Transcript conduction band

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
BAND STRUCTURE FOR ELECTRON ENERGIES
IN INSULATORS AND SEMICONDUCTORS
Conduction
band
Electron
energy
Conduction
band
Eg > 5eV
Valence
band
Eg > 1eV
Valence
band
INSULATOR
SEMICONDUCTOR
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
Energy levels of an organic molecule with -electron structure
Basic elements of a photomultiplier tube
Photo-electron
trajectory
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
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
d.c. amplifier – negative feedback
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.
COUNTING EQUIPMENT – SCHEMATIC DIAGRAM
I60Co –ray spectrum
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.
R
Pulses in
C
Reservoir
capacitor
Relationship between half-value thickness (g/cm2)
and –ray maximum energy
DECAY CURVE OF MIXED Cl-38 AND Na-24 SAMPLE