4.2 Ion transport efficiency

The LEBL has two isotope dependent parts: the Wien filter and the drift tube. See figure 3.2 for an overview of the various elements in the LEBL. The Wien filter consists of a crossed variable electric and fixed magnetic field. The force on an ion with kinetic energyK is F= q(E + vB), which results in a deflection of the ion beam with an angle θ = arctan(qd(2KE +2KBm)) for a Wien filter of length d. The electric field is tuned such thatθ = 0 for the particles of interest. The relative change in the electric field from21Na to23Na is thus 4%. The mass dispersion of the Wien filter is 4.2%/mm at 2 keV at the MCP focus[69] (the MCP is situated on position E3, see figure 3.2). This is insufficient for our use. Indeed, the filter was designed to be used with the Radio Frequency Quadrupole (RFQ) cooling and buncher[195] which has a much smaller entrance aperture than the drift tube. Therefore contaminations can be expected to be present, both for21Na and23Na.

The transmission of the l= 1 m long drift tube is mass dependent as well. The maximal transmission is 50%, which is achieved for a switching frequency of 1

2l

K 2m. The transmission decreases linearly with a higher or lower frequency. For a beam energy of 2.8 keV the expected optimal frequency for23Na is 76.6 kHz. Experimentally however, 68 kHz is found to be optimal, which implies a duty cycle of 44% at this energy.

The ion transport efficiency is the ratio of the ion implantation rate into the neutralizer foil and the ion extraction rate from the Thermal Ionizer,

εionRion

Rsource . (4.1)

A measurement of this ratio is non-trivial as explained below. Our strategy is to determine the ion transport efficiency in two different ways, providing a consistency check. For the stable isotopes we measure the electrical current at two positions in the Low Energy Beam Line (LEBL). For21Na we measure the transport efficiency by detecting the 511 keV photons originating from the annihilation of theβ+particle.

Transport of23Na

The hot materials of the Thermal Ionizer (TI) contain trace elements, among which is

23Na. We assume that the ion beam produced from these trace elements has identical properties to that produced from the radioactive particles stopped in the TI foils. We measure the current after the drift tube, using the microchannel plate as a Faraday cup. The second position that we have used is the neutralizer foil.

We have measured the transport efficiency for four mass groups as listed in table 4.1 by observing the image of the focused ion beam on a phosphorescent plate after the drift tube (position E3 in figure 3.2). It was checked that the transport efficiency of the drift tube scales linearly with the drift tube frequency. The H2O+setting is close to the23Na setting, which might affect the efficiency measurement for23Na, leading to

Table 4.1: The transport efficiency of the LEBL for stable isotopes. At the21Na setting also stable ions are coming onto the neutralizer (about 20 pA).

Cell Isotope setting LEBL

of the Wien filter efficiency (%)

Cross H2O+ 32(4)

[Contamination at21Na] [20(2)]

23Na 26(3)

39K/40Ca 22(2)

133Cs/137Ba 15(2)

Cubic 23Na 32(4)

an apparent lower efficiency. The observed dependence of the transport efficiency on mass may indicate a residual velocity dependence. We estimate the overall systematic error due to the secondary electron yield and possible beam contamination to be 10%.

After installation of the cubic cell the 23Na beam parameters were checked by maximizing the optical signal of the MOT. This eliminates contributions other than

23Na. All relevant voltages of the LEBL are controlled by computer, allowing for this multidimensional scan to be performed in a few hours. The optimal LEBL settings found for23Na using this procedure are the same as found by optimizing on the current.

From these values the LEBL setting for21Na is calculated and used as a starting point for further optimization. Measuring the transmission at the21Na settings shows that a clean beam of21Na is not possible with the current setup (see table 4.1). The transport efficiency into the cross setup was measured to be 26(3)% and into the cubic setup 32(4)% (see table). The most probable reason for the difference between these two cells is that for the cubic cell we aligned the neutralizer foil optically in line with the LEBL, for the cross setup we found that only 2/3 of the neutralizer foil was visible by eye from the TI extraction point.

Transport of21Na

The determination of the transport efficiency using21Na requires knowledge of the efficiency of the detectors used to measure the 511 keV photons from theβ+ annihila-tion. In table 4.2 their calibration with a22Na source and typical numbers from a21Na beam time are reported. It is important to note the difference between the calibration source22Na and21Na. The annihilation detection efficiency is different because the range of theβ+particles from21Na is much larger than the range of theβ+particles from the decay of22Na (where theβ+annihilates inside the source). For this reason we apply a reduction of 50% to the detection efficiency when21Na hits a thick stopper, i.e. only theβ+particles moving into the stopper contribute. The neutralizer foil is not thick enough to stop theβ+particles, therefore we measure here in singles mode, avoiding a strong dependence on position. For the efficiency of the pair detection at

4.2 Ion transport efficiency 79

Table 4.2: Decay detection efficiency for a calibrated22Na source and typical21Na beam time conditions.

Isotope

Decay properties 22Na 21Na

Meanβ+energy (MeV) 0.2 1.1

Range in pyrex (mm) 0.35 6.5

Thermal Ionizer extraction

β+coincidence detection efficiency 3· 10−3 1.5· 10−3

21Na current ITI(/s) - 3· 105

Collector chamber

β+detection efficiency in singles mode 1.3· 10−2

21Na current ICC(/s) - 5· 104

Transport efficiencyε = IICC

TI - 20(10)%

the position after the TI, the source was put at the cup position where also the23Na current was measured.

Putting the source inside the CC is not practical, so we average the efficiency from two positions. The source is first put close to the neutralizer and then on the other side at the entrance to the cell about 10 cm further away. The difference by a factor of 3 is taken as the uncertainty on the final value.

In figure 4.2 the 511 keV photon count rate in the cubic glass cell is shown, as function of the Wien filter setting. This establishes the dispersion at the glass cell.

The line in the figure is a fit of a Gaussian function, it gives a mean of 565 V, with a widthσ = 18 V. The value for the mean is the voltage predicted from the scaling of the optimal transmission of other isotopes and the optimized settings for the LEBL obtained by maximizing23Na fluorescence signal from the MOT related to the ion beam. The Wien filter setting of23Na, expected and found at 540 V predicts then a

23Na contamination of about 36% for the21Na setting.

Summarizing this section, we have used various methods to estimate the efficiency of the transport from the Thermal Ionizer to the glass cell. We find a value of 30(3)%

for the cross setup and 35(4)% for the cubic setup using23Na. In standard conditions the yield of21Na is 5· 104ions/s in the glass cell for the cubic setup. With21Na we find a transport efficiency of 20(10)% for the cubic setup.

During the writing of this thesis, it was found from off-line measurements that the transmission efficiency for Ba could be substantially increased (close to 100%) by increasing the trapping frequency of the RFQ, the reason being the details of the end trap of the RFQ[279]. It might be worthwhile to check for21Na if this variable has

L) V -Wien (WienR

540 550 560 570 580 590

511 keV count rate (1/s)

200 400 600 800 1000 1200

Figure 4.2: The 511 keV count rate from21Na decays in the glass cell walls (3.5 mm thickness) measured by a NaI detector as function of the Wien filter voltage. The line is a fit of a Gaussian function.

been correctly optimized in the first test with21Na. The transport efficiency of 35% for Na might then be brought close to 100% and would also result in less contamination in the ion beam. Using the RFQ instead of the drift tube thus might result in a factor 3 higher ion transport efficiency. Note that, although the RFQ has previously been used with Na, it was omitted as making the system unnecessarily complex. If the RFQ could only achieve a total transmission of 40%[69], similar to the drift tube, the latter should be preferred.

In document University of Groningen Laser trapping of sodium isotopes for a high-precision β-decay experiment Kruithof, Wilbert Lucas (Page 86-89)