Proton Calorimetry/Meetings/2017/05/11

Minutes for UCL Proton Calorimetry Meeting, 11th May 2017 (D17, Physics & Astronomy, UCL)

Simon Jolly, Laurent Kelleter, Anastasia Basharina-Freshville

ABF:

Summary of data taken at MedAustron.
First night:
- Rates generally too high for 5 second spill: lots of pile-up.
- For 30 second spill, pile-up seen for longer gates.
- Fit to resolution peak needs improvement: sometimes double peaks appearing on fitted peak.
- PMT may be saturating: running HV at -1200 V.
Second night:
- Some reduction in beam current through reducing final quad strengths; much better low current stability from increasing chopper current to scrape on chopper dump.
- 6mm collimator and chopper scraping gave 100–150 kHz rates.
- Ran energy scan from 252.4–62.4 MeV.
- Generally excellent linear response between energy and ADC peak; some odd results, probably due to pile-up.
- Energy is highly reproducible despite PMT current limit: good performance from UCL detector and MedAustron accelerator!
- Energy resolution as a function of energy shows √(E) dependence but data hand-selected...
- Clear double peaks seen in 112.4 MeV measurements: probably correlated to PMT current limit; lower current at start of spill, then higher current gives 2 rates.
Need to confirm how Caen ADC is doing pedestal subtraction.
Also finished running a whole battery of tests on PMT rate and current saturation: all sent to Hamamatsu.
Tested signal cables to see effect on resolution: longer cables degrade resolution.

LK:

Looking at dose build-up effect in proton therapy and secondary particles:
- Electron contribution to dose increases with smaller range cut: more short-range electrons produced.
- Threshold at ~1 micron: for shorter range cuts, no difference in dose. Probably due to 990 eV "safety threshold" for ionisation in Geant4.
Fun facts about Geant4:
- Deposited energy ignores energy of secondary particles.
- Energy loss records energy of tracked particle and its secondaries BUT in the place where they are produced! (not where the secondaries deposit their energy).
- Range cut: Only secondaries which are expected to travel further then the range cut will be tracked BUT this theoretically only applies to secondary electrons, positron, gammas and protons.
- To be more clear: exotic secondaries as neutrons will always be produced and tracked, independently from the range cut. No influence of the range cut on the number of secondary protons seen.
- There is a safety energy threshold for the production of secondary particles of 990 eV since otherwise Geant4 models might fail.
LET-weighted results show much higher contribution from hadronic secondaries, particularly at high energy.
Range cut effects not clearly understood: ABF suggests contacting Ben Morgan at Warwick.

SJ:

SJ and ABF visiting Nuvia a.s. on Monday 29th May. SJ needs to pull his finger out and book his flights...
SJ talking at UCL Medical Physics PART Group meeting; LK may be press-ganged into joining.
During 3-day trip to PSI, discussed collaboration with Ros Perrin, Tony Lomax and Sairos Safai to look at effect of lung motion interplay during PBT dose delivery:
- LuCa lung phantom is tissue equivalent with air-driven breathing cycle: photos and movies can be found here.
- Plastic scintillator disc would be inserted into phantom and irradiated during treatment: light emission imaged by camera.
- PSI would look after camera, mounting and optics.
- UCL (ie. LK) would write the code to reconstruct the dose distribution in each slice from camera movie by fitting to quenched light output.
- Approximately 3 months total work for LK: would include time spent at PSI as part of OMA placement.
- ABF also interested in spending time at PSI.