UV-B DETECTOR BASED UPON RYDBERG STATES

THE THREAT OF OZONE DEPLETION CAUSING INCREASED ULTRAVIOLET RADIATION AT MID-LATITUDES ON THE EARTH'S SURFACE, AND THE RESULTING HARM TO PLANT AND ANIMAL LIFE, HAVE PROMOTED THE U.S. DEPARTMENT OF AGRICULTURE TO ESTABLISH AN IMPROVED UV IRRADIANCE MONITORING PROGRAM, WITH EMPHASIS ON THE UV-B SPECTRAL REGION BETWEEN 290 NM AND 320NM. HOWEVER, THERE IS CONCERN OVER THE LONG-TERM STABILITY OF INSTRUMENTS USED TO MEASURE THE UV-B FLUX. THIS PROPOSAL DESCRIBES A DETECTOR THAT HAS AN INTRINSIC ABSOLUTE WAVELENGTH CALIBRATION AT ALL TIMES. THE DEVICE IS BASED UPON UV-B EXCITATION OF VAPOR-PHASE ATOMS. HIGH-ENERGY STATES (RYDBERG STATES) OF CERTAIN ALKALI METAL ATOMS ARE PRODUCED BY RESONANCE ABSORPTION F WAVELENGTH IN THE UV-B. THESE EXCITED-STATE ATOMS CAN BE IONIZED BY AN ELECTRIC FIELD AND THE RESULTING ELETRONS DETECTED IN AN MULTIPLIER. EACH RYDBERG STATE HAS A UNIQUE VOLTAGE THRESHOLD AND CAN BE IDENTIFIED BY THE VOLTAGE AT WHICH IONIZATIN OCCURS. THEREFORE, THE IONIZATION SIGNAL AT A PARTICULAR VOLTAGE IS PROPORTIONAL TO THE PHOTON FLUX AT THE (KNOWN) RESONANCE EXCITATION WAVELENGTH OF THE CORRESPONDING RYDBERG STATE. THE INSTRUMENT IS INSENSITIVE TO GAIN CHANGES BECAUSE PHOTON COUNTING IS USED.