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2026-02-24

Timestamp:: Feb 27, 2026, 10:15:58 AM (3 months ago)
Author:: ccd24
Comment:: --

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Research/LhARA/RadiationBiology/Meetings/2026-02-24

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 ==== Sparse Scintillating-Fibre Array ====
+Calvin has simulated the expected energy deposition in an individual fibre. It appears that many fibres will potentially be saturated, obviously depending on their position in the beamline. Generally, we want to avoid placing any electronics within the vacuum chamber as the EMP from the laser-target interaction will likely knock these out. Therefore, Peter discussed different methods of transporting the light outside of the chamber. Peter has simulated one fibre with a Polystyrene (Core) and PMMA (cladding) as per BCF20. He has tested using another fibre of wider diameter to transport it out, as well as using lenses to transport the light. The fibre transport 7% of the light while the use of two paraxial lenses transports 4.6%. The lateral, angular and longitudinal offsets of the transporting fibre were all tested for their sensitivity. The transport was most sensitive to the longitudinal offset. The next step is to consider how to transport the light from multiple fibres out of the chamber.
+==== Air Scintillation ====
+==== Optical Air Fluorescence Monitor ====
+This is similar to a gus curtain monitor. The beam causes excitations in the air that causes fluorescence. The emitting species can either be charged or neutral. The largest fluorescence comes from N_2 but since this is neutral the excitation is thermal and must be captured quickly. Once collected the count rate can indicate the current, with appropriate modelling. Colin and Narender are looking at completing a systematic study to understand how the intensity profile is related to a dose profile collected on a piece of EBT3. Current experiments use 10mm of air but this can be reduced down to a few mm and the main constraining factor is the equipment size. Peter has worked with N_2 fluorescence before and the main issue he found was the sunlight background. However, a pulsed beam like PoPLaR or LhARA might generate a high enough signal to avoid the background.
 ==== Tony's Suggestions ====
+Tony had a few different suggestions all building to a final idea that Sam Flynn at the NPL is working on.
+Firstly, Tony's project of CMOS. This is too thick to be used at PoPLaR, and also has an issue with the dose rate.
+A very thin phosphor sheet. This is work with Simon Joly. Can deposit on thin Kapton. Peter has also worked with phosphor experts at Brunel. This could in theory be added to the bottom of a newly designed cell dish. We are calling this the Instrumented Cell Dish Bottom technique. The main issue would be the optics and transport of the light. Could make the cell dish lid see through and carry the light out the other side?
+Ultra-thin Secondary Electron Emission. Will not work for our energies at the current spec. It is comprised of nm Al on um Kapton. Tony is going to test some at Birmingham. There will be a problem with putting this in the vacuum chamber due to the EMP and the electronics involved in the detector. It is possible that there would also be interest in this from CNRS. If it becomes sensitive enough for our aims, then it may be possible to generate an x-y profile on a 10x10 grid.
+Last Design: Secondary Standard Calorimeter. Sam Flynn at NPL has been designing a version for FLASH, with a final prototype built and the last tests before production were done in 2025. Currently, the lowest measurement is 2Gy. Sam believes it could reach 0.3Gy. Would be placed in front of the cell dish, so the fear of using electronics due to the EMP is less of an issue. There is still an issue in that an energy spectrum would be required to fully characterise the beam. This could be done with Nick's time-of-flight detector, but this runs into the same issue as the Thompson Parabola because it only samples a small portion of the beam.
 ==== RCF Use and Potential Diagnostic ====
 Diaza has written an RCF protocol explaining key issues, including film orientation and placement, scanner warm-up and the Callier effect. For the last one, we require a glass sheet to cover the film when being scanned. Further reading was undertaken on the RCF calibration and a technical document will be added to the wiki to describe the best practice. One key insight is that using the Polynomial Model but fixing c provides a much lower error when calculting the dose at little cost to the accuracy of the measurement. The error reduced from between 5 and 16% to 3-4.5% in a range of 0-18Gy. It was also suggested that a calibration could be undertaken using all three channels as this reduces the error further.
+Diaza has written an RCF protocol explaining key issues, including film orientation and placement, scanner warm-up and the Callier effect. For the last one, we require a glass sheet to cover the film when being scanned. Further reading was undertaken on the RCF calibration and a technical document will be added to the wiki to describe the best practice. One key insight is that using the Polynomial Model but fixing c provides a much lower error when calculating the dose at little cost to the accuracy of the measurement. The error reduced from between 5 and 16% to 3-4.5% in a range of 0-18Gy. It was also suggested that a calibration could be undertaken using all three channels as this reduces the error further.
 The larger RCF films were then used to determine whether placing RCF around the edge of the cell dish would allow you to estimate accurately the mean dose in the cell dish. It was found that measurement error alone could not explain the residuals from the linear fit, so a constant error term to measure the film-to-film variation was added. Once this uncertainty was increased to 0.23Gy the reduced chi-squared dropped from around 6 to around 1. This means that this error will become substantial at low doses.
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+Actions Required
+== Actions Required ==
 * Get a glass sheet
+* Investigate dose prediction from RCF infront of cells