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 (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 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
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,
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.
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)]
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.
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.
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
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. 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%, similar to the drift tube, the latter should be preferred.