0	4.3	2.5	1.0	3.3	0.7	1.3	1.3
40 keV Att. Leng. (1/e, mm)	0.106	0.106	0.407	0.127	0.314	0.439	0.394	0.111	0.185	0.251	0.314	0.319	0.214	0.334

¹ top/bottom row: slow/fast component. ² at the emission peak. ³ normalized to LYSO:Ce. ⁴ excited by alpha particles. ⁵ ceramic with 0.3 Mg at% co-doping. ⁶ density for composition Lu_0.7Y_0.3AlO₃:Ce.

compares the scintillation performance for various fast and ultrafast inorganic scintillators [4]. Among them, Lu₂O₃:Yb ceramics show the highest density and dE/dx, and the shortest radiation and nuclear interaction length. Its ultrafast decay time of 1.1 ns and the high U/T ratio of 61% make it promising for future TOF and ultrafast calorimetry applications. Additional work is needed to improve its light yield in the first ns while keeping the slow component under control.

4. Conclusions

Development of ultrafast heavy crystals with sub-nanosecond decay time is important to break the ps timing barrier for future HEP TOF system and ultrafast calorimetry, and for GHz hard X-ray imaging. All Lu₂O₃:Yb and (Lu,Y)₂O₃:Yb samples from RMD show XEL emission peaked at ~370 nm. Lu₂O₃:Yb ceramics show light yield up to 280 ph/MeV with negligible slow component. Mixing Lu₂O₃ with Y₂O₃ increases light yield but introduces a significant slow component of 100 and 2500 ns decay time. A sub-nanosecond decay time of 1.1 ns was measured by using MCP-PMT. With a high density, an ultrafast decay time, a light yield in the first nanosecond of 170 photon/MeV and an U/T ratio of 61%, Lu₂O₃:Yb ceramics are promising for future TOF and ultrafast calorimetry applications. This investigation will continue to optimize the composition of Lu₂O₃:Yb and increase its transparency and ultrafast light while keeping the slow component under control.