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BE-BI-PM Section


TV Beam Observation System

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Introduction

In order to set up and optimize the particle beams in an accelerator it is necessary to know how the particles inside the beam are distributed at any position of the accelerator. There are models and simulation programs that can predict how particles are transported from one point of the accelerator to another. The mathematical formulas that describe how the particles are transported from one point to another is called the optics of the machine. In fact the transport of light trough a lens system (optical system) has many similarities and this explains the name.

The simplest way of checking how the particles are distributed is by intercepting them with a scintillating screen and look at the light emitted by the screen. Typical scintillating screens are doped alumina plates (Al2O3:Cr). When a charged particle hits the atoms of the screen it excites them, later these atoms will return to the ground state emitting the exceeding energy in the form of photons. Only a very small portion of the electromagnetic spectrum constitutes the visible light, normally the de-excitation of the atoms happens with the emission of photons outside this range. The effect of the dopant material in the alumina is exactly to force a larger fraction of these decays to include the emission of photons in the visible range.

There are of course many other types of scintillating and fluorescent materials, the particular needs will favor the choice of one over the others. Important factors can be sensitivity (i.e. light intensity vs. number and energy of particles), thermal properties (the screen can get very hot due to the energy deposited by th particles), lifetime of the excited states (delay between the passage of the particles and the emission of the photons) etc.

There are of course also other ways of obtaining the same information. Another very popular technique is based on the Optical Transition Radiation OTR. This is a "weird" phenomena, when a charged particle crosses the boundary between materials with different dielectric constants it emits photons. This is the case for example when the charged particles of a beam trespasses a metallic foil (this can be very thin). The properties of the light emitted are quite complicated, but nevertheless this light can be used to obtain an image of the beam as with the scintillating screens.

Other techniques are based on the Cherenkov emission, or on the scintillation of the rest gas (residual particles left in the beam pipe, vacuum can never be perfect) or still other not mentioned here.

The TV Observation Systems

In the description given above of the techniques available for the imaging of the beams there are two component that are absolutely necessary in a Beam Observation System: a screen (alumina, OTR, etc.) and a detector (something to observe the image. Apart from very low intensity and low energy beams in fact it is not possible to look directly at the screen with your very eyes. This is because a particle beam is always accompanied by ionizing radiation (X-rays etc., mainly from particles lost during the transport). In any case a detector that allows the capture of the image for further analysis is preferable to the human look of a very fast event (flash). For many reasons the beams are transported in pipes under vacuum (the rest gas pressure inside can vary from 10-4 to 10-12 mbar). This means that the screens used for the imaging (often referred to as radiators) are also installed inside vacuum tanks. Moreover the interception of the particles by the radiator is often destructive, meaning that the beam is destroyed or at least degraded after the passage trough the screen. For this reason the screens should only be inserted when a measurement is required and retracted from the beam otherwise. This adds a component to the device as we need an actuator to move the screen in and out or to select between different screen choices or to do both. There is one additional very useful item to add, an illumination system. Inside the vacuum tank there is normally no light (the tank and the beam pipes are made of steel or other metal and only have small glass view-ports for the screen observation). The addition of an illumination system allows to view the screen, eventual reference marks and other useful features. The light is also useful to see the status of the screen and verify the functioning of the detector.

In short there are five main components for a BTV system:

  • Vacuum tank
  • Radiator (scintillating screen, OTR, Cherenkov, etc.)
  • Actuator for the positioning of the radiator
  • Detector (usually a TV camera, but other devices are also used)
  • Illumination system (adjustable lights)

In addition a system for the positioning of neutral density filters can be added. This last one is useful to adapt the light emitted by the radiator to the sensitivity of the detector and for extending the dynamic range of the detector itself (normally < 50dB).

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Page last modified on July 09, 2010, at 03:42 PM EST