Particle accelerator facilities are widespread around the world, both for the study of particle physics and medical applications.
Beam diagnostic devices are key tools for the correct operation of accelerators, since they allow understanding of the beam properties and explain how waves behave in an accelerator.
Beam diagnostics is a rich field; where a great variety of physical effects are made use of and imagination finds a wide playground. Today, there is a vast choice of diagnostic devices available, each employing different techniques.
The beam parameters that should be monitored in an accelerator are usually: intensity, position, size and shape, emittance, energy and polarization.
All of these measurements perturb the beam, some of these being negligible and others destructive.
It is common to say that an accelerator can never be better than its diagnostics and accurate equipment serves as a basis for this.
A particle accelerator can be operated with a high or low intensity beam. In the case of high intensity, it is possible to take direct measurements of the visible light emitted by the beam interaction with residual gas or fluorescent monitors.
Position measurements of the beam centroid in the beam pipe are of prime importance for the accelerator operation. On-line and non-interceptive measurements are required wherever possible. When the beam is in DC mode, a quantitative measurement of the beam centroid position can be done using a sCMOS camera.
In this case the beam excites the atoms of the residual gas in the beam pipe and they emit light. A sCMOS camera can sense this emitted light and its position can be deduced. Hamamatsu Photonics produces a range of sCMOS cameras designed particularly for this application.
The Cornell Electron-positron Storage Ring (CESR) is an electron-positron collider with a circumference of 768 meters, located 12 meters below the parking lot of the scenic Cornell University campus. It is capable of producing collisions between electrons and their anti-particles, positrons, with center-of-mass energies between 9 and 12 GeV. When an electron and positron collide and annihilate, the flash of energy results in the creation of new matter. This new matter, sometimes strange and exotic, is the main object of interest in the CLEO experiment.
One of the main aims of beam diagnostics is to measure the longitudinal and vertical beam dynamics of single colliding bunches for luminosity optimisation.
For the longitudinal beam diagnostics, the synchrotron light emitted both by electrons and positrons is reflected by a mirror into a streak camera. Hamamatsu Photonics C10910 Streak camera can measure electron bunch length with 1 ps time resolution.
Two PMT arrays are installed for electron/positron vertical beam size measurements. Each array has a 32 channel linear anode with an effective area per channel of 0.8x7mm. The PMT signal has a sub-nanosecond rise time that measures the individual vertical beam size of all the bunches in CESR.
Beam profile measurements exploit the residual-gas ionization profile measurement. The interaction between the beam and the background-residual gas creates electron-ion pairs that can be collected by a micro-channel plate (MCP) followed by a one-plane position sensitive device. As for beam centroid position measurements, the profile of the primary beam is deduced from the measurements of charges collected and amplified by the MCP.
Another technique for analyzing the beam profile measures the fluorescence of the residual gas interacting with the primary proton. The light in the visible range can be sensed by a sCMOS camera and allows for qualitative information on the beams size. In accelerators using this technique, special glass windows are used where the beam is intercepted by the monitor. The NIRS-HIMAC is a heavy ion synchrotron complex for medical use in Chiba, Japan. Precise control of position, intensity and shape of the beams is crucial in medical applications; where ion beams are used to irradiate tumours with millimetre precision or to produce large quantities of radioisotopes for diagnostics and therapy.
In this facility the beam profile monitoring system is composed by; a double stage MCP, an image intensifier and a sCMOS camera, as shown in the image below.
Ions generated by the beam at the inclined sheet beam target are collected with a radial electric field. This field is induced by semi-spherical electrodes and multiplied by a two stage MCP (Hamamatsu Photonics F2226: max. gain 1x107). The luminescent light from the phosphor screen (decay time of the light : 100 ns at 1/10) is detected by a sCMOS camera, which is attached to an image intensifier (I.I.: max. gain 1x104 : Hamamatsu Photonics C9546-04). An image of the beam profile at 8MeV and 5µA at the NIRS cyclotron is shown in the figure.
When the particle accelerator is operated with low intensity, in the case of radioactive ion beams for cancer therapy, the beam diagnostic device should have different characteristics. In this situation the diagnostic system is required to be very robust and cheap since it has to withstand beam tuning. This leads to long operating periods, human errors leading to overexposure, and all of the normal run operations.
This is why the adopted solution is usually a scintillation detector. It consists of a piece of scintillator material (usually NaI) optically coupled to a photosensor.
The scintillator intercepts the beam and produces an average of one photon per 100 eV of deposited energy. The light readout devices can be different depending on the purpose.
Photodiodes are better suited for current readout, photomultiplier tubes (PMTs) and avalanche photodiodes (APDs) are best for pulse counting and there are some PMTs which are calibrated specifically for single photon counting.
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