452
F. Bordry et al.
be achieved using Cherenkov or scintillation light created in long optical fibres
[151]. The fast response of the Cherenkov signal is detected using photomultipliers
at the ends of the fibre, allowing a longitudinal position resolution of 20cm to be
achieved.
8.6.5.2 Distributed BLM Systems
By far the most common, distributed BLM systems are comprised of many
individual beam loss monitors located throughout the accelerator, each acquired in
parallel. The active medium in these systems is tailored to the type of loss to be
measured. Ionisation chambers are commonly used due to their robustness and large
dynamic range (see e.g. [152]). The chamber provides some medium with which
the secondary particles created by the beam loss can interact, typically a gas such as
nitrogen or argon. This interaction produces electron-ion pairs that are collected by
a series of high voltage gaps along the length of the chamber. The resulting current
is then measured and is proportional to the beam loss at the location of the monitor.
The drawback of the ionisation chamber is its relatively slow response time,
dominated by the drift velocity of the ions created during a beam loss. In cases
where a fast response is required scintillation counters are often employed. These
come in plastic or liquid form and are read-out using a photomultiplier. The
disadvantages of these systems are the susceptibility of plastic scintillators to
radiation damage and long term drift of photomultiplier gain. It is also possible
to directly use the photomultiplier as a detector by replacing the photocathode
with an aluminium foil that works as a secondary electron emitter when irradiated.
Known as an Aluminum Cathode Electron Multiplier (ACEM) it gives response
times in the nanosecond regime [153]. Diamond detectors, consisting of a biased
monocrystalline or polycrystalline diamond wafer with an active area of a few cm 2 ,
have also recently been demonstrated to give nanosecond response times while
being relatively radiation resistant [154].
In order to distinguish beam losses in accelerators where there are sources of
background ionising radiation such as X-rays or synchrotron radiation, Cherenkov
or PIN photodiode detectors can be used. Cherenkov detectors are useful as the
threshold for light production is above the typical Compton-electron energies
produced by such background radiation, while back to back PIN photodiodes can
be used to distinguish between the hadronic shower created by beam losses and
synchrotron radiation [155]. In the latter case only the charged particles will interact
with both photodiodes, giving a coincidence signal, with the photons absorbed by
the first diode. In contrast to the charge detection of most other BLM systems,
PIN photodiode detection depends on counting coincidences, with the count rate
proportional to the loss rate so long as the number of overlapping coincidences is
small.
F. Bordry et al.
be achieved using Cherenkov or scintillation light created in long optical fibres
[151]. The fast response of the Cherenkov signal is detected using photomultipliers
at the ends of the fibre, allowing a longitudinal position resolution of 20cm to be
achieved.
8.6.5.2 Distributed BLM Systems
By far the most common, distributed BLM systems are comprised of many
individual beam loss monitors located throughout the accelerator, each acquired in
parallel. The active medium in these systems is tailored to the type of loss to be
measured. Ionisation chambers are commonly used due to their robustness and large
dynamic range (see e.g. [152]). The chamber provides some medium with which
the secondary particles created by the beam loss can interact, typically a gas such as
nitrogen or argon. This interaction produces electron-ion pairs that are collected by
a series of high voltage gaps along the length of the chamber. The resulting current
is then measured and is proportional to the beam loss at the location of the monitor.
The drawback of the ionisation chamber is its relatively slow response time,
dominated by the drift velocity of the ions created during a beam loss. In cases
where a fast response is required scintillation counters are often employed. These
come in plastic or liquid form and are read-out using a photomultiplier. The
disadvantages of these systems are the susceptibility of plastic scintillators to
radiation damage and long term drift of photomultiplier gain. It is also possible
to directly use the photomultiplier as a detector by replacing the photocathode
with an aluminium foil that works as a secondary electron emitter when irradiated.
Known as an Aluminum Cathode Electron Multiplier (ACEM) it gives response
times in the nanosecond regime [153]. Diamond detectors, consisting of a biased
monocrystalline or polycrystalline diamond wafer with an active area of a few cm 2 ,
have also recently been demonstrated to give nanosecond response times while
being relatively radiation resistant [154].
In order to distinguish beam losses in accelerators where there are sources of
background ionising radiation such as X-rays or synchrotron radiation, Cherenkov
or PIN photodiode detectors can be used. Cherenkov detectors are useful as the
threshold for light production is above the typical Compton-electron energies
produced by such background radiation, while back to back PIN photodiodes can
be used to distinguish between the hadronic shower created by beam losses and
synchrotron radiation [155]. In the latter case only the charged particles will interact
with both photodiodes, giving a coincidence signal, with the photons absorbed by
the first diode. In contrast to the charge detection of most other BLM systems,
PIN photodiode detection depends on counting coincidences, with the count rate
proportional to the loss rate so long as the number of overlapping coincidences is
small.