FISICA AMBIENTALE 1 Lezioni 29-30 Radioattività: misure 2 Marie Curie
RIVELATORI A STATO SOLIDO Measurement of radiation energy is achieved by mechanism of ionization of fixed atoms in a solid semiconductor. The e- exist in definite energy bands. Radiation can extract an e- to the conduction band, leaving a hole in the valence band. Electron-hole pair can be compared with ion pairs in a gas
Conduction band Conduction band Electron energy Eg > 5eV Eg > 1eV Valence band Valence band INSULATOR SEMICONDUCTOR BAND STRUCTURE FOR ELECTRON ENERGIES IN INSULATORS AND SEMICONDUCTORS
SILICON JUNCTION DETECTOR IN CROSS SECTIONAL VIEW A B C D E Surface barrier mount with coaxial connector (E) at rear. The silicon wafer (C) is mounted in a ceramic ring (A) with electrical contact made between either side of the junction and opposite metalized surfaces. The front surface is connected to the center conductor of the coaxial connector (E). Cutaway view of a transmission mount, in which both surfaces of the silicon wafer are accessible. The coaxial connector is placed at the edge of the ceramic ridge
Photo-electron trajectory Basic elements of a photomultiplier tube
EFFETTI FOTOGRAFICI Ionizing radiation affects photographic film as visible light does. With the aid of special film holders incorporating filters, the photographic film enables information on the type and energy of radiation
DOSE-DENSITY CURVE After processing, the film is read by passing a beam of light through it and measuring the optical density which is converted to radiation dose by means of a calibration curve
d.c. amplifier – negative feedback ELETTRONICA Amplification of a very low current to a high enough value to operate a conventional ammeter. A negative feedback reduces instability due to temperature fluctuation caused by very high gains
FUNCTION OF A DISCRIMINATOR A discriminator rejects all pulses below a certain level which is set by applying a discriminator bias voltage
PLATEAU FOR G-M COUNTER In order to set up the equipment, a small source is placed near the detector. The graph obtained by plotting the count rate is called plateau, relatively independent on the applied V over a certain range.
R Pulses in C Reservoir capacitor BASIC RATIMETER CIRCUIT Each pulse feeds a charge into the capacitor C, which then slowly discharges through the resistance R, giving a reading on the meter.
Relationship between half-value thickness (g/cm2) and –ray maximum energy