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Properties of Bismuth Germanate and Its Use for Electromagnetic Calorimetry *

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Properties of Bismuth Germanate and Its Use for Electromagnetic Calorimetry * PROPERTIES OF BISMUTH GERMANATE AND ITS USE FOR ELECTROMAGNETIC CALORIMETRY * M. Cavalli-Sforza, D.G. Coyne, S. Cordes, J. Hawley, C. Newman-Holmes, R. Sonnenfeld Physics Department, Princeton University, Princeton, NJ, 08540 G. I. Kirkbride High Energy Physics Laboratory, Stanford University, Stanford, CA, 94305 Summary TABLE I BGO - NaI(Tt) COMPARISON The calorimetric and fluorescence properties of Bismuth Germanate are discussed and compared to those BGO NaI(Tt) of NaI(Tt). Results on the energy resolution of BGO are presented for energies up to 50 MeV; the energy and General Properties position resolution are studied by Montecarlos at high­ er energies. The performance of a 4n BGO e.m. calor­ Specific Gravity 7.13 3.67 imeter is compared to that of NaI and is specified for Hardness -5 (soft glass) -2 (rock salt) a compact fieldless calorimeter designed for the zO Stability rugged cleaves, energy region. Cost, optical uniformity and radiation shatters easily sensitivity are areas needing further work before a Chemical Stability good poor large BGO detector can be built. Solubility (H20) none very hygroscopic Introduction Calorimetric Properties Large solid angle, highly segmented sodium iodide detectors have emerged over the last few years as a Radiation Length, Xa 1.12 cm 2.59 cm novel and fruitful way to measure many of the paramet­ Mel iere Radius 2.24 cm 4.4 cm ers of the final states produced in e+e- annihilation. dE/dx (min) -9 MeV /cm 4.8 MeV/cm Two such detectors are at present active in e+e- phy­ Nuclear Absorption sics: the Crystal Ball, that after more than three Length A -23 cm -41 cm years of very productive existence at SPEAR is about to begin exploring b-quark physics at DORIS II, and the CUSB detector, working in the region of the ~·s at Optical and Fluorescence CESR. Properties As plans for detectors at the next generation of e+e- machines are getting more detailed, considerable Refractive Index 2.13 1.85 attention has been devoted to Bismuth Germanate (BGO) A.maxEmission 480-500 nm 420 nm as a possible alternative to NaI(Tt) for large solid Fall Time 300 ns 250 ns angle detectors that seek superior energy resolution Photoelec- coupled with good angular resolution. t rons/MeV 300-600(tube depn'd't) Light Output 16 100 General and calorimetric properties of BGO Bisnruth Germanate (Bi4Ge3o12 ) is a transparent, crystalline material of high density and high atomic materials and carefully controlled growth conditions. number with large scintillation light yield. BGO crys­ The optical quality of BGO crystals is not yet on a par tals are grown from a melt of a 2:3 stoichiometric with that of NaI(Tt), although it must be pointed out mixture of Bisnruth Oxide (Bi2o3) and Germanium Oxide that the technology for BGO is still developing. (Geo ). Crystal boules of up to 30 cm in length and 4 rangLng in diameter from 5 to 10 cm are pulled from the Optical and fluorescence properties melt by the Czochralski method; the boules are then cut, machined and polished to the desired dimensions. The fluorescence of BGO has been assigned to the Crystals are presently available from at least two 3p1 + lg transition of the Bi+++ ion. The large manufacturers.l Table 1 lists the properties of BGO in Stokes s~ift between the absorption spectrum2 (peaked comparison to NaI(Tt). in the near UV) and the emission spectrum makes the The calorimetric properties of BGO are of course material highly transparent to its own light. The the main reason for its emergence as a particle detec­ index of refraction is about 2.15 over the visible tor. The radiation length of BGO is 2.3 times shorter range (compared to n=l.85 for NaI(Tt)). Such a high than that of NaI(Tt), and the Moliere radius (determin­ index makes the light transfer through photomultiplier ing the scale of the lateral spread of e.m. ·showers) windows (typically n=l.46) rather inefficient; graded is a factor 1.75 less than NaI(Tt). The nuclear absor­ index couplings do not help significantly. On the ption length, A = 23 cm, prohibits its use as a hadron other hand, the high index helps in keeping most of absorber, although the ratio A/X0 is marginally higher the light into a long parallelepipedal crystal; in than for NaI(Tt) and may thus improve slightly the fact, all of the light that is within the critical hadron-electron separation. angle for transmission through the interface to a PMT The crystals are mechanically rugged, do not is totally reflected off the sides for parallelepipedal cleave, are reasonably hard to scratch and are imper­ crystal geometry. This circumstance suggests that in vious to most chemical solvents - water in particular. parallelepipedal crystals light collection uniformity These latter properties are in marked contrast to along a crystal could be very good. Scattering centers NaI(Tt), that is very delicate and difficult to handle · in crystals would worsen the uniformity of light on all ~hese counts. On the other hand, growing long collection; we see examples both of macroscopic (~20 cm) crystals of uniform transparency appears to be scattering centers and of the expected effect in 20 cm a challenging task, requiring high-purity starting long crystals we have under test. These issues have been investigated in detail by Montecarlo for smaller crystals in view of applications in positron emission . *Work supported. under NSF Grant PHY-8--02409 and PHY- tomography. 3 79-16461. The emission spectrunf.•4 •5 peaks at 480 to 500 nm, The electron beam was run at 1 particle per ~ith a substantial tail in the green and red. It is pulse with energies of 30 and 50 MeV; it had thus not ideally matched to the response curve of an energy width of 0.4%. bialkali photocathodes, while matching very well the response of Silicon photodiodes. 14 The fluorescence decay time has long been known FWHM to be about 300 ns at room temperature. A recent, RESOLUTION ( PEAK ) VS. ENERGY accurate measurement of the light pulse shape, done 'with the single photon methodlS, confirms that the 'decay time is 300 nsec at room temperature but shows in 20 addition an additional component decaying with a 60 ns ·time constant and accounting for -10% of the total ~ 15 light. The risetime is measured in the same study to w be 2.8 ns and is probably dominated by light collection ~ 10 time in the crystal used. w The integrated light output of BGO is often meas­ a.. 7 ured relative to NaI(Ti); it depends of course on the ~ 6 PMT response curve. Furthermore, it is very sensitive <t 5 to crystal purity and optical quality, as well as sur­ w 4 face treatment. Using bialkali PMT's, light yields of a.. up to 16% of NaI(Ti) have been reported.6 We have ''.::!: 3 measured the photoelectron yield using an RCA 8850 :r: Quantacon tube on a 1 inch diameter x 1 inch long, :;: 2 cylindrical crystal, with polished sides; we observe LI.. ~ ~ 1 7 SOURCES VAN '-.,.-J 192±5 photoelectrons using 0.662 photons (Cs 3 ), with DE GRAAFF LINAC a FWHM/peak resolution of 18±1%. Within the errors, the resolution is explained by photoelectron statis­ 10 100 tics. On the same crystal and similar ones, but using Hamamatsu bialkali PMT's, we get typical FWHM/peak ENERGY ( MeV) resolution of 12.5% with cs137 gamma rays. This reso­ lution indicates a higher photoelectron yield, roughly Fig. 1. Energy resolution measured with the 4 inch x 3 600 electrons/MeV. inch BGO crystal. It is known that the energy resolution of BGO For all measurements, one of the two crystals was scales roughly like E-112 up to a few Mev.1 4 The reso­ grease-coupled to a Hamamatsu Rl069 5 inch PMT. lution falls more slowly for NaI(Ti) crystals; these In the Van de Graaff data, the photons were colli­ circumstances encouraged us to investigate the resolu­ mated to impinge axially on an area of approximately 1 tion of BGO at higher energies. inch square, offset by 1 inch from the crystal axis. Before closing this review of material properties, This was to assure that the scintillation light would we note that both fluorescence decay time and integra­ neither be produced nor transmitted in the impurity ted pulse height of BGO decrease rapidly with tempera­ "cloud" that faced the incoming photons. Different ture. Both quantities are approximately linear vs. cloud-beam geometries gave worse resolutions than the temperature from l0°C o 55°C, with AP/P -l%/°C at = arrangement described. At 30 and 50 MeV, the crystal 20°C, where P is the pulse height.7 The light output was complemented with an array of 6 large NaI(Ti) saturates at -200°K, where it is -5 times more than at detectors, arranged to maximize the solid angle viewed room temperature.2 While all this may speak for cool­ from the center of the BGO crystal. The array was used ing BGO crystals, it also demands that any future as a veto to eliminate events in which more than 0.3 detector be carefully temperature controlled, and that MeV of the shower energy would escape the crystal and the uniformity be better than l°C across the detector. thus measure the intrinsic resolution of the crystal. We note that, at these energies, it is not possible to Energy resolution measurements for E ~ 50 MeV keep the shower or its light from the "cloud" in the crystal.
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