Abstract
Future high-energy physics experiments pose new challenges for crystal calorimetry, particularly in sustaining precision under harsh radiation environments. This paper examines the impact of radiation damage in scintillating crystals and concludes that the primary effect is radiation-induced absorption, or color center formation, rather than a reduction in scintillation light yield. The significance of preserving light response uniformity and the feasibility of constructing precision crystal calorimeters in radiation environments are discussed in detail. The mechanisms of radiation damage in different scintillators are also addressed: in alkali halides, damage is attributed to oxygen or hydroxyl contamination, while in oxides, it arises from structural defects such as oxygen vacancies. The material analysis methods employed to reach these findings are described comprehensively.
Introduction
Total absorption shower counters constructed from inorganic scintillating crystals have been recognized for decades for their exceptional energy resolution and detection efficiency. In high-energy and nuclear physics, large arrays of such crystals have been deployed for precision measurements of photons and electrons. The pioneering potential of crystal calorimetry was first demonstrated by the Crystal Ball detector [1], which studied radiative transitions and decays of the Charmonium family [2]. Over the past decade, building upon the Crystal Ball and CUSB experiments [3,4], larger crystal calorimeters have been developed, significantly contributing to the success of experiments such as L3 at LEP [5], CLEO II at CESR [6], and the Crystal Barrel at LEAR [7].
Currently, several crystal calorimeters are being designed for the next generation of high-energy physics experiments. Examples include a CsI calorimeter for the KTeV experiment at Fermilab [8], two CsI(Tl) calorimeters for B Factory experiments—the BaBar experiment at SLAC [9] and the BELLE experiment at KEK [10]—and a lead tungstate (PbWO₄) calorimeter for the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) [11], where electroweak symmetry-breaking studies demand the precision offered by crystal calorimetry [12,13,14]. Table 1 summarizes the design parameters of these calorimeters, all of which require several cubic meters of high-quality crystals.
The unique physics capabilities of crystal calorimeters arise from their excellent energy resolution, hermetic coverage, and fine granularity [15]. However, future high-energy experiments pose a new challenge: radiation damage due to increased center-of-mass energies and luminosity. For instance, CsI(Tl) crystals at B Factories may experience dose rates of a few rad per day, whereas PbWO₄ crystals at the LHC could be exposed to 15–500 rad/h.
This paper explores the effects of radiation damage in scintillating crystals, highlights critical issues affecting crystal calorimeter precision under radiation, and examines the causes and potential remedies for such damage. Section 2 provides a brief overview of crystal scintillators commonly used in high-energy and nuclear physics experiments. Section 3 discusses radiation damage phenomena, with particular emphasis on light response uniformity, a key factor in maintaining crystal precision in situ. Sections 4 and 5 analyze damage mechanisms in alkali halides (e.g., BaF₂ and CsI) and oxides (e.g., bismuth germanate, Bi₄Ge₃O₁₂ (BGO), and PbWO₄), respectively. Section 6 presents the paper’s summary and conclusions.
Unless otherwise specified, all measurements were performed at Caltech using samples sourced from the Beijing Glass Research Institute (BGRI, Beijing, China) [16], Bogoroditsk Techno-Chemical Plant (BTCP, Russia) [17], Crismatec (France) [18], Hilger (UK) [19], Amcrys-H (Ukraine) [20], and Shanghai Institute of Ceramics (SIC, Shanghai, China) [21].
Table 2 summarizes the basic properties of several widely used heavy crystal scintillators, including NaI(Tl), CsI(Tl), undoped CsI, BaF₂, BGO, and PbWO₄. These crystals have all been employed in high-energy and nuclear physics experiments and are commercially available in large quantities. Among them, CsI(Tl) is notable for its high light yield, exceeding that of NaI(Tl) for photon detection. The relative light yield of 45% listed in Table 2 reflects the quantum efficiency of the Bi–alkali cathode used in the readout system.
All known crystal scintillators are susceptible to radiation damage. The most prevalent effect is the formation of radiation-induced absorption bands caused by color centers, which reduce the crystal’s light attenuation length (LAL) and consequently decrease light output. However, the formation of color centers does not always degrade light response uniformity. Radiation can also induce phosphorescence (afterglow), which contributes to increased readout noise.
Material analysis is crucial for understanding radiation damage mechanisms. Glow Discharge Mass Spectroscopy (GDMS) performed at Charles Evans and Associates was used to examine trace impurities in CsI(Tl) crystals and their correlation with radiation hardness. Samples were collected 3–5 mm below the surface to avoid contamination. A survey of 76 elements, including all lanthanides, revealed no clear correlation between trace impurities and susceptibility to radiation damage, indicating that other intrinsic factors govern the damage mechanism.
Similarly, GDMS analysis of PbWO₄ crystals showed no specific correlation between trace impurities and radiation sensitivity [27]. This suggests that intrinsic defects in the crystal lattice, such as oxygen vacancies, play a major role in radiation damage. Table 4 presents GDMS results for trace elements (ppmw) in several PbWO₄ samples. While GDMS cannot detect structural defects, these findings highlight their critical influence on crystal performance under radiation.
Future high-energy colliders will expose crystal calorimeters to severe radiation due to increased center-of-mass energies and luminosity. Despite decades of experience, crystal calorimetry faces new challenges: without stringent quality control, crystals may experience significant radiation damage, leading to degraded performance. Potential effects of radiation damage include radiation-induced absorption, color center formation, afterglow, and non-uniform light response, all of which can impact calorimeter precision.
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