FISICA
AMBIENTALE 1 Radioattività:
misure 2 Lezioni 29-30 Marie Curie
FISICA
AMBIENTALE 1 Radioattività:
misure 2 Lezioni 29-30 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
BAND STRUCTURE FOR ELECTRON ENERGIES
IN INSULATORS AND SEMICONDUCTORS INSULATOR SEMICONDUCTOR Valence
band Valence
band Eg > 1eV Eg > 5eV Conduction
band Conduction
band Electron
energy
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 A B C D E SILICON JUNCTION DETECTOR IN CROSS SECTIONAL VIEW 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).
Energy levels of an organic molecule with -electron structure
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
ELETTRONICA d.c. amplifier – negative feedback 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
A discriminator rejects all pulses below a certain level
which is set by applying a discriminator bias voltage FUNCTION OF A DISCRIMINATOR
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.
COUNTING EQUIPMENT – SCHEMATIC DIAGRAM
I60Co –ray spectrum
BASIC RATIMETER CIRCUIT C Reservoir
capacitor R Pulses in 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
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