Lawrence._ Berkeley Laboratory Lead Carbonate, a New Fast, Heavy Scintillator

We describe the scintillation properties of Lead Carbon ate (PbC03), a newly discovered, heavy (6.6 g/cm3), in organic scintillator. Its fluorescence decay lifetime, mea sured with the delayed coincidence method, is predomi nantly (69%) a single exponential with a 5.6 ± 1 ns time constant, but has smaller contributions at 27 ± 2 ns (28%) and 155± 10 ns (3%). The emission spectrum peak is cen tered at a wavelength of 475 nm, and drops to less than 10% of its peak value at 370 nm and 580 nm. We have been unable to obtain an optical quality crystal of pure PbC03, but when a 3 mm cube of a cerussite (a naturally occurring mineral form ofPbC03) is excited with 511 keV photons, a photopeak with a 42% full width at half maxi mum is observed at approximately 9% the light output of a Bismuth Germanate (BGO) crystal with similar geometry. The light output increases rapidly with decreasing tem perature, plateauing at twice the light output of BGO at approximately -40° C. Lead Hydroxide (Pb(OH)2) can be mixed with PbC03 in a 1:2 ratio without significantly af fecting the scintillation properties. This adds 0.26% hydro gen, which may provide compensation for hadronic shower calorimeters. The short fluorescence lifetime, high density, and reasonable light output of this new scintillator suggest that it would be useful for applications where high count ing rates, good stopping power, and nanosecond timing are important, such as medical imaging and nuclear science.

This paper describes the scintillation properties of Lead Carbonate (PbC0 3 ), a newly discovered inorganic scintillator. The physical characteristics of PbC0 3 are well suited for use as a gamma radiation detector. It has a density of 6.6 gjcm 3 , is not hygroscopic, and is birefringent with indices of refraction of 1.80 and 2.08 [1]. The crystal structure is rhombic [1], its attenuation length for 511 keV photons is 1.1 em, and it is colorless, transmit-  ting wavelengths down to approximately 270 nm. We have been unable to obtain an optical quality synthetic crystal of pure Lead Carbonate, and therefore have performed our measurements using crystals cut from a naturally occurring mineral form of PbC0 3 known as cerussite. Also, all measurements were performed at room temperature (24° C) unless otherwise indicated.  The results of this measurement are shown in Figure 2. A good fit to the data (the chi-squared per degree of freedom is 0.95) is obtained with a sum of three exponential decay constants plus a constant background. Note that although the data Figure 2 is displayed in 8 ns bins, the data was acquired and fit using 0.4 ns bins. The majority of the light (69% of the emitted photons) is produced with an 5.6 ± 1.0 ns decay constant, while 28% of the emitted photons are produced with a 28 ± 2 ns decay constant and a small fraction (3%) are produced with a 155± 10 ns time constant. The errors in this m~asurement are dominated by correlations between the three exponential terms.

Coincidence Timing
The coincidence resolving time of PbC0 3 was measured by exciting two crystals of PbC0 3 , each coupled to a quartz windowed Hamamatsu R-2059 photomultiplier tube operated at -2500 V, with 511 keV photons resulting from positron annihilation from a 22 Na source placed between the two crystals. A timing signal from each photomultiplier tube was generated using two channels of a Tennelec TC-454 constant fraction discriminator, and the time difference between the two timing signals was digitized with an Ortec 457 time to amplitude converter and a LeCroy 3512 ADC. The resulting timing distribution, which has a FWHM of 1.3 ns and a full width at tenth maximum (FWTM) of 3.2 ns, is plotted in Figure 3. The same apparatus measures a timing distribution FWHM of 0.5 ns for Barium Fluoride (BaF2).

Emission Spectrum
The emission spectrum of PbC0 3 was obtained using a 0.125 meter Jarrell-Ash MonoSpec 18 monochromator with a 1200 line/mm grating blazed for 500 nm. The entrance and exit slits of this monochromator were 500 pm wide, resulting in a spectral resolution of 12 nm. The 511 keV photons from a 3.0 mCi 68 Ge source were used to excite a small (5 mm typical dimension) crystal that was cut from a cerussite crystal. The faces of this crystal were polished and covered on 5 sides with a reflective coating of white Teflon tape. The sixth side was placed at the entrance slit of the monochromator, and a quartz windowed Hamamatsu R-2055 photomultiplier tube (spectral range 200 nm to 600 nm) was placed at the exit slit. The resulting photomultiplier count rate is plotted, after background subtraction, as a function of monochromator wavelength in Figure 4. This emission spectrum peak is centered at 475 nm, and drops to 10% of its maximum intensity at 370 nm and 580 nm. Note that absence of light below the 300 nm cutoff of borosilicate glass implies that fused silica or UV glass windowed photomultiplier tubes are not necessary in order to collect all of the PbC03 scintillation light. Figure 4 also shows the transmission spectrum of a 3 mm .,, LBL-27899 thick sample of PbCOg, as measured with a Shimadzu Spectronic 200UV spectrophotometer. The transmission is relatively uniform for wavelengths greater than 400 nm, with a gradual decrease between 400 nm and the ultimate cutoff of 270 nm, demonstrating the PbC03 is transparent to its own emissions.

Light Output
The light output of PbCOg was measured by comparing its response to 511 keV photons to the response of a Bismuth Germanate (BGO) crystal under the same conditions. A 3 mm cube of cerussite was coated on five sides with a reflective coating of white Teflon tape, then optically coupled to a quartz-windowed Hamamatsu R-1306 photomultiplier tube with General Electric Viscasil 600M silicone fluid. The crystal was irradiated with 511 keV positron annihilation photons from a 22 N a source, and the output of the photomultiplier tube amplified with a Tennelec TC-222 amplifier with 1.2 psec shaping time and digitized with a Lecroy 3512 ADC. The resulting pulse height spectrum is plotted (after pedestal subtraction) in Figure 5(a). The PbC03 crystal was removed and the same experiment was performed on a 3 mm cube ofBGO, and the resulting spectrum is shown in Figure 5(b).
The photopeak corresponding to the 511 keV photon is seen in each plot in Figure 5. Note that the units used for the horizontal scale are the same for both plots. The 511 keV photopeak in PbC03 is centered at a pulse height that is 9% of the 511 keV photopeak pulse height in BGO. Using the BGO light output of 8200 photons/MeV reported by Holl, et al. [4], this implies that the light output of PbC03 is approximately 760 photons/MeV. The full width at half maximum (FWHM) in PbC0 3 of the 511 keV photopeak is 42%, which is consistent with a light output that is 9% of BGO.
The light output of PbC0 3 increases significantly when it is cooled. Figure 6 compares the light output of PbC0 3 at several temperatures to the light output of BGO at room temperature (24° C). The PbC0 3 light output at room temperature is 9% of BGO, exponentially increasing to twice that of BGO at approximately -40° C, and stabilizing at temperatures below -40° C. The 22 Na excited pulse height spectrum of cooled PbC0 3 is shown in Figure 5(c). Although we have not measured the effect of temperature on .decay time, observation of oscilloscope traces indicates that the fast component of the decay time is not significantly changed as the temperature is reduced, but an additional slow (""' 1ps) component is responsible for the increased light output. 6 Hadronic Compensation Two molecules of Lead Carbonate can combine with a single molecule of Lead Hydroxide (Pb(OH)2), to form a rna-5 Count 1 0 II"T"T""T"T"l""T"T""~"'T"T'"T"T"T""''"T"1r-rT"T""T""1M""rT"T"T"T"III Rate  Temperature (°C) Figure 6: Light Output of PbC0 3 vs. Temperature terial known as basic Lead Carbonate. Although the density of basic Lead Carbonate (6.2 g/cm 3 ) is slightly less than pure PbC0 3 , it contains a small amount (0.26% by weight) ofhydrogen. The presence ofhydrogen in this scintillator suggests that it may be useful for hadronic shower calorimeters, as the hydrogen helps detect otherwise unobservable low energy neutrons, and thus provides some compensation in hadronic showers [5]. Therefore, we have measured the scintillation properties of a powdered sample of pure basic Lead Carbonate, and found them to be very similar to those of cerussite crystals (i.e. pure PbC0 3 ). The fluorescence decay time of a powdered sample of basic Lead Carbonate was measured using the delayed coincidence method [2] on a sample excited with a 1 ns burst of 22.7 keV synchrotron x-rays from beamline X23-A2 at Brookhaven National Laboratory. The resulting spectrum is very similar to the distribution shown in Figure 2, and when fit with three exponentials as in Section 2, shows 75% of the photons emitted with an 6.6 ns decay time, 22% emitted with a 32 ns decay time, and 3% produced with a 138 ns time constant (x 2 jDOF = 1.1). The emission spectrum of the powdered sample was obtained by exciting the sample with synchrotron x-rays, then measuring the spectral output with the apparatus used to measure the emission spectrum of the cerussite crystal in Section 4. The resulting spectrum peaks at 480 nm, and has approximately the same width as the emission spectrum in Figure 4.
In addition, we checked powdered samples of Lead Hydroxide (Pb(OH)2) for scintillation, and found that its scintillation light output is less than 1% of basic Lead Carbonate [6]. Since basic Lead Carbonate differs from pure PbC0 3 only by the addition of Lead Hydroxide and the scintillation properties of cerussite are very similar to those of basic Lead Carbonate, we therefore conclude that the PbC0 3 molecule is the active scintillator in both cerussite and basic Lead Carbonate and that the inclusion of Lead Hydroxide does not affect its scintillation properties.

Synthetic Crystal Growth
As mentioned earlier, all measurements were made with crystals cut from a single naturally formed sample of PbC0 3 , pictured in Figure 7. This sample has many black inclusions and internal cracks, which reduce the clarity of the crystal and lower its light collection efficiency. Therefore, the light output measurement is likely to be affected by the relatively poor optical quality of the cerussite crystal and the other measurements presented in this paper will probably not be affected. We hope that the light output presented here is a lower limit, and that the light output of this material will increase once it is learned how to grow optical quality single crystals of pure PbCOs.
Growing pure synthetic crystals is complicated because the decomposition temperature of PbCOs is lower than its melting point. However, it may be possible to grow opti- cal quality crystals using a hydrothermal method, similar to the process used to grow single quartz crystals. Lead Carbonate is slightly soluable in hot water, with its solubility increasing with increasing temperature. Therefore PbC0 3 is dissolved in 300° C water at approximately 1000 atmospheres pressure, and a 50° C temperature gradient is placed across this solution, forming PbCOs crystals at the colder end of the container [7].
It may also be possible to grow PbCOs crystals from the melt by driving the reverse of decomposition reaction. PbC0 3 breaks down into PbO and C0 2 when it is heated, so if it were heated under several thousand atmospheres of C0 2 pressure, the rate of the reverse of the decomposition reaction may increase enough to equal or exceed the decomposition rate. If this can be achieved, optical quality crystals can be grown using more conventional zonerefinement or Czochralski crystal growth techniques. This method has been applied with some success to grow optical quality CaCOs crystals [8].

Conclusions
Lead Carbonate is a newly discovered, heavy, inorganic scintillator. Its density of 6.6 gjcm 3 is similar to that of BGO (7.1 gjcm 3 ), and the attenuation length for 511 keV photons is the same as BGO (1.1 em). Its primary decay time of8.5±1 ns is slowP-r than the 0.8 ns ''fast" component ofBaF 2 , but considerably faster than BGO (300 ns) or the "slow" component of BaF 2 ( 620 ns). The PbCOs emission spectrum peak is centered at 475 nm, and so its emissions can be detected with good efficiency both by borosilicate glass photomultiplier tubes and PIN photodiodes. The scintillation light output is fairly low, approximately 9% of BGO at room temperature but increasing to twice that of BGO at -40° C. The light output measurements presented here were made with poor optical quality natural crystals, so we hope that the light output will be greater in synthetically grown pure PbC03 crystals. The scintillation properties are not appreciably affected when Pb(OH)2 is added to form basic Lead Carbonate.
The combination of high density, short fluorescence lifetime, and reasonable light output suggest that PbC0 3 would be useful for applications where high counting rates, good stopping power, and nanosecond timing are important, such as medical imaging and nuclear science. The absence of a significant "slow" fluorescent decay component implies that PbC0 3 would be well suited for applications where counting rates as high as 10 Mhz are expected. The inclusion of a small (0.26%) fraction of hydrogen in basic Lead Carbonate may allow this material to be sensitive to low energy neutrons, and thus making it an attractive material for constructing compensating hadron shower calorimeters.