HOLA LORA

The PMTs are the light sensitive part of the detector. Light incident on the photocathode of a PMT produces electrons through the photoelectric effect. The electrons propagate to a dynode under the influence of an electric field, which imparts enough additional kinetic energy to stimulate secondary emission in the dynode. This process repeats in a cascade over multiple dynodes, and a wired anode collects the electronic current.

Mounted on the end of each light guide is a Hamamatsu R5912 high quantum effi- ciency (HQE) PMT with a peak quantum efficiency of 35% at 400 nm (compared to 25% for a standard R5912). The PMTs are operated at voltages between 1500 V and 1900 V, matched such that the PMTs have a mean charge from a single photoelectron of 9.47 pC, with an RMS variation of 0.12 pC [132].

2.3. THE INNER VESSEL CHAPTER 2. THE DEAP-3600 DETECTOR

Figure 2.15: An exploded view rendering of a fully installed and constructed PMT mount. Ren- dering by Koby Dering.

Each PMT is attached to a light guide using a mounting assembly, shown in Figure 2.15. The gap between the PMT glass and light guide face is filled with silicone oil of viscosity 1000 cSt, with a refractive index of 1.403 specified in Ref. [133]. This min- imises reflections when light encounters boundaries with acrylic and PMT glass, which have refractive indices of 1.501 [134] and 1.458 [135] respectively for 440 nm light. The PMTs are connected to cables which power the PMTs and return low voltage signal from the PMTs to the electronics discussed in Section 2.6. The AV is shown with a complete set of PMTs installed and with all cables and foam blocks installed in Figure 2.16.

The PMTs used in DEAP-3600 were studied using a 532 nm pulsed laser by T. Cald- well et. al. in Ref. [136]. The time dependent current of each pulse is described by either a double or triple log normal pulse shape (82% and 18% of pulses respectively):

IPMT(t) = Q n

∑

For each component i, the τi are the mean arrival time of an electron at the anode; σi

are the root mean squared arrival time; and Ni are normalisation factors controlling the

Figure 2.16: Photographs showing the acrylic vessel: a) with all PMT mounts, copper collars and filler blocks installed, b) PMT cabling installed between two layers of polyurethane foam blocks. Photographed by Mark Ward.

double and triple log normal pulse shape respectively. Q is the total charge of the pulse, described by an analytic function fitted to the charge distributions of single and multiple PE pulses. The function parameters are determined using data from the AARF in-situ optical calibration source as discussed in Section 2.7.1.

Photoelectron production processes produce a set of pulse types observed in Ref. [136], listed below. The characteristic timing distribution the four pulse types is shown in Figure 2.17, relative to the peak of the prompt timing distribution shown in blue.

Prompt The result of a simple cascade from dynode to dynode towards the anode (blue curve at -18<t<24 ns). The pulse has a charge Q described by the model discussed in Section 2.7.1. The mean transit time from photocathode to first dynode has been measured at 25.26 ns. Occurs for 91.2% of photoelectrons produced at the photo- cathode.

Late Occurs when a photoelectron elastically scatters off the first dynode in the chain, travelling backwards before being accelerated again towards the first dynode by the electric field in the PMT (green curve at t>24 ns). The timing distribution is peaked at twice the measured mean transit time, or 50.52 ns, and the pulse has the same

2.3. THE INNER VESSEL CHAPTER 2. THE DEAP-3600 DETECTOR , ns) PMT t - PMT

Relative Transit Time (t

20 − 0 20 40 60 80 100 120 ) PMT t- PMT P(t 6 − 10 5 − 10 4 − 10 3 − 10 2 − 10 1 − 10 1 Prompt Late Double Early

Figure 2.17: PMT pulse timing distributions for prompt, late, double and early pulse types, with each distribution normalised to unit area under the curve. Pulse times are shown relative to the peak of the prompt PDF shown in blue. Reproduced from data in Ref. [136].

charge Q as a prompt charge. Occurs for 3.2% of photoelectrons produced at the photocathode.

Double Occurs when a photoelectron at the first dynode inelastically recoils away from the first dynode in the chain, producing a first photoelectron and returning after being accelerated towards the dynode to produce a second photoelectron (pink curve at t>24 ns). The two photoelectrons produce two pulses with charges Qi that sum

to the prompt charge, ∑2i=1Qi= Q. The first pulse timing distribution follows the

prompt distribution and the second pulse time is distributed with a peak at 50.52 ns, or twice the mean transit time. Occurs for 5.5% of photoelectrons produced at the photocathode.

Early When a photon passes the photocathode and PMT glass without interacting. The photon then creates a photoelectron directly at the first dynode earlier than the prompt peak by approximately the mean transit time (purple curve at −30< t < −10.5 ns). The cascade takes place as usual, but over fewer dynodes, resulting in a pulse with a smaller charge than a prompt pulse. Occurs for 1.2% of photoelec- trons which reach a PMT photocathode. The other 98.8% produce prompt, late and

double pulses.

Afterpulsing The result of a photoelectron ionising residual gas in the vacuum inside a PMT. The positive ions move towards the photocathode under the influence of the electric field and produce photoelectrons. Afterpulses are detected several mi- croseconds after prompt pulses, with small charges Qi which satisfy ∑n_i=1APQi= Q

for nAP afterpulses.

Dark Pulses The result of thermal electron emission at the photocathode. These occur uniformly distributed in time, at a rate of 500 Hz, with the same charges Q as a prompt charge.

Additionally the neck is wrapped with four bundles of wavelength-shifting optical fibres with a peak absorption wavelength of 430 nm and a peak re-emission wavelength of 476 nm. The fibres cover the first 10 cm above the point where the neck meets the AV, and are connected to four Hamamatsu High Quantum Efficiency R7600-300 PMTs. The assembly is used to observe light produced by alphas near to or inside of the neck.