The accuracy of the calibration of the CEM detectors to absolute efficiencies from the measu- red single countsNi,e andNbi,be at the corresponding detectors depends mainly on the single count to background ratios Ni/Nbi and Ne/Nbe (see subsection 5.1.2). This follows as an elevated background Nbi,be compared to a small signalNi,e will significantly affect the single counts Ni,e0 for detector calibration (eq. 5.5), and its associated errors ∆ηi,e. In the follow- ing therefore, possible sources of an enhanced background in the CEM detectors albeit from intrinsic CEM detector based dark counts (see subsection 3.2.3) are analysed and discussed. This follows as particularly for a long term operation of the joint CEM detection system for a future loophole-free Bell test [18, 21], stable and reproducible background rates are an important criterion.
Typical background rates for the joint CEM detector
In the calibration measurements, a significant increase in the background counts of the CEM detectors is almost entirely caused by the blue laser radiation of the ionising laser beam (fig. 4.1). The single application of the red laser beam(λ12= 780 nm)only with corresponding laser powers (see section 4.5) does not contribute to any increased background18. In contrast to that, already the sole presence of the blue laser beam (λ2i = 473 nm) only significantly increases the background count rates, particularly the electron background rate. Accordingly, the rate nbe rises proportionally to an increase in laser intensity (nbe ∼I2i) for the ionising transition. This effect is generally observed in several different calibration measurements.
18For an explicit definition of background countsN
bi,be and dark countsNdi,deat a CEM detector, see sub-
For a typical coincidence measurement as illustrated in fig. 5.4 (∆Uacc= 3.8 kV), the CEM detector based19dark count ratesndi,deare in the range ofndi= 20−30 s−1 for the ion-CEM, and nde = 250−350 s−1 for the e−-CEM. In contrast to that, the laser induced electron background rates nbe exceed the electron dark count rates nde by more than an order of magnitude (i.e., nbe = 2464 s−1 compared to nde = 250 s−1 in fig. 5.4). However, in relation to that the ion background rate nbi compared to the ion-CEM dark count rate ndi remains almost unaffected (nbi= 38 s−1 compared to ndi= 35 s−1).
Possible sources for background counts at the CEMs
For the detector efficiency calibration measurements, the continuous operation of the laser beams will become unfavourable, if the background count ratesnbi,be induced by the ionising laser radiation(λ2i = 473 nm) are too high. Enhanced background rates result in prolonged measurement times for the system, and even in an eventual electrical breakdown of the de- tection system caused by the elevated charge background. Therefore in the following, some possible sources of background enhancement in the CEM detectors are discussed. However, an elevated detector background in the calibration efficiency measurements will not constrain the future application of the joint CEM system as single atom readout in the single atom trap. This results as the neutral 87Rb-atom in the optical dipole trap will be photoionised by a pulsed readout scheme with both laser beams being blocked for most of the time during the experiment.
Primarly, an obvious source for enhanced background countsNbi,be in the CEM detectors will originate from photonic stray particles entering the CEMs. As the detectors have an non-vanishing detection efficiency for any photons in the visible and infrared spectrum (see subsection 2.4.2), additional background counts induced by stray light of the two laser beams
(λ12, λ2i) will be observed. However, this possible source can mainly be excluded as possible source for the enhanced detector background. This follows as if one interchanges both CEM detectors, using the previously used ion-CEM as e−-CEM and vice versa, again only the electron background rate Nbe now at the ’new’ e−-CEM significantly increases, while the ion-CEM shows comparable rates to the previous CEM detector configuration.
A second source of background will possibly stem from photoelectric emission of ambient surfaces next to the CEM detectors in the UHV induced by stray photons from the two laser beams. Even more, the photoelectric emission will further particularly explain the increased electron background for the ionising laser light(λ2i)in contrast to the non-significant increase in the ion-CEM background. Generally, photoelectric emission sets in at a threshold energy Eth∼Φof the specific surface, withΦbeing the electron work function of the corresponding bulk material. However, only the work function of alkali20 metals exceed the single photon energy (E12=~ω12= 1.59 eV, E2i =~ω2i = 2.62 eV) of the two laser transitions used in the calibration experiments of this thesis (e.g., Rubidium: ΦRb ∼ 2.26 eV). The work function of all other elements lies significantly beyond the single photon energy (e.g., Copper: ΦCu ∼
4.53−5.10 eV). Consequently, photoelectrons will thus only be emitted from Rubidium21 doped or coated surfaces in the UHV, caused by the increased Rubidium background vapour from the dispenser sources (see section 4.2).
19
Assuming all laser beams are blocked with no light in the glass cell setup.
20The values for the electron work function of alkali metals range fromΦ
Cs∼2.14 eVtoΦLi∼2.93 eV[237].
21
5.2. Electron-ion correlation measurements 10 3 10 -1 ions [s ]
dispenser current [A]
3 4 5 6 7 8 9 2 10 (a) electrons -1 [s ]
dispenser current [A]
3 4 5 6 7 8 9 2 10 (b) 3 10 1 10 3 10 -1 coincidences [s ]
dispenser current [A]
3 4 5 6 7 8 9 2 10 -2 10 -1 10 (c) 4 10 -3 10 -4 10 0 20 40 60 ions [#] TOF-ion [ns] 500 1000 1500 2000 0 (d)
Figure 5.8:Measured count rates per second for various dispenser currents at one particular relative position (x = 1.2 mm, y = −0.4 mm; fig. 5.13). In the figures (a-c), the values are displayed for measurements with the count ratesni (black scatter) and background count rates
nbi (red) for ions (a), for electrons (ne, nbe; (b)), and true/accidental coincidences (nc, nac; (c)). (d) Correlation histogram of measured time differences ∆t similar to fig. 5.4(a), but at a dispenser current of I = 3A (see values in (a-c)). Although the partial 87Rb-background pressure is vanishingly low at these currents (<10−10mbar), still a distinct photoelectron-ion correlation peak can be identified in the correlation histogram, demonstrating the spectroscopic sensitivity of the photoionisation detection method [176].
A third source for an increased CEM background is certainly related to outgassed volatile impurities out of the dispenser source which are emitted during dispenser operation. These outgassed impurities will either already be charged due to the thermal emission or will be subsequently photoionised in combination with the two laser beams of the calibration mea- surements, producing an enhanced charged particle background in the UHV. Referring to outgassed impurities without the presence of any laser radiation in the UHV, an increase in the background count rate of a CEM detector by enhanced degassing of impurities out of a dispenser source is experimentally observed by [164]. Nevertheless, the measurements show that a relevant increase in CEM detector background only starts at considerable higher pres- sures ofp >10−6mbar. In the aspect of photoionisation of outgassed dispenser impurities in the UHV, in fig. 5.8 the count rates and background rates for87Rb-ions (ni (black),nbi(red); fig. 5.8(a)), electrons (ne, nbe; fig. 5.8(b)), and the coincidences to accidental coincidences (nc,nac; fig. 5.8(c)) for various applied dispenser currents I are depicted. The measurements are performed at one particular relative position (x = 1.2 mm, y =−0.4 mm; fig. 5.13) with identical CEM operation parameters as used in fig. 5.4, while only altering the current I of the dispenser source. For high currents, an increased background is observed, although the residual background pressure in the UHV chamber remains still low (p < 7.8×10−9mbar
atI = 8.3 A). However, the background increase occurs simultaneously in both background count rates nbi, nbe (red traces; fig. 5.8(a,b)), also excluding this as possible source for an enhanced CEM detector background in thee−-CEM only.
For the particular detection scheme of this thesis (see section 4.1), photoionisation of atoms or molecules from the residual vapour background of the UHV is in general comparably unlikely as possible source for an increased stray charge background in the CEMs. For single photon transitions to the continuum, the ionisation threshold of any neutral atom22 is too large compared to the single photon energy(~ω12= 1.59 eV,~ω2i= 2.62 eV)of the used laser transitions. Moreover, also the photoionisation due to multi-photon transitions is extremely unlikely at the laser beam intensities used in the experiments (see section 4.1). Furthermore, photoionisation will always affect both background count rates in the CEMs, the ion and
the electron background, as it generates charged particle pairs. Therefore, atomic/molecular photoionisation out of the residual background is also not responsible for an enhanced electron background in thee−-CEM, too. This leaves the photoelectric emission from ambient surfaces in the UHV coated or doped with87Rb-atoms as possible source of the increased background count rate. However, an additional bakeout of the entire UHV system (see section 4.2) should in this case substantially reduce the elevated CEM detector background.