CHAPTER 5 ABSORPTION AND FLUORESCENCE OF CsI(Tl) AND CsI(Tl)In CRYSTALS 93
ABSTRACT In order to obtain a laser action in doped alkali halides crystals, fluorescence measurements have been performed in visible region under second harmonic of copper vapor laser (CVL) excitation. High value of gain is detected at small value of power density shows that CsI(Tl) and CsI(Tl)In crystals seems to be promising active materials in the UV / Visible region. 94
5.1 INTRODUCTION Cesium iodide (CsI) with thalium (Tl) and thalium - indium ( Tl )In doped crystals are widely used in high energy physics experiments due to their high quantum yield, short radiation length and for good optical properties. CsI (Tl) is also known to be scintillating crystal that has been widely used with silicon PIN photodiodes; its use has been limited due to the poor match of its emission spectrum with the radiant sensitivity of commercially available photo cathodes [43,68]. The emission was thought to be caused by intracenter transitions in Tl + ion, donor-acceptor recombination between an electron center and a hole center and the radiative annihilation of two halide self-trapped excitons perturbed by Tl + ions [70-72]. These excitons are expected to be revealed in the shortlived optical absorption spectra, like self-trapped excitons in non-doped alkali halide crystals. These crystals are also promising materials for solid-state laser materials in the visible region of the spectrum [73]. 5.2 EXPERIMENTAL DETAILS The experimental setup for fluorescence measurement in CsI(Tl) and CsI(Tl)In crystals is shown in Fig. 5.1.The absorption spectra of the crystals were obtained using Varian Cary 50 UV-Visible spectrophotometer. Here, second harmonic of copper vapor laser (λ = 255.3 nm, 1 W average power, 5.6 khz) has been used as an excitation source for fluorescence measurement Fluorescence of these crystals has been observed in visible region of the electromagnetic spectrum using plug and play fiber coupled Spectrometer (USB 2000, Ocean optics). The fluorescence spectrum at different pump power of second harmonic of copper vapor laser (CVL) has been observed at room temperature. 95
Power meter were used to measure the power varies from 50 to300 mw and a spherical lens of about 25 cm focal length were used. 2 1 3 4 5 6 1. Second Harmonic Copper Vapor Laser. 2. Power meter. 3. Spherical lens (of focal length 25cms.). 4. Samples. 5. Plug and Play Fiber Coupled Spectrometer. 6. Personal Computer. Fig. 5.1 Experimental Arrangement for Fluorescence Measurement 96
The detailed descriptions about the devices used in the absorption and fluorescence measurement are given below: 5.2.1. COPPER VAPOR LASER: The copper vapor laser is useful because the wavelengths fall in the visible region and average power output is about 100 W or more. It is a self-terminating laser and Resonant- Metastable (RM) metal vapor laser as the upper laser level is resonant to the ground state while the lower level is metastable (self-terminating character). In copper vapor laser, copper atoms are excited, through neon as a buffer a gas, by electric field. The neon ions collide with copper atoms; in this collision charge transfer from neon gas ions to copper atoms takes place, M Ne M * Ne (Gas-discharge) In this collision copper atoms are also excited to upper level. These excited copper ions take part in laser transition A CVL had a discharge tube made of alumna, which sustain a high temperature (melting point 1800 0 C). This tube was wrapped with alumina fiber mat and placed co- axial in glass jacket. A double wall jacket around the glass discharge tube was used to remove the heat flowing out of the glass surfaces. This is kept in glass vacuum enclosure that can be filled with a buffer gas, which is needed to initiate the discharge when the tube is in cold condition. The electrodes are placed at the two end of tube for the electrical discharge. The water jacket also provides the coaxial current return path needed to minimize the discharge circuit s inductance. The schematic of CVL along with excitation circuit is shown in Fig.5.3 97
The copper vapor laser needs a high temperature enclosure in which copper vapor can be generated. Since melting point of copper is 1028 0 C, it requires sufficient electrical power to heat the discharge tube. It is pumped by pulsed electrical discharge in the lasing medium. It requires fast excitation pulses since the effective lifetime of upper laser level (ULL) is few tens of nanoseconds. Copper vapor laser (CVL) requires fast excitation pulses with rise time less than 100 ns. Optimum repetition rate for CVL depends upon size of the laser tube, gas-gas mixture used. For a typical 30W average output power CVL using 45 mm diameter of discharge tube and neon as the buffer gas, typical pulse power supply requirements are as mentioned below. Table 5.1: Pulse Power Requirements of CVL Rise time Voltage magnitude (open circuit voltage) Peak Current Pulse Repetition Rate Average output power 80 ns 10-20 kv 800 Amp 5-6 khz 4-5 kw Capacitor-to-capacitor charge transfer circuits or L - C inversion circuits are used as an excitation circuit of CVL. Thyratron switches are commonly used in these circuits. Thyratron has limitation of short lifetime (typical 1000 hrs to 2500 hrs) and high cost (~ 2.5 lakhs to 3 lakhs). 98
Table 5.2: Typical output parameter of CVL Pulse width Pulse repetition rate 30-80 ns 5-12 khz Operating temperature of the laser tube 1500 o C. Warm up time of laser Average output power range 60 to 90 minutes 40-60 W 5.2.2. PLUG AND PLAY FIBER COUPLED SPECTROMETER (USB 2000, OCEAN OPTICS). The USB2000 Spectrometer connects to a notebook or desktop PC via USB port or serial port. When connected to the USB port of a PC, the USB2000 draws power from the host PC, eliminating the need for an external power supply. Ocean Optics fiber optic spectrometer systems consist of low-cost, modular data acquisition components. A typical USB2000-based sampling system contains four core elements: USB2000 Spectrometer OOIBase32 operating software Light source Sampling optics (varying, depending on application requirements) 99
How Sampling Works The following list explains the function of Ocean Optics sampling components in the sampling process: 1. The user stores reference and dark measurements to correct for instrument response variables. 2. The light from the light source transmits through an optical fiber to the sample. 3. The light interacts with the sample. 4. Another optical fiber collects and transmits the result of the interaction to the spectrometer. 5. The spectrometer measures the amount of light and transforms the data collected by the spectrometer into digital information. 6. The spectrometer passes the sample information to OOIBase32. 7. OOIBase32 compares the sample to the reference measurement and displays processed spectral information. 5.2.3. VARIAN CARY 50 UV-VISIBLE SPECTROPHOTOMETER The innovative design of the Cary 50, which incorporates a Xenon flash lamp, enables it to offer many advantages over traditional UV-Vis spectrophotometers. The Cary 50 is controlled by the new Cary WinUV software. This Windows based software features a modular design which makes it easy to use. 100
The Cary 50 features are: 1. The maximum scan rate is 24000 nm per minute. That means we can scan the whole wavelength range of 190-1100 nm in less than 3 seconds. 2. The Cary 50 can have a collection rate of 80 points per second. 3. The Xenon lamp flashes only when acquiring a data point, unlike a diode array which exposes the sample to the whole wavelength range with each reading, causing degradation of photosensitive samples. 4. The Cary 50 is unaffected by room light. We can operate with the sample compartment open or closed, we won t notice the difference. 5.3 RESULTS & DISCUSSIONS Figure 5.2 shows the absorption spectrum of CsI(Tl) and CsI(Tl) In crystals. Absorption has been observed in ultra violet (UV) region below 350 nm for both the crystals. Figure 5.3(a) and 5.3(b) show the typical record of fluorescence spectrum of these crystals at different pump power levels. The fluorescence spectrum of these two crystals shows that fluorescence increases with the increase of pump power in both the crystals, which is a clear evidence of an optical gain greater than unity [69]. The value of gain G = exp (ανl) which are 11and 5 for CsI(Tl)In and CsI(Tl) crystals respectively has been obtained using the following relation [47] I F (N) = const x {exp [ανl (Ip/ I o ] - 1} where I o is the maximum of Ip, N being the population of excited centers and α is the coefficient depending on the properties of the crystals. These values of high gain are obtained in the pump power range from 50 to 300 mw at room temperature. 101
10 B C 8 Absorption (a.u) 6 4 2 0 20 0 4 00 600 80 0 100 0 W a vele ng th (n m ) Fig. 5.2 The absorption spectrum of CsI(Tl) [B] and CsI(Tl)In [C] crystals. 102
Fig. 5.3 (a) Fluorescence spectra of CsI(Tl) crystal at different power of second harmonic of CVL. Fig. 5.3(b) Fluorescence spectra of CsI(Tl)In crystal at different power of second harmonic of CVL 5.4 CONCLUSIONS The high value of gain at low pump power shows that CsI (Tl) and CsI (Tl) In crystal are promising active materials in the UV / visible region. 103