Trapping of 21 Na and 23 Na from a neutralized ion beam

In document University of Groningen Laser trapping of sodium isotopes for a high-precision β-decay experiment Kruithof, Wilbert Lucas (Page 109-118)

We first discuss time dependent backgrounds which we observed when studying the collection efficiency of the setup with23Na. This can be relevant information in understanding the diffusion process and possible chemistry on the neutralizer surface. Then we discuss the impact of the Doppler background fluorescence from

23Na for an optical signal from a21Na MOT cloud. Finally we discuss in this section the observation of a MOT signal with a neutralizer at room temperature.

23Na; time dependent background

We observed that23Na is present in the neutralizer foil upon installation and this amount increases when not all ions from an ion beam are released. Therefore, we have to distinguish between the atoms that originate from the freshly deposited ions and those that originate from previous depositions or were already present in the foil when it is built into the setup.

Without any ion beam present, just starting pulsing the neutralizer already shows a time dependent signal, see figure 4.16. Here the cycle is 20 s long and the neutralizer is heated 3 s long with a current of 6.3 A. Fitting individual peaks in the spectrum shows that the first MOT signals have a short lifetime, after ten pulses it becomes longer and reaches a constant value. We additionally observed that if the neutralizer is not heated for a few cycles the situation of the cold start is reproduced, though it is faster it still takes hours to reach the peak value again (in figure 4.16 the heating is not interrupted). Apparently in absence of heating the surface rapidly returns to its original state.

In figure 4.17a we show a measurement where we change the cycle time from 10 s to 20 s and to 40 s, the neutralizer is heated during 3 s. An additional time constant of

∼ 500 s is involved each time the cycle length is changed. In figure 4.17b the number

4.5 Trapping of21Na and23Na from a neutralized ion beam 101

Time since neutralizer on (s)

0 5 10 15 20 25 30 35 40

511 keV count rate (1/s)

225 230 235 240 245 250

(a)

Time since neutralizer on (s)

0 5 10 15 20 25 30 35 40

511 keV count rate (1/s)

286 288 290 292 294 296 298 300 302

(b)

Figure 4.14: The 511 keV count rate for two CsI detectors, near (a) and further away (b) from the periodically heated neutralizer foil.

Time since neutralizer on (s)

0 5 10 15 20 25 30 35 40

511keV count rate (1/s)

830 832 834 836 838 840 842 844

Figure 4.15: The 511 keV rate as detected in the cubic cell setup.

of trapped atoms and the lifetime of the MOT cloud is shown as function of the length of the cycle. We see from figure 4.17b that the MOT lifetime gets longer when going to longer cycle times. This is expected as less outgassing takes place. However, the MOT signal is higher for shorter cycle length.

A possible explanation for this behavior is the surface condition of the neutralizer.

During a longer cycle more deposits from gas molecules are building up at the surface.

For a Zr surface the incident rate of oxygen molecules at a pressure of 10−6mbar is 1/s per adsorption site [289]. For the typical 3 · 10−9 mbar, derived from the MOT lifetime, this means an adsorption timescale of order 300 s for a single monolayer.

This timescale is comparable with∼ 500 s timescale we observe. At low temperatures of the neutralizer this monolayer might form a significant barrier to diffuse through.

Doppler background fluorescence from23Na

In a MOT a strong isotope selectivity is naturally provided by the narrow bandwidth of the laser frequencies and narrow linewidth of the atomic transitions. This is also the case for the combination of21Na and23Na. Nevertheless Doppler fluorescence from the atomic vapor is much less dependent on the actual laser frequency and therefore

23Na can contribute to the signal of the21Na MOT.

Such background signals are shown in figure 4.18 for four different detunings with only pump laser light present. The signal does not dependent on the presence of the MOT magnetic quadrupole field, as expected as no repump light is present.

4.5 Trapping of21Na and23Na from a neutralized ion beam 103

Time (s)

102

×

4 103 2×103 3×103 104 2×104 3×104

/s)6 PMT count rate (10

0 2 4 6 8 10 12

Figure 4.16: The MOT fluorescence rate for a periodically heated neutralizer. The dropouts are due to lasers which lost their frequency lock. The data points are the average of the PMT rate on the time interval 2.5− 3.5 s within a cycle, note the logarithmic horizontal scale.

The large blue and red detuning prove that the fluorescence signal is due to Doppler background fluorescence.

For a blue detuning of the pump laser and a red detuning of the repump laser a signal of about 1.5· 103counts/s can be seen (figure 4.19). This figure corresponds to the right most point from figure 4.6a where the MOT atom number appears to be zero, but in fact the signal is just small. Therefore we can compare the MOT fluorescence rate and the rate observed here. The ratio between the maximal fluorescence rate in figure 4.6a to the rate for this detuning is 1.3· 103.

We now do an order of magnitude estimate of the expected signal for Doppler background fluorescence: An atom with velocityv spends a time of order d

v in the detection region approximated by a sphere of diameter d. Its fluorescence is recorded with a detection efficiencyε. The scattering rate from a laser beam is given by equation 4.2. With the polar angleθ to the laser beam, the laser frequency detuning due to the Doppler shift is given byδ = 2π/λ = kv cos θ (for example 6 m/s gives a 10 MHz Doppler shift forθ = π). The velocity is distributed according to the Maxwell-Boltzmann distribution, equation 2.8. The total number of photons Nc for a single

Time (s)

0 500 1000 1500 2000 2500 3000 3500 4000

/s)6 PMT count rate (10

(a) Transition of 10 s to 20 s cycle at∼700 s and at ∼2000 from 20 s to 40 s (the neutralizer started pulsing at t=-28000 s). The dropouts are due to unlocked lasers. The data points are the average of the PMT rate on the time interval 2.5− 3.5 s within a cycle.

Cycle period (s)

(b) The23Na MOT signal and MOT lifetime as function of the cycle length. The lines are to guide the eye.

Figure 4.17

4.5 Trapping of21Na and23Na from a neutralized ion beam 105

Figure 4.18: The Doppler background fluorescence signal from23Na, only pump laser light is present. The detuning from the21Na pump transition is indicated in the figures, a constant background rate is subtracted from the data.

laser beam is thus

m the most probable velocity. Integrating numerically we have for six laser beams, d= 1 cm, s0= 1 and T = 900 K on average 60 photons scattered by an atom. Not all the atoms fly through the detection volume, for the cubic cell setup the detection volume is about 40 mm away from the neutralizer foil. The solid angle is therefore about 8· 10−3, including an extra factor of two as the atoms are released into a solid angle of 2π.

Table 4.6 collects the data relevant for the discussion on the Doppler background fluorescence. From table 4.4 we take the number of implanted ions, the released fraction and the trapping efficiency. For21Na two laser frequencies are present which

Table 4.6: Estimated23Na Doppler background and MOT fluorescence rates for the cubic cell setup.

Number of implanted ions 1· 109

Released fraction 40%

Fluorescence mechanism Doppler MOT

Photons/atom/s 60 107

Collection efficiency - 10−5

Solid angle atom release 8· 10−3 -Fluorescence detection efficiency 10−5 Count rate (/s) 2· 103 2· 106

Time since neutralizer on (s)

0 1 2 3 4 5 6 7 8 9 10

/s3 (PMT count rate - 53) 10

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Figure 4.19: The PMT count rate for a 4 MHz pump and -5 MHz repump laser frequency detuning for23Na.

4.5 Trapping of21Na and23Na from a neutralized ion beam 107

both contribute to the Doppler background fluorescence. The largest contribution comes from the repump laser frequencies of21Na, which is 199 MHz above the pump frequency of23Na. This results in about 50 photons/atom/s (equation 4.12).

The pump frequency of21Na is 1648 MHz below the pump frequency of23Na, this gives 20 photons/atom/s. In total about 70 photons/atom/s can thus be expected from23Na background gas for21Na laser settings. As this is close to the value of 60 photons/atom/s we found for23Na in table 4.6. The same ratio can thus be used. The count rate due to Doppler background fluorescing23Na when using21Na laser settings is about a factor of 1000 smaller than the count rate from the23Na MOT count rate, keeping the LEBL settings for21Na.

We conclude that the Doppler background fluorescence rate of23Na contributes to the21Na MOT signal. For the laser frequency setting of21Na a background rate due to Doppler fluorescence from23Na of the order of 100 counts/s can be expected for a current of 1 pA of23Na, see discussion in section 4.2. This is about 10% of the expected MOT fluorescence rate on basis of the input currents of23Na and21Na respectively. For a fluorescence rate of 1000 counts/s for a21Na, as can be expected from measurements with23Na, the rate from Doppler broadened scattering from23Na atoms would then be about 100 counts/s.

Ion beam induced MOT cloud with a room temperature neutralizer The diffusion at room temperature is negligible (section 4.4). Surprisingly we observe with a cold (room temperature) neutralizer clearly a MOT cloud (cf. figure 4.20). The extraction of the 14 pA ion beam is switched on for 20 s, the lifetime of the MOT cloud is 3.9 s. We use different ion beams to see whether the23Na MOT is due to the

23Na ion beam or if any ion beam shows this effect. The results are listed in table 4.7.

Although the K, Ca ion beam gives also an ion related MOT cloud, the effect is a factor of 30 lower than for a23Na ion beam.

We optimized the ‘cold’ neutralizer MOT signal by changing the LEBL parameters.

These settings do not coincide with the optimal settings for a heated neutralizer.

This setting reduces the beam current measured on the neutralizer with about 70%.

Especially this last observation indicates that the neutralization might take place on the glass surface; in that case the neutralizer is not playing any role here.

For the francium setup at Legnaro, also a ‘cold’ neutralizer effect was observed for Fr by De Mauro et al., see page 63 of[78]. With the ion beam continuously on and the neutralizer continuously heated, they observe about 350 trapped Fr atoms in a steady state. With a cold neutralizer they observe about 80 trapped atoms in a steady state. The MOT lifetimes can be quite different in both situations and therefore the steady state atom number most probably changed for that reason as well. De Mauro et al. suspect that the ‘cold’ neutralization effect might be due to sputtering and that this effect depends on the intensity of the ion beam (local melting of the bulk material).

They observed the ‘cold’ effect only for high ionic beam intensities and never observed it with the Rb ionic beam (where the ions have, in contrast to Fr, a mass comparable to the neutralizer material Y, which has a mass of about 90 amu, Zr has a mass of

Table 4.7: The number of trapped23Na atoms with a room temperature neutralizer for different isotope settings of the Wien filter.

Wien Current Trapped Trapped filter (pA) atoms atoms (/pA)

23Na 11.5(0.3) 512(30) 45

K,Ca 35(0.5) 44(3) 1.3

Cs,Ba 2(0.5) 0(3) 0

about 91 amu). Finally, they state that the back-scatter fraction for Fr at Y at their beam energy is very low2.

To consider this further we calculate the combined neutralization and collection ef-ficiency from the cold neutralizer by converting the ion current assuming no secondary electron emission:

ε = NMOT

τI , (4.13)

where I is the number of ions/s, τ the MOT lifetime and NMOT is the number of continuously trapped atoms. From the measurement shown in figure 4.20 we find a combined neutralization and collection efficiency of 2.2· 10−6on a cold neutralizer surface. As 27% of this ion beam is not hitting the neutralizer foil (section 4.2), it may be that the atoms are neutralized at the glass surface. Then this process would have an efficiency which is at least 3 times larger than neutralization on the cold neutralizer.

Further, a possible explanation might be that these trapped atoms are originating from the neutralization of back scattered ions. According to simulations about 20%

of the 2.8 keV beam will be back scattered with a recoil energy of about 1 keV (see section 2.6). The back scattered fraction of Rb and Fr is negliglible, but still the ‘cold’

neutralizer effect is observed. It remains unclear what the underlying mechanism is.

For practical purposes the origin of the neutralization of ions for a room temperat-ure neutralizer is not very important. The fraction of the beam lost during transport is comparable to the expected back scattered fraction, therefore the calculated efficiency does not depend on which of these two processes we assume. The atoms which are neutralized continuous are not available for trapping in pulsed mode, where the atoms are released at once. Therefore it has a minor impact on the pulsed trapping efficiency, as can already be concluded from the measurements shown in figure 4.3b (cycle 1).

2We did a simulation to determine the stopping distribution and to determine the back scattered fraction of a 2.8 keV Na ion beam on a Zr target (see figure 2.8 for the results). We also did the simulation for a 3 keV Rb and Fr ion beam on a Y target. The ion range for Rb is about 51 Å and the back scattered ion fraction is about 2%, for Fr the values are an ion range of about 54 Å and no back scattered ions.

In document University of Groningen Laser trapping of sodium isotopes for a high-precision β-decay experiment Kruithof, Wilbert Lucas (Page 109-118)