Cryogenic Beam Loss Monitoring for the LHC

A Beam Loss Monitoring (BLM) system was installed on the outside surface of the LHC magnet cryostats to protect the accelerator equipment from beam losses. The protection is achieved by extracting the beam from the ring in case thresholds imposed on measured radiation levels are exceeded. Close to the interaction regions of the LHC, the present BLM system is sensitive to particle showers generated in the interaction region of the two beams. In the future, with beams of higher energy and brightness resulting in higher luminosity, distinguishing between these interaction products and possible quench-provoking beam losses from the primary proton beams will be challenging. The particle showers measured by the present BLM configuration are partly shielded by the cryostat and the iron yoke of the magnets. The system can hence be optimised by locating beam loss monitors as close as possible to the protected element, i. e. the superconducting coils, inside the cold mass of the magnets in superfluid helium at 1.9 K. The advantage is that the dose measured by the Cryogenic Beam Loss Monitor (CryoBLM) would more precisely correspond to the dose deposited in the superconducting coil. The main challenges of this placement are the low temperature of 1.9 K and the integrated dose of 2 MGy in 20 years. Furthermore the CryoBLM should work in a magnetic field of 2 T and at a pressure of 1.1 bar, withstanding a fast pressure rise up to 20 bar in case of a magnet quench. The detector response should be linear between 0.1 and 10 mGy/s and faster than 1 ms. Once the detectors are installed in the LHC magnets, no access will be possible. Hence the detectors need to be available, reliable and stable for 20 years. Following intense research it became clear that no existing technology was proven to work in such conditions. The candidates under investigation in this work are diamond and silicon detectors and an ionisation chamber, using the liquid helium itself as particle detection medium. All the selected detector technologies are based on ionisation and subsequent charge carrier transport within the detector bulk. Therefore laboratory measurements were performed to measure the charge carrier characteristics in the detector material in the temperature range from 1.6 to 300 K. In the silicon detector, charges were generated using laser light and - particles. For diamond detectors the measurements were done with -particles only. The temperature dependence of the drift velocity and of the mobility of the charge carriers was measured. To measure the detector’s characteristics with respect to particle detection at liquid helium temperatures, low intensity beam tests with minimum ionising protons were carried out. They allowed to prove that all tested detectors work at 1.9 K. The silicon detector Full Width Half Maximum (FWHM) of the signal from a MIP is 2.5 ± 0.7 ns at liquid helium temperatures. For the diamond detector the FWHM is 3.6 ± 0.8 ns. The signal width decrease from room temperature to liquid helium temperatures is of 54 % for silicon material and 28 % for diamond material. This allows bunch by bunch resolution of the LHC losses, as already demonstrated at room temperature. The radiation hardness of the solid-state detectors over 20 years of LHC operation was addressed during high intensity beam tests carried out at CERN in a liquid helium environment. A complete cryogenic system was installed in the irradiation area of the CERN East Hall. Data from the continuous monitoring of the signal development during irradiation and measurements from test cycles enabled the advantages and disadvantages of each detector technology to be identified. The expected reduction in detector sensitivity over 20 years (2 MGy) of LHC operation is of a factor of 14 ± 3 for the diamond detector. For the silicon detector the expected signal reduction is of a factor of 25 ± 5. Using liquid helium as particle detection medium has the advantage of no radiation hard- ness issues. The downside is the low electron and ion mobility in superfluid helium, which leads to a slower detector response. With the current design of the liquid helium chamber a successful protection from losses with a time constant above 180 s is ensured. These results show that the diamond and silicon detectors satisfy the criteria for use in a fast protection and feedback system, while the simultaneous use of the liquid helium chamber enables the calibration of the solid-state detectors and the reliable protection from steady state losses

Particle Shower Simulations and Loss Measurements in the LHC Magnet Interconnection Regions

Particle losses in the LHC arcs are mainly expected in the interconnection region between a dipole and quadrupole magnet. The maximal beam size, the maximal orbit excursion and aperture changes cause the enhancement of losses at this location. Extensive Geant4 simulations have been performed to characterise this particular region to establish beam abort settings for the beam loss monitors in these areas. Data from first LHC beam loss measurements have been used to check and determine the most likely proton impact locations. This input has been used to optimise the simulations used for the definition of thresholds settings. The accuracy of these settings is investigated by comparing the simulations with actual loss measurements

Investigation of the use of Silicon, Diamond and liquid Helium detectors for Beam Loss Measurements at 2K

At the triplet magnets, close to the interaction regions of the LHC, the current Beam Loss Monitoring (BLM) system is very sensitive to the debris from the collisions. For future beams with higher energy and higher luminosity this will lead to a situation in which the BLM system can no longer distinguish between these interaction products and quench-provoking beam losses from the primary proton beams. The solution investigated is to locate the detectors as close as possible to the superconducting coil, i.e. the element to be protected. This means putting detectors inside the cold mass of the superconducting magnets at 1.9 K. As possible candidates for such loss monitors, diamond, silicon and a liquid helium chamber have been tested in a proton beam at liquid helium temperatures. The initial promising results from these tests will be presented and discussed in this contribution

Operation of Silicon, Diamond and liquid Helium Detectors in the range of Room Temperature to 1.9 K and after an Irradiation Dose of several Mega Gray

At the triplet magnets, close to the interaction regions of the Large Hadron Collider (LHC), the current Beam Loss Monitoring (BLM) system is sensitive to the debris from the collision points. For future beams, with higher energy and intensity the expected increase in luminosity implicate an increase of the debris from interaction products covering the quench-provoking beam losses from the primary proton beams. The investigated option is to locate the detectors as close as possible to the superconducting coil, where the signal ratio of both is optimal. Therefore the detectors have to be located inside the cold mass of the superconducting magnets in superfluid helium at 1.9 Kelvin. Past measurements have shown that a liquid helium ionisation chamber, diamond and silicon detectors are promising candidates for cryogenic beam loss monitors. The carrier parameter, drift velocity, and the leakage current changes will be shown as a function of temperature. New high irradiation test beam measurements at room temperature and 1.9 Kelvin will reveal the radiation tolerance of the different detectors

Quench Protection with LHC Beam Loss Monitors

To prevent from beam-induced quenches of the superconducting magnets a system of about 4000 beam loss detectors is installed on the magnets cryostat. These detectors, being ionisation chambers, measure the particle shower starting inside the magnet. Examples of simulations linking the heat deposited in the superconducting coils with signals in the ionisation chambers are presented. A comparison of the simulations to the data is done. Limits of the present system are discussed

Characterisation of SI Detectors for the Use at 2 K*

It is expected that the luminosity of the Large Hadron Collider (LHC) will be bounded in the future by the beam loss limits of the superconducting magnets. To protect the superconducting magnets of the high luminosity insertions an optimal detection of the energy deposition by the shower of beam particles is necessary. Therefore beam Loss Monitors (BLM) need to be placed close to the particle impact location in the cold mass of the magnets where they should operate in superfluid helium at 1.9 Kelvin. To choose optimal detectors n-type silicon wafers have been examined at superfluid helium temperature whilst under irradiation from a high intensity proton beam. The radiation hardness and leakage current of these detectors were found to be significantly improved at 1.9 Kelvin when compared to their operation at room temperature

Cryogenic Beam Loss Monitors for the Superconducting Magnets of the LHC

The Beam Loss Monitor detectors close to the interaction points of the Large Hadron Collider are currently located outside the cryostat, far from the superconducting coils of the magnets. In addition to their sensitivity to lost beam particles, they also detect particles coming from the experimental collisions, which do not contribute significantly to the heat deposition in the superconducting coils. In the future, with beams of higher energy and brightness resulting in higher luminosity, distinguishing between these interaction products and dangerous quench-provoking beam losses from the primary proton beams will be challenging. The system can be optimised by locating beam loss monitors as close as possible to the superconducting coils, inside the cold mass in a superfluid helium environment, at 1.9 K. The dose then measured by such Cryogenic Beam Loss Monitors would more precisely correspond to the real dose deposited in the coil. The candidates under investigation for such detectors are based on p+-n-n+ silicon and single crystal Chemical Vapour Deposition diamond, of which several have now been mounted on the outside of cold mass of the superconducting coil in the cryostat of the Large Hadron Collider magnets. This contribution will present the mechanical and electrical designs of these systems, as well as the results of their qualification testing including results of a cryogenic irradiation test

Degraded collagenase deteriorates islet viability.

OBJECTIVE: The utilization of purified enzyme blends consisting of collagenase class I (CI) and II (CII) and neutral protease is an essential step for clinical islet isolation. Previous studies suggested that the use of enzyme lots containing degraded CI reduced islet release from human pancreata. The present study sought to assess the effect of degraded collagenase on islet function in vitro and posttransplantation. MATERIALS AND METHODS: Crude collagenase was chromatographically separated into CI, CII, and a mixture of degraded CI and CII isomers. Subsequently, classes were recombined to obtain a CII/CI ratio of 0.5. Rat islets were isolated utilizing neutral protease and 20 units of recombined collagenase containing either intact (Ci) or degraded isomers (Cd). RESULTS: Digestion time was reduced utilizing Cd (P < .001). The highest islet yield and lowest islet fragmentation were obtained with Ci (P < .01). Utilization of Cd corresponded to a reduction in viability and in vitro function (NS). Islet transplantation reversed hyperglycemia in diabetic nude mice, but revealed an absence of weight gain in recipients receiving islets isolated using Cd (P < .01). CONCLUSION: This study suggested that islet function posttransplantation is affected by degraded collagenase isomers. This finding has to be considered for the purification process of collagenase

The ratio between collagenase class I and class II influences the efficient islet release from the rat pancreas.

BACKGROUND: Previous studies indicated different roles of collagenase class I, class II and neutral protease in the enzymatic islet release from pancreatic tissue. Because no information has been available, this study was aimed to investigate the isolation efficiency of different ratios between collagenase class II and I (C-ratio) in the rat pancreas serving as model for the human pancreas without being restricted by the large variability observed in human donors. METHODS: Rat pancreata were digested using a marginal neutral protease activity and 20 PZ-U of purified collagenase classes recombined to create a C-ratio of 0.5, 1.0, or 1.5. Collagenase efficiency was evaluated in terms of isolation outcome and posttransplantation function in diabetic nude mice. RESULTS: The highest yield of freshly isolated islets was obtained using a C-ratio of 1.0. Purity and fragmentation of freshly isolated islets were not influenced by the C-ratio. After 24-hr culture performed for quality assessment, a marginal but significant reduction of viability was observed in islets isolated by means of a C-ratio of 0.5 and 1.5. Islet in vitro and posttransplantation function revealed no negative effect mediated by different C-ratios. CONCLUSIONS: The present study demonstrates that the C-ratio is of significant relevance for the outcome after enzymatic rat islet isolation. The data indicate further that purified collagenase class I or class II does not damage islet tissue even if used in excess. The present study can serve as a start for subsequent experiments in the human pancreas