MASTER'S THESIS M-642 HECKER, Irwin. FAST SPECTROPHOSPHORIMETER. The American University, M.S., 1963 Physics, general University Microfilms, Inc., Ann Arbor, Michigan PAST SPECTKOPHOSEHÜRIMETER Irwin Hecker Submitted to the Faculty of the College of Arts and Sciences of the American University in Partial Fulfillment of the Requirements for the Degree of Master of Science Signatures of Committee )eair o e College Date : Date; /^i3 A:izmcm umversit 1^55 LIBRARY The American University SEP 3 01964 Washington, D .G. WASHINGTON. D £ i:) ACKNOWLED GEMENTS I wish to thank the faculty and fellow students of the American University Physics Department for their stimulating discussions and suggestions during the development of the phosphorimeter, Dennis Robin­ son, an undergraduate research assistant at the univer­ sity, was very helpful in the construction and testing of the phosphorimeter. I am particularly indebted to Professorial Lecturer Samuel Moss for his direction and valuable criticisms. This research was sponsored by the American Instrument Go. Inc. of Silver Spring, Md. Finally, I wish to thank my wife, Barbara Hecker, for typing all of the manuscripts and for help with the proofs. CONTENTS LIST OF ILLUSTRATIONS........................ 1 1. INTRODUCTION............................. 2 1.1 Phosphorescence.................. 2 1.2 Present Phosphorimeters........... 4 1.5 Need For Fast Phosphorimeter...... 5 2. GENERAL DESCRIPTION..................... 7 2.1 Design Of System................... 7 2.2 System Operation................... 7 3. LIGHT SOURCE SUBSYSTEM.................. 10 5.1 Function Of Light Source........... 10 5.2 Explanation Of Circuit............. 10 4. PHOTOMULTIPLIER DETECTION SUBSYSTEM 16 4.1 Requirements On Detection Subsystem........................ 16 4.2 Theory Of Photomultiplier Tubes,... 16 4.3 Explanation Of Circuit............. 17 5. SWITCH SUBSYSTEM......................... 25 5.1 Timing ....................... 25 5.2 Delay Line........... 25 5.5 Monostable Multivibrator And Amplifier........................ 25 5.4 Shockley 4-Layer Diodes............ 27 5.5 High-Speed Switch.................. 50 6. CONCLUSION............................... 55 BIBLIOGRAHIY................................. 34 ILLUSTRATIONS 1. Block Diagram Of System.................. 8 2. Flash Tube Firing Circuit................ 11 3. Exciting Light Source Subsystem Schematic......... 12 4. Successive Light Pulse From Plash Source..... «................... 14 5. Gate Pulse Caused By Flash............... 14 6. Picture Of Flash Light Source.,.......... 15 7. Operation Of Secondary-Emission Multiplier Tubes....................... 17 8. Schematic Of Photomultiplier Detection System................................. 19 9. Voltage Across Phototube ........... 21 10. Pictures Of Detection System Components............. 22 11. Timing................................... 24 12. Delay Line, Multivibrator And Amplifier................. 26 1 3 . Characteristic V-1 Curve Of Shockley Diodes................................. 28 14. Construction Of Shockley Diode........... 28 1 5 . Picture Of Shockley Diode String......... 32 INTRODUCTION 1.1 PHOSPHORESCENCE When a substance absorbs energy a fraction of the absorbed energy may be re-emitted as electromagnetic radiation in the visible or near visible region of the spectrum. This phenomenon is called luminescence^. Luminescence involves at least two steps: the exci­ tation of the electronic system of the sample and the subsequent emission of photons. The initial excita­ tion may be caused by light, electron or ion bombard­ ment, mechanical strain, chemical reaction, or direct heating. Fluorescence is luminescence in which the light is emitted during excitation. Luminescence in which the light emission occurs after the excitation has ceased is referred to as phosphorescence or afterglow. The afterglow period may be from microseconds to hours in duration. A decay time of 10"^ seconds is frequent­ ly taken as the demarcation line between fluorescence and phosphorescence. This period is the approximate p lifetime for dipole radiation from an excited atom. 1 This term does not include the emission of block-body radiation, which obeys the laws of Kirchbbff and Wien. 2 Adrianus J. Dekker, Solid State Physics. (Englewood Cliffs, N.J., Prentice-Hall, Inc., 1961). Luminescent solids are usually referred to as phos­ phors . Several models have been successfully applied to the study of phosphorescence. The property of lu­ minescence is known to be due to the presence of small amounts of impurities, called activators, in the com­ pound. The phosphor chemists have been able to syn­ thesize, under carefully controlled conditions, much A. more efficient phosphors than those found in nature^. Photoluminescent phosphors are substances which absorb electromagnetic energy, usually ultraviolet light, subsequently re-emitting this energy in the form of visible light. The specific properties of a phosphor (e.g., the color of the light it emits, the wavelength of light it must absorb to be excited to fluorescence, the brightness of the emitted light, the duration of the phosphorescence, or afterglow) are a function of the physical nature of the material and the activator it contains. The decay characteristics of a phosphorescence substance are of great importance in the study of luminescence. According to the models, fluorescence and phosphorescence are both first-order 3 J.S. Prener, D.B. Sullenger, "Phosphors", Scientific American, (Oct. 1954). processes and follow exponential laws of decay. The identification of the luminescence centers and their energy levels is simplified if the decay characteris­ tics are knownt 1 .2 PRESENT PÏÏÜSPHÜRIMETERS Some important types of apparatus designed for investigation of photoluminescence are the phosphori- meters (phosphoroscopes) and fluorimeters. These serve for measuring the duration of short decaying e- mission process. These instruments are based on the principle of permitting the observation of the phos­ phorescence for a short, and if desired, a variable time after the end of the excitation-^. Although there is a wealth of equipment now avai­ lable for measurement of decay times, there is a need for a phosphoroscope v/hich is accurate, inexpensive, flexible, and capable of measuring decay times as short as 1 microsecond. Most phosphoroscopes now avai­ lable are limited by the speed of their mechanical components^. These instruments cannot measure decay 4 Dekker, op. cit. 5 For a detailed history and description of these devi­ ces see Peter Pringsheim, Fluorescence and Phospho­ rescence , (Interscience Idblishers, Inc., N.Y., 1949) ■ times of less than 10~^ seconds. For the measurement of shorter decay periods (down to 10 ^ seconds) fluo- rimeters are used. These devices are not limited in speed but have other disadvantages 7 . For example, fluorimeters which make use of Kerr cells have the li­ mitation that Kerr cells absorb certain wavelengths of light. Other fluorimeters require the incident light beam to be diffracted periodically by means of supersonic waves. These "supersonic-cell" 8 fluorime­ ters are very space consuming. Both types of fluo­ rimeters mentioned above are difficult to operate and expensive. 1.3 NEED FOR FAST PHOSPHORIMETER To more fully understand the structure a lumines­ cent compound, it is desirable to measure precisely the decay of both the fluorescence and the phosphores­ cence radiation. Since the fluorescence decay time for some substances is as long as 1 0 ”^ seconds, a fast, accurate phosphorimeter is required to be able to dis­ tinguish the phosphorescence radiation from the radia­ tion of fluorescence. 6 Such as the rotating discs used by Becquerel or the revolving mirrors used by Vavilov and Levshin. Ibid. 7 Ibid. 8 Ibid. Phosphors are widely used in fluorescent lamps, cathode ray oscilloscopes, radar and television pre­ sentation, and nucleon and radiation detectors. Better understanding of the nature of phosphors may lead to improvements of these devices. In addition, phosphorimeters have been applied to a number of problems involving both identification and quantita­ tive assay of many organic compounds. It was felt that it is within the state of the art to build a phosphoroscope which could measure decay times as short as lO"^ seconds and which would be inexpensive, easily operated, flexible, and accu­ rate. This device was designed to be used in conjunc­ tion with a system of two grating monochromators^ for selecting excitation and phosphorescence wave­ lengths. This paper describes the design and opera­ tion of the electronically switched phosphoroscope which resulted. 9 Such as the Aminco-Keirs Spectrophosphorimeter described in Instruction Book No. 816. (American Instrument Co. Inc., Silver Spring, Md., I960). 2 GENERAL DESCRIPTION 2.1 DESIGN OP SYSTEM The system can be logically divided into three subsystems: a source of light to excite the sample, an electronic switch to handle the delicate timing problem, and a unit which detects the phosphorescent radiation of the sample. Pig. 1 is a block design of the general features of the system. The exciting light source (A) was a Xenon flash tube. This tube produced a flash of high intensity and short dura­ tion. The electronic switch (B) was designed to switch very high voltages on to a load in less than one microsecond. The detection system (C) was de­ signed so that it could be sensitized a short time after sample excitation. 2.2 SYSTEM OPERATION The system functioned in the following manner: