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 > 1eVEg > 5eV

Conduction band Conduction

bandElectronenergy

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-electrontrajectory

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

Reservoircapacitor

RPulses 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

DECAY CURVE OF MIXED Cl-38 AND Na-24 SAMPLE