The kinetics of the combustion-relevant reaction of the allyl radical, a-C 3 H 5 , with molecular oxygen has been studied in a flow tube reactor at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source storage ring, using the CRF-PEPICO (Combustion Reactions Followed by Photoelectron Photoion Coincidence Spectroscopy) setup. The ability to measure threshold photoelectron spectra enables a background-free detection of reactive species as well as an isomer-specific analysis of reaction products. Allyl was generated by direct photodissociation of allyl iodide at 266 nm and 213 nm and indirectly by the reaction of propene with Cl atoms, which were generated by photolysis from oxalyl chloride at 266 nm. Experiments were conducted at room temperature at low pressures between 0.8 and 3 mbar using Ar as the buffer gas and with excess O 2 to maintain nearly pseudo-first-order reaction conditions. Whereas allyl was detected by photoionisation using synchrotron radiation, the main reaction product allyl peroxy was not observed due to dissociative ionisation of this weakly bound species. From the concentration–time profiles of the allyl signal, second-order rate constants between 1.35 × 10 11 cm 3 mol −1 s −1 at 0.8 mbar and 1.75 × 10 11 cm 3 mol −1 s −1 at 3 mbar were determined. The rates obtained for the different allyl radical generation schemes agree well with each other, but are about a factor of 2 higher than the ones reported previously using He as a buffer gas. The discrepancy is partly attributed to the higher collision efficiency of Ar causing a varying fall-off behavior. When allyl is produced by the reaction of propene with Cl atom, an unexpected product is observed at m / z = 68, which was identified as 1,3-butadienal in the threshold photoelectron spectrum. It is formed in a secondary reaction of allyl with the OCCl radical, which is generated in the 266 nm photolysis of oxalyl chloride.
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Characterization of Multianode Photomultiplier Tubes for use in the CLAS12 RICH detector
We present results of the detailed study of several hundred Hamamatsu H12700 Multianode Photomultiplier
Tubes (MaPMTs), characterizing their response to the Cherenkov light photons in the second Ring Imaging
Cherenkov detector, a part of the CLAS12 upgrade at Jefferson Lab. The total number of pixels studied was
25536. The single photoelectron spectra were measured for each pixel at different high voltages and light
intensities of the laser test setup. Using the same dedicated front-end electronics as in the first RICH detector,
the setup allowed us to characterize each pixel’s properties such as gain, quantum efficiency, signal crosstalk
between neighboring pixels, and determine the signal threshold values to optimize their efficiency to detect
Cherenkov photons. A recently published state-of-the-art mathematical model, describing photon detector
response functions measured in low light conditions, was extended to include the description of the crosstalk
contributions to the spectra. The database of extracted parameters will be used for the final selection of the
MaPMTs, their arrangement in the new RICH detector, and the optimization of the operational settings of the
front-end electronics. The results show that the characteristics of the H12700 MaPMTs satisfy our requirements
for the position-sensitive single photoelectron detectors.
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- Award ID(s):
- 2012413
- NSF-PAR ID:
- 10475818
- Publisher / Repository:
- NIM A
- Date Published:
- Journal Name:
- Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
- Volume:
- 1044
- Issue:
- C
- ISSN:
- 0168-9002
- Page Range / eLocation ID:
- 167446
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.nima.2022.167446
