UK Calibration Activities Lee Thompson University of Sheffield 3rd Hyper-Kamiokande EU Open Meeting CERN 27th -28th April 2015

Calibration topics PMT gain calibration PMT timing calibration Water attenuation length measurement Water scattering measurement Simulating the Cerenkov ring Need to understand: Coverage Redundancy Intensity Wavelength Light profile

Proposed systems (UK) Pulsed light sources Short duration light pulses from LEDs Light coupled into optical fibres (possibly via lens and standard fibre) Graded index multimode fibre from pulser to injection point Fibre ends inject light directly into the detector (via a diffuser) Illuminate multiple PMTs on other side of a tank Continuous low pulse rate operation during data taking No risk to the detector (mechanical, radioactivity) Electronics (which may require intervention) is easily accessible Pseudo-muon light source Create a Cerenkov light cone at a fixed angle Mimic expected Cerenkov spectrum No use of radioactive source to generate Cherenkov light

Calibration requirements PMT amplitude and timing monitoring of t0 of the pulse, width of the pulse diffuser profile - needs to be uniform in terms of: time, width, intensity, angular occupancy (a single PMT should be hit by >4 diffusers) dynamic range (to permit single and multiple photoelectron measurements) single wavelengths Scattering in water multiple wavelengths down to ~300nm narrow beams of order  1 degree (no diffuser?) monitoring of intensity

Calibration requirements Attenuation in water multiple wavelengths down to ~300nm intensity monitor fine control over light level wider beam than for scattering - or order 30 degrees (needs simulation studies), uniform in intensity and angle good pulse-to-pulse reproducibility

Pulsed light sources: Why LEDs? Pros LEDs are cheap (few $/£/€ each), low cost driver electronics Exist in the wavelength range of interest to us Possible to cover a wide spectral response with multiple LEDs Spectral emission in a narrow wavelength range (e.g. 10-15 nm) Can be pulsed at few nanoseconds pulse width Number of photons per pulse at fibre end from 103 to 105 Significant UK expertise in using LEDs for calibration systems (ANTARES, SNO+) In principle relatively easy to couple to optical fibres to deliver light to a remote location

Pulsed light sources: Why LEDs? Cons (LEDs) LEDs aren’t designed to be pulsed, no guarantee that a particular LED will deliver ns pulses – need to identify suitable models Variations within a batch of nominally identical LEDs Higher drive currents required to illuminate compared to laser diodes Coupling of fibre to LEDs leads to large optical power losses, need to improve on current method for small core diameters Cons (Laser diodes) high cost of laser diodes with individually optimised coupling into graded index fibre laser mode hopping – need to strictly control temperature and current to avoid this, leads to amplitude variations   coherent light, possible unwanted interference patterns in light output, non uniformity within light cone  fast laser modulation relies on a bias current – may lead to unitended light emission between pulses.

Pulsed light sources: flow diagram

Pulsed light sources: illumination mode

Pulsed light sources: illumination mode Features Front end uses programmable FPGA which controls multiple LED driver (pulser) circuits on one board LEDs will be coupled to graded-index multi-mode optical fibre, low dispersion (100ps per 100m) and low attenuation. Large core step index fibre would lead to unacceptable 14 nanosecond optical pulse width over 100 m. Fibre to be coupled to a suitable diffuser at the detector end for PMT and water attenuation measurements, different or no diffuser for scattering measurements Aims Inject of order 103-104 photons per flash Minimum optical pulse width (few nanoseconds) Minimize losses between LED and diffuser

LED, fibre and couplings LED will be directly coupled to the graded index fibre either via a drilled hole in the LED plastic lens via a step index fibre and lens First option enables the fibre to be placed close to the diode emission region Graded-index multimode fibre small diameter (60 micrometer) presents a challenge for efficient coupling, various coupling mechanisms under consideration

Pulsed light sources: monitoring modes Propose to monitor the light flashes at 2 different points in the chain: At the LED Will have a small SiPMT coupled directly to the LED to measure the characteristics of each individual light pulse. At the diffuser Directly coupled return fibre of the same type as delivers the light to monitor the average optical pulse shape and round trip time. Both systems will constantly monitor light pulse characteristics, e.g. amplitude, timing, pulse-to-pulse reproducibility, longevity, variation with environmental parameters, Known and stable pulse characteristics essential, e.g. to measure water properties including attenuation and scattering

Pulsed light sources: monitoring at the LED

Pulsed light sources: monitoring at the diffuser

LED pulser designs: status Three different LED pulser designs under consideration based on MOSFETs and BJT (bipolar junction transistors): Quad MOSFETs in a H-bridge arrangement with on chip driver circuits Quad OpAmps – may struggle to deliver sufficient current Zener diode Monitoring circuit also prototyped White Rabbit tests underway

LED pulser designs: status The baseline design is a quad-MOSFET circuit. A dual MOSFET driver in SNO+ gave < 1 ns at 104 photons per pulse into the fibre Investigating discrete transistors (BJT) circuits as alternative Several prototype LED pulsers now constructed and under test

LED couplings: status Have checked l460nm light over 100m of graded index multimode fibre. Series of tests to scan the light profile across a standard SM LED (that can be pulsed) in progress Using a 3D scanning stage with 0.01mm positioning accuracy and Thorlabs optical meter 300 micron beam spot close to device These tests will help to guide the final decision on which coupling method between the LED and graded index fibre

Pseudo-muon light source Objective is to inject a Cerenkov-like cone of light into the detector Simple application of refraction across a boundary of different refractive indices Can be achieved using a short, narrow transparent (acrylic) tube along with a light source which produces almost parallel light Different muon momenta can be achieved by using different lengths In the limit of q1 -> zero and n1 -> b (≈1) then q2 -> qCerenkov Independent of ni and wavelength 2.5m line source simulates 500MeV muon Vertical scale ≠ Horizontal scale

Pseudo-muon source: next steps Optical simulation of the source to drive the design Identification of a suitable selection of LEDs that, when their output light is convoluted, will give a good approximation to a Cerenkov spectrum Tests of a prototype in a large water tank in the UK to evaluate angular profile and optical output Ultimately deploy in 1 kton prototype

Simulation work: scattering calibration Fibres mounted on PMT support structure with collimated (~few degree opening angle) beams fired across detector Fast pulses (~2ns, LED or laser) at 4-5 different wavelengths to probe λ4 Rayleigh scattering dependence. Operate at low intensity so in-beam PMTs mostly in single pe regime.

Simulation work: scattering calibration Analyse through summation of hits over run Use timing and spatial cuts to select analysis regions dominated by direct, reflected and scattered hits. Scale scattering in MC to match data scattered hits Use in-beam hits for normalisation

Simulation work: PMT timing calibration Propose similar approach to SNO+ SNO used centrally deployed isotropic laserball SNO+ will use embedded fibres, ~30 degree opening angle, sharp LED pulses To account for different delays for each LED driver relative to trigger, ensure each PMT sees light from ≥ 2 fibres. (more for redundancy) Cycle through all fibres at high rate Scan LED intensities to sample whole charge range.

Simulation work: PMT timing calibration Plot PMT hit times vs charge for each channel Linear offset = PMT cable delay Low charge turn up = time walk effect

Timelines 2015 2016 2017 LED PULSER BOARDS, TESTING 2015 2016 2017 LED PULSER BOARDS, TESTING Board Prototyping Pulse characterisation with LEDs Final board choice and tests LED – FIBRE COUPLING, DIFFUSERS Optimise LED-fibre coupling Diffuser performance and choice Characterisation of light pulses in water PSEUDO-MUON SOURCE Optical modelling and simulations Design and build prototype Water tank tests SIMULATIONS

Summary and next steps UK activities in this area have started in earnest A large number of UK universities involved: Liverpool, Sheffield : LED pulsers, characterisation, testing Warwick: LED-fibre couplings Lancaster: DAQ-calibration interface Imperial, Edinburgh, Liverpool, Sheffield: Muon source QMUL: simulations Initial simulation studies underway First prototype LED pulsers now constructed and under evaluation LED-fibre coupling schemes under study