MANUSCRIPT NOT RECEIVED ARE 1 METER SQUARE CALCIUM FLUORIDE LASER WINDOWS POSSIBLE? C. William King Ontario H. Nestor Harshaw Crystals & Electronics 6801 Cochran Road Solon, Ohio 44139 ABSTRACT Calcium Fluoride (CaF2) crystals are grown in a vacuum at approximately 1400 Deg C via a Bridgman-Stockbarger method. This process traditionally has limitations in the size of the single crystals that can be produced. Another limitation, in current practice, is the randomness of the crystallographic orientation of the crystals. A new process for producing large, single, crystallographically oriented crystals of calcium fluoride that overcomes these limitations is described. This process has been reduced to practice to produce single oriented crystals 17.5 x 17.5 x 5 cm3. Currently under construction is a system for producing 35 x 35 x 7.3 cm3 crystals. This process is considered scalable to 1 meter square. The concept employed in the growth process plus seeding the crystal growth provides sufficient control over the entire process to produce a single oriented crystal. This crystal growing process makes possible the fabrication of very large CaF2 windows suitable for high power lasers. Manuscript Received Radiation Damage in Barium Fluoride Detector Materials* P. W. Levy, J. A. Kierstead and C. L. Woody Brookhaven National Laboratory 60 co To develop radiation hard detectors, particularly for high energy physics Key Words: radiation damage; radiation damage "protection;" barium fluoride; In recent years, a number of programs, e. g. SDI, have needed optical materials that are resistant to radiation damage, particularly materials that are resistant to the radiation induced coloring that reduces light transmission. Most recently, the advent of colliding beam facilities for high energy physics research has created a new need for radiation resistant optical materials, particularly for luminescent light emitting particle detectors. In these particle detectors it is necessary to preserve certain luminescent features, such as the fast fluorescent lifetimes and to minimize other features, such as long-lived phosphorescence, in addition to preserving light transmission. To develop a luminescent particle detector that is resistant to radiation damage, and to obtain improved understanding of the mechanisms involved in reducing radiation induced coloring by the addition of impurities, radiation induced coloring is being studied in barium fluoride (BaF2) crystals, both pure and doped with La, Ce, Nd, Eu, Gd and Tm rare earth elements. Pure BaF2 is currently used as a high energy detector. Recently it was shown that lanthanum doping of BaF2 reduces the ratio of the long-lived, or slow, component to the short-lifetime, or fast, luminescent emission without causing any additional coloring [1]. *Research supported in part by the US Department of Energy, Contract DE-AC02-CH7600016, and in part by a grant from the Government of Japan. The qualitative features of radiation induced coloring in glasses and most transparent crystals is reasonably well understood [2]. However, there is very little information on the processes that occur to reduce the luminescent light emission when scintillators are exposed to radiation other than the reduction in light intensity that can be attributed to absorption by radiation induced coloring. For exposures to purely ionizing radiation, gamma rays and/or fast electrons, the radiation induced coloring is, with only a few exceptions, due to the formation of color centers by the trapping of (ionization) electrons and holes on the defects and impurities in the material. The number of defects produced by doses of 10° rad (104 Gray) or less is negligible when compared with those originally present before irradiation, except in a few materials, such as the alkali halides. Thus, for doses less than 106 rad, curves of radiation induced coloring against dose usually increase, with continuously decreasing slope, to a saturation level. For larger doses the coloring curves often contain a saturating component superimposed on a slowly increasing component. The latter component is due to the trapping of charges --to form color centers -- on the defects introduced by radiation damage processes. It is well known that the luminescence emitted by a scintillator, when traversed by ionization producing radiation, usually contains both fluorescent and phosphorescent components. The fluorescence is emitted by a center, usually a defect or an impurity, that is, with few exceptions, excited directly by the incident particle produced ionization event (electron and/or holes). More specifically, the normal fluorescence process probably does not include a charge migration step. If the emission center excited state lifetime is sufficiently short the center decays by the emission of short lifetime fluorescence. This fast emission is an essential characteristic of high energy particle detectors. If the light emission process includes a charge migration step, such as the migration of an electron to a trapped hole, the emission is comparably long-lived and is called phosphorescence. Both of these mechanisms are affected by radiation, i.e. they show radiation damage effects. However, phosphorescence processes are almost always altered more rapidly by radiation damage than the fluorescence processes. Both of the luminescence processes mentioned above appear to be occurring in pure barium fluoride crystal high energy particle detectors [1]. A comparably long-lived phosphorescent emission is seen with an emission band that peaks at roughly 320 nm and with a lifetime of approximately 625 nanoseconds (ns). Also occurring is a short-lived emission that has peaks at roughly 195 and 220 nm and has a life time of approximately 0.6 ns, which appears to be a fluorescent process. This fast, or short-lived, emission is used for particle detection. The long-lived emission gives rise to an unwanted background and the usefulness of BaF2 particle detectors would be greatly improved if it could be eliminated. Many articles contain statements like, "the addition of cerium prevents glass from being colored by radiation," or, "protection against radiation damage is provided by adding cerium." Such statements are usually gross oversimplifications or incorrect. The addition of cerium to the typical hot-cell window lead glass creates absorption bands in the UV and upon exposure to radiation additional UV bands form and continue to grow as long as the radiation continues. In the visible absorption bands appear as soon as radiation commences and continue to increase throughout irradiation. However, the bands in the visible grow much more slowly than bands in similar lead glass without cerium. Consequently, the cerium does provide "protection" against coloring in the visible --actually it slows down the coloring rate in the visible-- but at the expense of increasing the coloring rate in the UV. The cerium appears to do two things to provide "protection." First, it provides traps, which trap ionization electrons and holes to form color centers in the UV, that have much larger trapping cross-sections than the traps that produce color centers in the visible. This process diverts the ionization produced charges from creating color centers in the visible to the creation of color centers in the UV. Second, the cerium functions as a large cross section electron-hole recombination center. The recombination process "uses-up" ionization to color centers. pairs and slows down the conversion of traps than similar counters without cerium. An example of cerium provided protection occurs in lead glass Cerenkov counters, for high energy particle detection [3]. New cerium protected Cerenkov counters emit less light However, the emission efficiency is quite sufficient for normal use. As the counters are used, in radiation fields strong enough to induce nonnegligible coloring, the Cerenkov emission continuously decreases. Most importantly, however, at doses that would render the cerium free counters completely useless the protected counters would still be useful. Thus cerium provides useful protection but "at a price" in this case reduced light output like the Mafia. 3. Present objectives In terms explained in the previous section, the goals of this study can be described succinctly. Namely, find a dopant, such as cerium, that can be incorporated into BaF2 to accomplish the following: 1) Reduce or eliminate the radiation induced coloring or cause the coloring to occur in a wavelength region(s) that will not interfere with the fast emission. 2) Reduce or eliminate the slow emission component, most likely by providing large cross section nonradiative recombination centers that "out compete" the slow emission luminescent centers for ionization electrons and holes. All of the BaF2 samples were purchased from the Optavac. When given, the dopant concentrations are in molar percent of the material added to the melt, not a measurement of the dopant incorporated into the crystal. 4.2 Fast to slow luminescence emission ratio measurements Because they differ in lifetime, the ratio of the two emission components can be 137 measured by counting pulses, produced by irradiating with a Cs gamma-ray source, with a gated detector. Details are given elsewhere [4]. 4.3 Apparatus used for optical absorption measurements Optical absorption measurements were made before and/or after irradiation, but not during irradiation, with a Cary spectrophotometer. Optical absorption measurements were made in a facility for making optical - and other measurements before, during and after irradiation with Co-60 gamma rays. This facility has been described elsewhere [5] and numerous studies made with it are included in reference 2. 5. Measurements 5.1 Absorption spectra of doped BaF2 crystals before irradiation The absorption spectra of both undoped and rare earth doped BaF2 crystals shown in figure 1 were recorded with a Cary spectrophotometer. These spectra show the sharp lines and occasional broad absorption bands expected for rare earth impurities in crystals like BaF2. 5.2 Measurements on the ratio of the fast to slow luminescent intensities As mentioned above, the fast luminescent emission is used for high energy particle detectors and it is important to maximize the fast to slow intensity ratio. Table 1 shows the ratios measured with BaF2 containing the specified rare earth impurities. samples used for table 1 were irradiated. None of the The radiation induced absorption spectra of undoped BaF2 and BaF2 doped with various rare earth impurities, shown in figures 2 through 11 were recorded with the apparatus for making optical measurements during irradiation. However, not all of these absorption spectra were recorded during irradiation. Some were recorded, during a long sequence of irradiations, immediately after a number of interruptions of irradiation. This rarely used procedure was employed since some samples emit very strong luminescence and the spectrophotometer could not be operated reliably in the far UV below approximately 300 nm. Consequently, the irradiation had to be interrupted to obtain meaningful absorption measurements. (With this apparatus reliable absorption measurements can be made in the far UV during irradiation at dose rate levels that produce even stronger luminescence. However, this requires additional equipment that, unfortunately, was not installed when these measurements were made.) These spectra show the expected characteristics. Information on each of these figures is given below. It should be mentioned that similar spectra were also recorded for additional BaF2 samples containing other dopants. Samples with these other dopants developed more radiation induced absorption than the pure and La doped samples and, at least at this time, they do not appear to provide useful radiation damage protection for high energy particle detectors. |