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In pulse counting mode, the particular shape of the observed pulse height distribution fur- ther enables to experimentally obtain the maximum attainable quantum yieldηdetector with a given CEM detector as stated by eq. 2.6. In space charge saturation, this results due to the shifted and compressed quasi-Gaussian pulse height distribution at the CEM anode (see subsection 3.1.2). In pulse counting mode with the pulse height distribution shifted to higher pulse height amplitudes (see subsection 3.3.3), the trigger level of any subsequent pulse proces- sing circuitry can thus be adjusted to considerably high pulse height amplitudes. Moreover, this additionally prevents the eventual counting of any spurious counts usually associated with a low trigger level (see subsection 3.4.2, subsection 3.4.3, and subsection 3.4.4). In space charge saturation therefore, the particular shape and position of the pulse height distribution assures that literally no pulses are lost by counting, or are additionally introduced by a low trigger level.

From a calibration measurement point of view, the observed CEM quantum yield will thus only suffer from detector-related properties as, e.g., an inefficient conversion of the incident primary particle at primary particle impact, or an early secondary electron avalanche die-out in the CEM channel. As a result from the measurements of this section and the previous section 3.3, the observed quantum yield ηdetector of this thesis will therefore not be associa- ted with any pulse loss or with additional spurious counts induced by the subsequent pulse processing electronics22. Remarkably, this leaves the count rate measurements and the cor- responding absolute detector efficiency calibrations of this thesis work unencumbered by any non-detector based counting errors. By this, the raw quantum yieldηdetector of a CEM detec- tor can be calibrated to absolute values23 , and compared to the cascaded dynode detector theory of chapter 2.

21

If single pulse traces are recorded as displayed in fig. 4.4, there is no associated pulse processing dead time within the time window of the observed trace. In contrast to the pulse counting mode with a discriminator unit, this allows to measure single pulse trains consisting of two or more consecutive pulses within a short time window oft <100 ns.

22

This implies that the observed count rateN0will be identical to the true count rateN (N0=N), whereN

corresponds to the incident flux of primary particles at the CEM detector entrance.

23_{This results from the absolute efficiency calibration via counting coincidences (see section 5.1), and the}

4. Joint Channel Electron Multiplier Detector

In this chapter, the concept of photoionisation detection and the experimental benchmarks for the joint CEM detection system of this thesis are introduced. The CEM detection system is primarily intended to serve as a readout unit for single neutral atoms in an optical dipole trap in the context of a loophole-free Bell test experiment under strict Einstein locality con- ditions with two remote 87Rb-atoms [18, 21]. For the implementation of the photoionisation detection scheme into the actual single atom trap configuration, the experimental setup of the CEM detection system is explicitly described and theoretically examined. By means of scan measurements using photoionisation, the imaging of the ionisation fragments in the CEM detection system is calibrated and investigated for a stable and reproducible operation of the CEM detection system over several months.

In section 4.1, a brief introduction is given which illustrates the potential of photoionisation detection with CEM detectors in comparison to alternative detection approaches. It further highlights the conceptional design and basic operation criteria of the joint CEM detection system of this thesis work. Accordingly, the experimental realisation of the system is described in section 4.2. Here, the vacuum system, the glass cell setup for the single atom trap, and the integrated CEM detection system are individually characterized. Additionally, initial design criteria and construction considerations are discussed which are required for the specific application of the CEM detection system as single atom readout unit in the actual single atom trap environment [45].

A simulation of the electric potentials of the integrated CEM detection system is performed in section 4.3 by means of numerically solving the corresponding differential equations on ba- sis of the finite element method. The simulation of the potential distribution allows to model the expected flight timesti,e of the generated photoionisation fragments until their impact in the corresponding CEM detector (see section 4.4). It further enables to compare the calcu- lated values with experimentally obtained flight times out of the correlation measurements in chapter 5. Together with the transit time ttransit of the secondary electron avalanche in the CEM (see section 3.2), this permits to calculate the detection time tdet of the two ionisation fragments with the joint CEM detection system, and thus the detection time to detect a single neutral atom.

The calculated flight times ti,e of section 4.4 will further yield an estimate of the impact position of the photoionisation fragments in the CEM. In combination with the correlation measurements and the measured flight time difference ∆t of chapter 5, this will enable to experimentally determine the explicit impact position of the incident primary 87Rb-ions in a CEM detector (see section 5.3). The impact position of the incident primary particles is particularly important in the aspect of obtaining a high quantum yield ηdetector with a given CEM detector (see section 2.4). The knowledge of the explicit impact position thus permits to evaluate the associated impact parameters Ekin and θ, enabling to relate the corresponding parameters to secondary electron emission yield values δ0 at isolated CEM surfaces. It therefore allows a general comparison of the observed CEM detector quantum yield ηdetector with cascaded dynode detector theory as introduced in chapter 2.

The potential simulations of section 4.3 additionally show that the influence of the inter- nal CEM potential on the incident primary particle becomes macroscopic for small kinetic particle energiesEkin <1 keV at CEM cone entrance forany CEM detection system. This is particularly interesting as the kinetic particle energy at CEM cone entrance is usually stated as reference parameter for efficiency calibration curvesηdetector(Ekin)of CEM detectors in the literature (see section 2.4). However, the kinetic energy at primary particle impact in relation to the kinetic energy at CEM cone entrance is significantly different as for low kinetic par- ticle energies. This generally leaves different efficiency calibrations in the literature difficult to compare with each other even for identical CEM models if the impact positions of the primary particles in the CEM are not additionally stated [48].

The potential simulations and the flight time model also allow to evaluate an eventual particle loss during imaging of the photoionisation fragments into the CEMs. This effectively provides a theoretical estimation of the collection efficiencyηcolof the joint CEM system of this thesis. The calculations show that the collection efficiency of the joint CEM detection system for the generated photoionisation fragments approaches unity. In contrast to any conventional particle detection system, this enables to observe the raw quantum yieldηdetector of a single CEM detector with the current CEM detection system. As a remarkable property of this detection system, this leaves the efficiency calibration of this thesis work unencumbered by any eventual particle loss associated with non-detector related properties or counting errors (see section 3.4).

In section 4.5, the imaging of the photoionisation fragments in the CEMs is experimentally calibrated using 2D-scan measurements at a fixedz-position between the CEM detectors. For this, the photoionisation of neutral atoms in a defined ionisation volume as correlated particle pair source is employed. The simultaneous imaging of both photoionisation fragments in the CEMs allows to observe coincidences of the correlated charged particle pair. The counting of these coincidences enables to calibrate the CEM detectors to absolute efficiencies (see section 5.1). In additional measurements, the temporal and spatial performance stability of the CEM detection system is investigated. The calibration measurements indicate a stable and reproducible operation of the CEM detection system for several months of detector use. The joint CEM detection system thus fulfills the basic operation requirements as an atomic readout unit for a future loophole-free Bell test experiment under strict Einstein locality conditions [18, 21]. The beam overlap measurements of section 4.5 further experimentally demonstrate the spectroscopic and the spatial selectivity of the photoionisation detection scheme as introduced in section 4.1. This leaves photoionisation in a defined volume as the unique realization of a correlated particle pair source for the calibration of charged particle detectors (see section 5.1).

Conceptionally, the particular design and configuration of the joint CEM detection system enables a large optical access to investigate and optically manipulate the particles to be detected with external laser sources. In contrast to common charged particle detection systems integrated in bulk metal systems, the large optical access due to the glass cell environment thus opens the opportunity for a wide range of, e.g., spectroscopic applications with the CEM detection unit in future experiments, particularly in combination with cold atom beam or trap systems. For an experimental implementation of the detection scheme however, the spatial limitations of the current single atom trap setup imposes significant experimental challenges for the design and integration of the charged particle detection system in the actual UHV

4.1. Concept of photoionisation detection