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Cesium iodide - CsI(TI), CsI(Na), undoped CsI
Properties of Cesium iodide, CsI(Tl), CsI(Na), undoped CsI
General description Cesium iodide is a material with a high g-ray stopping power due to its relative high density and Z value. For scintillation counting, it is used either in its undoped form or doped with sodium or thallium. Cesium iodide is noted for its high resistance to thermal and mechanical shock due to the absence of a cleavage plane. Most physical characteristics of CsI are independent of the activator used. Compared to NaI(Tl), it is relatively soft and plastic. It is easily fabricated into a variety of detector geometries. Because of its rugged character, Cesium iodide has been extensively used for well logging, space research or other applications where severe shock conditions are encountered. Cesium iodide itself is soluble in water but is not hygroscopic in the real sense. However, when in contact with materials to which water vapor can adhere, or when used in atmospheres with a high relative humidity, surface degradation can occur. For undoped CsI and CsI(Tl), resurfacing the crystal will generally restore the original performance. CsI(Na) is hygroscopic and must be hermetically sealed at all times just as NaI(Tl). CsI(Tl) CsI(Tl) is a scintillator with the highest light output of all presently known scintillators. However, the maximum of the broad emission is situated at 550nm and the emission is, therefore, not well matched to a bialkali photocathode photomultiplier tube. This results in a photoelectron yield for g-rays which amounts to 45% of the value for NaI(Tl). Fig. 1 shows the emission spectrum. Due to its higher Z-value, the photofraction of CsI(Tl) is higher than that of NaI(Tl). For some applications this can be advantageous. The scintillation intensity as a function of temperature can be found in Fig. 2. CsI(Tl) is a relatively slow scintillator with an average decay time of about one microsecond for g-rays. Electronics with suitable shaping times (4-6 ms) should therefore be used. This limits the count rate the detector can handle. The decay time of CsI(Tl) consists of more than one component. The fastest component has a value of about 0.6 ms, the slowest 3.5 ms. For excitation with highly ionizing particles, such as a-particles or protons, the ratio between the intensity of these two decay components varies as a function of the ionizing power of the absorbed particle. CsI(Tl) scintillation crystals can therefore be used for particle discrimination using pulse shape analysis. It has been demonstrated that particles up to Z=3 can be identified this way. Radiation damage of CsI(Tl) scintillation crystals may become significant above doses of 10 Gray (103rad). However, some of the damage is reversible. Since most of the damage is caused by optical absorption bands which manifest themselves especially at low wavelengths, the use of photodiodes for readout of the scintillation light decreases the effect on the light output and on the pulse height resolution. CsI(Na) As shown in Fig. 2, the emission maximum of CsI(Na) peaks at 420nm and is well matched to the photocathode sensitivity of a bialkali photomultiplier. The photoelectron yield for y-rays amounts to 85% of that of NaI(Tl). The decay time of CsI(Na) is smaller than that of CsI(Tl). Fig. 2 shows the dependence of the scintillation light output as a function of the temperature. The maximum scintillation emission intensity is measured at about 80°C(353K) which makes this crystal suited to operate at high temperatures, such as in well logging or in space applications.
Undoped CsI Undoped CsI, also called CsI(pure) has an emission maximum at 315 nm (see Fig. 1) with an intensity much smaller than both the activated crystals. The scintillation of undoped CsI is characteristic of the material in its pure form. The 315 nm emission is characterized by a relatively short decay time of 16 ns. The material can therefore be used for fast timing applications. Next to this fast 315 nm component, a much slower component with a decay time of about 1 ms is present which represents about 15-20% of the total light output. The intensity of this slow component depends very much on the purity of the crystal since contamination with certain trace elements tends to degrade the fast-to-total ratio. The photoelectron yield in combination with bialkali photocathodes amounts to about 400 photoelectrons per MeV y-rays. For small crystals an energy resolution of 17-18% can be expected for 662 keV y-rays. The material is suited for medium to high energy photon spectroscopy and time-of-flight applications. Undoped CsI can be used in combination with standard (glass) PMTs. However, slightly better results are obtained using quartz window PMTs. The scintillation intensity of undoped CsI as a function of the temperature is shown in Fig. 2. The intensity increases strongly with decreasing temperature, as does the decay time. Measurements indicate that undoped CsI is much more radiation hard than doped CsI and can recover from radiation damage after some time. For doses up to 1000 Gray (105 rad), no severe radiation damage has been observed. Photodiode readout Photodiodes offer some advantages over photomultiplier tubes for certain applications. Since CsI(Tl) has most of its emission in the long wavelength part of the spectrum (>500 nm), the material is well suited for photodiode readout. A significant contribution to the energy resolution of such a detector is caused by the noise of the photodiode/preamplifier combination. This noise contribution to the resolution depends on the ratio between the noise level (electrons) and the amount of free charge carriers (electron-hole pairs) created in the photodiode per amount of radiation absorbed in the scintillation crystal. When you have optimum collection of light, a typical electron hole pair yield between 3.5 x 104 and 4.5 x 104 can be obtained. Note that all graphs in this section regarding the temperature dependence of the scintillation intensity are for PMT readout. For certain crystals, the shape of the emission spectrum is a function of the temperature. The response for crystals with photodiode readout can therefore be different. Photodiodes are presently available in a variety of sizes. When choosing a photodiode in combination with a scintillation crystal, the size of the diode should be such that a maximum amount of scintillation light can be detected by the diode. Our standard scintillation photodiode detectors are equipped with 10 x 10 mm2 or 18 x 18 mm2 photodiodes.
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