- Email: [email protected]
Upgrades to the collinear laser spectroscopy experiment at the IGISOL
Nuclear Inst. and Methods in Physics Research B xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Upgrades to the collinear laser spectroscopy experiment at the IGISOL R.P. de Grootea, , A. de Roubina, P. Campbellc, B. Chealb, C.S. Devlinb, T. Eronena, S. Geldhofa, I.D. Moorea, M. Reponena, S. Rinta-Antilaa, M. Schuhd ⁎
a
Department of Physics, University of Jyväskylä, PB 35(YFL) FIN-40351 Jyväskylä, Finland Department of Physics, University of Liverpool, Liverpool L69 7ZE, United Kingdom School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, United Kingdom d Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany b c
ARTICLE INFO
ABSTRACT
Keywords: Collinear laser spectroscopy RFQ cooler-buncher IGISOL
We give an overview of recent changes to the collinear laser spectroscopy beamline in the IGISOL laboratory. We present a new data acquisition system, commissioning of a newly installed charge exchange cell, and coolervoltage calibration measurements. Currently ongoing modifications to the RFQ cooler-buncher are also discussed.
1. Introduction Collinear laser spectroscopy is applied at the IGISOL-IV facility [1,2]. By overlapping a fast (30 keV) ion or atom beam with a highresolution continuous-wave laser in a collinear geometry, Doppler-free hyperfine spectra can be measured. From these spectra, the nuclear magnetic dipole and electric quadrupole moment, nuclear spin, and changes in the mean-squared charge radius can be determined in a nuclear-model independent way [3]. These observables provide deep insight into nuclear structure far from stability and offer stringent tests of modern nuclear theories [3,4]. Over the years, many improvements to the technique were pioneered at the IGISOL. The development of a radio-frequency quadrupole (RFQ) cooler-buncher [5] has led to a reduction in background by up to four orders of magnitude [6,7]. In-cooler optical pumping, also developed at the IGISOL, has furthermore extended the reach of the method [8–10]. Novel trapping techniques have been developed as well [11], aimed at for instance in-vacuum pumping before injection into the collinear line. Despite these developments, a number of elements have so far been inaccessible to the collinear apparatus at the IGISOL, in particular those elements that do not have suitable optical transitions in their ionic state. For such cases, a charge-exchange cell (CEC) can be used to neutralize the ion beam and thus produce a fast atom beam. Such CEC units have been used in several laboratories around the world, but have so far not been used for online experiments at the IGISOL. This article is structured as follows. In Section 2 an overview of the experimental apparatus will be given. Then, in Section 3.1 a new data
⁎
acquisition system will be presented in brief. In Section 3.2, the first offline results using a newly installed CEC will be presented. In order to investigate the systematic errors to be placed on future online measurements, offline tests with stable ytterbium beams were performed as well. These measurements will be discussed in Section 3.3. In parallel to these developments, the RFQ cooler-buncher has also been upgraded. Currently, ion bunches extracted from the cooler have a temporal length of about 10 μs. In order to maximize the mass resolving power of the multi-reflection time-of-flight mass spectrometer (MR-ToF MS), to be installed soon at the IGISOL, this bunch length needs to be compressed. The design goal of the modified cooler is to reach a bunch length of 100 ns, while keeping the energy spread of such short bunches below 40 eV. For collinear laser spectroscopy shorter bunch lengths are also advantageous, but since an increase in the energy spread of the ion beam results in a loss in resolution and efficiency, the trade-off between energy spread and bunch length needs to be investigated carefully. In Section 4, the first in a series of such systematic energy spread measurements will be presented. 2. Collinear laser spectroscopy at the IGISOL In the IGISOL laboratory, ion beams can be produced either by offline sources or by the IGISOL itself. Typically, beams are mass-separated using a dipole magnet with a mass resolving power of 400, except for one of the surface ion sources and the laser-ablation ion source which can be installed after the magnet prior to the RFQ coolerbuncher. In all cases, beam is injected into the RFQ cooler-buncher, which is discussed in more detail in Section 4. Ions are extracted from
Corresponding author. E-mail address: [email protected] (R.P. de Groote).
https://doi.org/10.1016/j.nimb.2019.04.028 Received 14 January 2019; Received in revised form 21 March 2019; Accepted 10 April 2019 0168-583X/ © 2019 Published by Elsevier B.V.
Please cite this article as: R.P. de Groote, et al., Nuclear Inst. and Methods in Physics Research B, https://doi.org/10.1016/j.nimb.2019.04.028
Nuclear Inst. and Methods in Physics Research B xxx (xxxx) xxx–xxx
R.P. de Groote, et al.
Fig. 1. Schematic view of the collinear beamline, laser systems, ion optics and diagnostic elements. The ion beam produced by the IGISOL enters the beamline from the right side.
the RFQ cooler-buncher at an energy of 800 eV. After passing through two quadrupole benders, the beam is then accelerated to 30 keV and injected into the collinear beamline. A schematic overview of this beamline is shown in Fig. 1. Electrostatic deflectors guide the ions into the charge exchange cell (CEC) and the light collection region (LCR). A quadrupole triplet is used to bring the beam to a focus in front of the (segmented) Photo-Multiplier Tube (PMT). Ion and laser beams are overlapped by tuning both through a removable 0.9 mm aperture. The beamline furthermore has two Faraday cups and a silicon detector which are used to optimize the transport efficiency. An off-axis Micro-Channel Plate (MCP) detector and a new MagneTOF mini-detector mounted on a linear actuator are installed after the CEC and LCR, which are used for both beam tuning and monitoring the neutralization efficiency. Several knife-edge baffles are placed near the LCR to reduce the amount of scattered light that reaches the PMT. All experiments at the IGISOL so far have used light produced by a Spectra Physics 380 dye laser, which uses intra-cavity doubling to generate up to 1 mW of UV laser light. This laser is pumped by a 5 W Coherent Verdi 532 nm pump laser. The dye laser is locked to an external cavity, which in turn can be locked to an iodine absorption line, or to a HighFinesse WSU/10 wavemeter to ensure long-term wavelength stability. The UV light is focused into a 1 mm spot in front of the PMT using a telescope system. Background rates due to scattered laser light are typically 100 Hz per 100 μW of laser light.
Python and EPICS-based control software for the whole IGISOL. Synchronization of cooler release, the stepwise voltage scan and the TDC card is achieved using a PulseBlaster PB24-100-32k-PCI Programmable TTL Pulse Generator. This pulse generator triggers the TDC card, cooler release, and the digital multimeter. Other information which does not require this precise synchronization, such as the cooler voltage or laser wavelength readout, is obtained from EPICS variables that are periodically published on the internal IGISOL network. The wavemeter lock of the dye laser also relies on the EPICS-published wavemeter readout, since the wavemeter is connected to a different computer in another part of the laboratory. A software PID-loop stabilizes the wavelength of the laser by supplying a ±5 V signal to the external stabilization cavity of the laser. This voltage is generated by a Measurement Computing USB-230 data acquisition board. The laser can be scanned around modehop-free over a range of 1 cm−1. Larger wavelength changes require manual intervention. 3.2. Charge exchange cell A charge exchange cell was added to the beamline at the start of 2018. This charge exchange cell, and the vacuum chamber that houses it, were originally designed for the collinear laser spectroscopy beamline at the TRIGA facility in Mainz, Germany [12]. The cell consists of a cylinder which can be filled with a neutralizing element of choice (e.g. K or Na). The cell is mounted on a platform which is electrically insulated from the chamber, so that it can be floated to ± 10 kV. This is used to Doppler-tune the atoms onto resonance with the laser. It is electrically heated using a variable AC power supply, while the ends of the cylinder are kept at a fixed temperature using an Lauda RA-8 oil circulator. This circulator is filled with GALDEN HT-230 heat transfer fluid, chosen for its good thermal conductivity, low electrical conductivity and low flammability. Ions not neutralized in the CEC are deflected from the atom beam using a pair of electrostatic deflector plates. The first tests with the CEC at the IGISOL were performed using a stable beam of 58Ni and using K as the neutralizer. Nickel was studied in detail using the BECOLA beamline at NSCL [13], making it an ideal first test case. In this work it was found that the two meta-stable levels, the 3d9(2D) 4s 3D (J = 3) at 204.787 cm−1 and 3d9(2D) 4s 3D (J = 2) 879.816 cm−1 are populated approximately equally. From these two levels, there are two strong optical transitions, at conveniently close wavelengths of 300.363 nm and 300.249 nm. Since the CEC and the light collection region of the BECOLA setup differ from the setup at the IGISOL, efficiencies and signal-to-background ratios are expected differ, but similar atomic populations after charge exchange can be expected. The temperature of the cell, measured by a thermocouple attached to the top of the cell (a few cm away from the reservoir itself), was 130 degrees during these measurements, while the ends of the cell were cooled to 70 degrees. The potassium reservoir is likely hotter, since it is wrapped tightly with heating wires. Fig. 2 shows typical resonances obtained using these two transitions. These plots show the photon rate, normalized to the beam intensity (measured with an unsuppressed Faraday cup) and UV laser power. Very similar signal rates and signalto-background ratios are obtained for both transitions. These
3. Recent changes and upgrades 3.1. New data acquisition system Signals from the PMT segments are amplified by a Fast Timing Amplifier (ORTEC FTA820A) and discriminated by a constant-fraction discriminator (ORTEC model 935). These signals are then time-stamped by a Cronologic TimeTagger 4-2G Time-to-Digital-Converter (TDC). This board has four channels, each with a 500 ps single-shot resolution. The hyperfine structure of an isotope is scanned by applying an acceleration or deceleration potential to the ion beam. Ideally, this potential is changed with precise reproducibility and with a short settling time. To achieve this, a voltage between −4 and 4 V, generated by one of the 16-bit analog outputs of a Measurement Computing USB3102 data acquisition board, is amplified using a ×1000 TREK 609E-6 high-voltage amplifier. The large slew rate (specified as 150 V/ µ s by the manufacturer) and fast settle time of both devices makes fast voltage sweeps possible. The settle time for a 5 kV voltage change was measured to be less than 200 μs, well within experimental requirements. After every scan, a calibration of the voltage scanning range is performed by a Keysight 34465 digital multimeter which reads out the scanning voltage after a 1:1000 voltage divider. The cooler platform voltage can be similarly monitored using either a Solartron 7071 voltmeter or a second Keysight 34465 multimeter combined with a 1:10000 voltage divider. Calibration measurements of this voltage divider stack are presented in Section 3.3. Control software was written in Python and integrated into the 2
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well with a previous (unpublished) calibration performed with the same voltage divider stack, where an offset of 29 V was required for a beam energy of 50 kV. Table 1 summarizes the corrected isotope shifts and hyperfine parameters, which are shown to be in good agreement with literature. 4. RFQ cooler-buncher modifications The installation of the RFQ cooler-buncher in 1998 has given access to higher quality beams for both the mass spectroscopy and collinear laser spectroscopy programs [5]. This device is positioned downstream of the magnet separator on a high voltage (HV) platform that is typically 50 V below the 30 kV of the IGISOL HV platform. The device consists of a gas-filled rf-quadrupole, longitudinally divided into 16 segments. The voltage applied on opposite pairs of rods is V = Vrf cos t + Udci where Udci is the dc voltage on the segment i. The characteristic radius of the quadrupole is r0 = 10 mm. A miniature quadrupole (also named mini-RFQ) with a characteristic radius of rmini = 1.5 mm is installed at the extraction side with an identical rf-field to prevent a radial spread of the ion beam until it is transported into high vacuum. The commissioning of this original configuration of the RFQ coolerbuncher was reported in 2001 by Nieminen [5]. The transmission efficiency and the energy spread were measured to be 60% 70% and below 1 eV, respectively, in a non-bunching mode. The characterization of the bunching mode was done with an ion beam of Hf+, at an intensity of 10 4 ions/s and an accumulation time of 1 s. The observed bunch width was 18 22 μs with an energy spread on the order of 1 eV and a transverse emittance estimated at 3 mm mrad. A more detailed description can be found in [5,6]. In order to enhance the capabilities of the JYFLTRAP Penning trap at IGISOL, an MR-ToF MS is currently being developed. The maximum mass resolving power of the MR-ToF MS is determined by the minimum width of an ion bunch delivered by the RFQ cooler-buncher. Therefore, to operate the MR-ToF MS at its full potential ion bunches should have a maximum time dispersion of 100 ns. As decreasing the time dispersion of the ion bunches comes at the cost of increasing the energy spread, crucial for the collinear laser spectroscopy work, two different extraction modes are thus needed at IGISOL; the original one providing a low energy-spread for collinear laser spectroscopy experiments and the new one with a short ToF-spread for mass measurements. The easiest way to implement a new bunching mode in the RFQ cooler-buncher, while keeping the original one, is to add a new bunching region after the existing RFQ cooler-buncher. A schematic of the modifications of the cooler-buncher extraction is shown in Fig. 4. The new bunching section, named mini buncher, takes place right after the mini-RFQ. It consists of two small axial sets of quadrupoles, with a total length of 5 mm, narrowing down the effective trapping region from 15 mm to 2–3 mm. In order to optimize the design parameters, simulations were performed using SIMION. A system of push/pull DC plates has also been installed before and after the mini buncher, respectively, to increase the extraction-field strength thus constraining the ion bunch width below 100 ns. A full re-commissioning of the RFQ cooler-buncher is currently ongoing using an ion beam of 107Ag+ provided by the IGISOL spark ion source [18]. During the measurements presented in the following, the transmission efficiency in non-bunching mode was 30% for a beam of 4–5 nA intensity. Since then, other experiments, both offline and online, have reached the usual transmission efficiency of > 60%, indicating the new bunching region does not negatively impact the performance of the main cooler. The shortest bunch width obtained from the mini buncher is 50 ns. However, this mode of operation decreased the transmission by several orders of magnitude. The source of this transmission loss is currently under investigation. By performing laser spectroscopy on the ion beam, extracted from the cooler under different conditions, its energy spread can be
Fig. 2. Two resonances of 58Ni, obtained with the 300.363 nm (left) and 300.249 nm (right) transitions. Photon counts were scaled with the beam intensity (13 and 10 pA of neutral beam current on an unsuppressed FC respectively) and laser power (0.5 and 0.45 mW respectively).
observations are in good agreement with the experimental and theoretical results in [13]. No obvious lineshape asymmetry, often observed in non-resonance charge exchange, is observed in this work. Higher CEC temperatures may be required to make the asymmetry more apparent in these measurements. 3.3. Calibration of the beam energy In order to extract accurate isotope shifts and hyperfine splittings from the measurements, the beam energy needs to be known precisely and accurately. Calibration of the beam energy can be achieved through measurements of the structure of a series of isotopes with well-known hyperfine structure and isotope shifts - only if the correct beam energy is used in the analysis will the hyperfine splittings agree with literature. To this end, the structure of the stable 168,170-174,176Yb isotopes was measured. There are two odd-A isotopes, which have a large and very precisely known hyperfine splitting [14,15]. The isotope shifts were also measured in [16] and more recently in [17]. The stable Yb isotopes were produced using an offline discharge ion source. The measured cooler platform voltage was nominally 29915 V. In Fig. 3, the difference of the fitted hyperfine splitting of 171,173Yb and the literature value is plotted as function of the offset on beam energy that was assumed in the analysis. The band shows the combined statistical uncertainty obtained from the fits. Best agreement with literature for both 171,173Yb isotopes is obtained for an offset of 15.1(14) V from the nominal voltage (i.e., a total beam energy of 29930 eV), where the uncertainty represents one standard deviation. This value agrees
Fig. 3. Difference of the measured hyperfine separation and literature, as function of cooler voltage. 3
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Table 1 Isotope shifts and hyperfine parameters compared to literature [14–17]. For the isotope shifts, the weighted mean of [16,17] was used. All values are in MHz. Mass
IS
ISlit
Alower
Alit lower
Aupper
lit Aupper
B upper
Blit upper
168 170 171 172 173 174 176
3004.2(25) 1452.8(15) 919.7(18) 0 −517.1(16) −1151.4(16) −2250.2(28)
3007.8(30) 1458.6(17) 920.7(13) 0 −535(38) −1153.4(9) −2257.2(21)
– – 12643.0(4) – −3497.2(1) – –
– – 12642.8121184682(4) – −3497.24007985(3) – –
– – 875.5(2) – −241.64(4) – –
– – 875.8(8) – −245(9) – –
– – – – 1456.1(2) – –
– – – – 1460(50) – –
400 ns. This increase in linewidth is due to an increase in the energy spread of the ion beam. For this particular mass and laser wavelength, this represents an increase from 85 to 130 MHz (to be compared to the natural linewidth of 14 MHz). In addition to a line broadening effect, the new extraction method may also introduce asymmetries and line shifts. Understanding and, if possible, mitigating these effects is crucial for optical spectroscopy. Measurements have already begun at the IGISOL to explore these effects, and yet more tests are planned to better understand the trade-offs and to define optimal running parameters. 5. Future outlook The developments presented in this paper pave the way for new measurements at the IGISOL. The new data acquisition and control systems have readied the experiments for the coming years. In the coming year, the 380 dye laser will be replaced with a new Sirah Matisse Dye DS, and first experiments with the Sirah Matisse Ti:Sapphire laser will be performed. Together with the newly installed charge exchange cell, many elements that have so far been out of reach are now accessible. This paper presented offline measurements on stable nickel as a first example of the expanded capabilities of the collinear line. Since these offline tests, measurements on beams of radioactive silver have already been successfully performed. The short temporal length of the bunches extracted from the modified RFQ cooler-buncher will permit an improved suppression of the background due to scattered laser light. The beam-related background mostly due to isobaric contaminants in the beam is not affected by this reduced bunch length, but could in the future be reduced by extracting cleaned beams from the new MR-ToF MS. These two factors combined could greatly enhance the experimental sensitivity and will allow measurements at the IGISOL to push towards more exotic and challenging cases.
Fig. 4. Schematic illustration of the new electrode configurations on the extraction side of the cooler-buncher.
Acknowledgement We gratefully acknowledge W. Nörtershäuser for the use of the charge exchange cell. References Fig. 5. Optical (top panels) and time-of-flight spectrum (lower panels) using the original (left) and modified (right) cooler-buncher obtained with a beam of 107 Ag. The optical resonance the F = 0 to F = 1 singlet transition.
[1] L. Vormawah, M. Vilén, et al., Phys. Rev. A 97 (2018) 042504 . [2] B. Cheal, D. Forest, Three decades of research using IGISOL technique at the University of Jyväskylä, Springer, 2012, pp. 83–91. [3] P. Campbell, I. Moore, M. Pearson, Prog. Part. Nucl. Phys. 86 (2016) 127. [4] R. Neugart, et al., J. Phys. G: Nucl. Part. Phys. 44 (2017) 064002 . [5] A. Nieminen, et al., Nucl. Instrum. Meth. A 469 (2001) 244. [6] A. Nieminen, et al., Phys. Rev. Lett. 88 (2002) 094801 . [7] P. Campbell, et al., Phys. Rev. Lett. 89 (2002) 082501 . [8] B. Cheal, et al., Phys. Rev. Lett. 102 (2009) 222501 . [9] K. Baczynska, et al., J. Phys. G: Nucl. Part. Phys. 37 (2010) 105103 . [10] C. Babcock, et al., Phys. Lett. B 760 (2016) 387. [11] S. Kelly, et al., Hyperfine Interact. 238 (2017) 42. [12] J. Ketelaer, J. Krämer, et al., Nucl. Instrum. Methods Phys. Res. Section A 594 (2008) 162. [13] C. Ryder, K. Minamisono, et al., Spectrochimica Acta Part B: At. Spectrosc. 113 (2015) 16. [14] P. Phoonthong, M. Mizuno, K. Kido, N. Shiga, Appl. Phys. B 117 (2014) 673. [15] A. Münch, M. Berkler, C. Gerz, D. Wilsdorf, G. Werth, Phys. Rev. A 35 (1987) 4147. [16] R. Berends, L. Maleki, JOSA B 9 (1992) 332. [17] T. Feldker, et al., Phys. Rev. A 97 (2018) 032511 . [18] S. Rahaman, et al., Phys. Lett. B 662 (2008) 111.
estimated. Fig. 5 shows examples of such measurements, performed on an atom beam of stable 107Ag, using the 328.0679 nm transition from the 4d105s 2S (J = 1/2) state at 0 cm−1 to the 4d105p 2P° (J = 3/2) level at 30472.66516 cm−1. By fitting the optical resonances with a Gaussian function, their width can be approximately determined. Note that the resonances obtained for the shortest bunch lengths can display some asymmetry, caused by a corresponding asymmetry in the energy distribution of the ions, which is not taken into account in this way. From the old bunching region, a typical time distribution measured at 10 μs yields resonance linewidths of about 7 V. Using the new bunching region, a total linewidth of 11 V is obtained for a bunch length of 4
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Upgrades to the collinear laser spectroscopy experiment at the IGISOL R.P. de Grootea, , A. de Roubina, P. Campbellc, B. Chealb, C.S. Devlinb, T. Eronena, S. Geldhofa, I.D. Moorea, M. Reponena, S. Rinta-Antilaa, M. Schuhd ⁎
a
Department of Physics, University of Jyväskylä, PB 35(YFL) FIN-40351 Jyväskylä, Finland Department of Physics, University of Liverpool, Liverpool L69 7ZE, United Kingdom School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, United Kingdom d Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany b c
ARTICLE INFO
ABSTRACT
Keywords: Collinear laser spectroscopy RFQ cooler-buncher IGISOL
We give an overview of recent changes to the collinear laser spectroscopy beamline in the IGISOL laboratory. We present a new data acquisition system, commissioning of a newly installed charge exchange cell, and coolervoltage calibration measurements. Currently ongoing modifications to the RFQ cooler-buncher are also discussed.
1. Introduction Collinear laser spectroscopy is applied at the IGISOL-IV facility [1,2]. By overlapping a fast (30 keV) ion or atom beam with a highresolution continuous-wave laser in a collinear geometry, Doppler-free hyperfine spectra can be measured. From these spectra, the nuclear magnetic dipole and electric quadrupole moment, nuclear spin, and changes in the mean-squared charge radius can be determined in a nuclear-model independent way [3]. These observables provide deep insight into nuclear structure far from stability and offer stringent tests of modern nuclear theories [3,4]. Over the years, many improvements to the technique were pioneered at the IGISOL. The development of a radio-frequency quadrupole (RFQ) cooler-buncher [5] has led to a reduction in background by up to four orders of magnitude [6,7]. In-cooler optical pumping, also developed at the IGISOL, has furthermore extended the reach of the method [8–10]. Novel trapping techniques have been developed as well [11], aimed at for instance in-vacuum pumping before injection into the collinear line. Despite these developments, a number of elements have so far been inaccessible to the collinear apparatus at the IGISOL, in particular those elements that do not have suitable optical transitions in their ionic state. For such cases, a charge-exchange cell (CEC) can be used to neutralize the ion beam and thus produce a fast atom beam. Such CEC units have been used in several laboratories around the world, but have so far not been used for online experiments at the IGISOL. This article is structured as follows. In Section 2 an overview of the experimental apparatus will be given. Then, in Section 3.1 a new data
⁎
acquisition system will be presented in brief. In Section 3.2, the first offline results using a newly installed CEC will be presented. In order to investigate the systematic errors to be placed on future online measurements, offline tests with stable ytterbium beams were performed as well. These measurements will be discussed in Section 3.3. In parallel to these developments, the RFQ cooler-buncher has also been upgraded. Currently, ion bunches extracted from the cooler have a temporal length of about 10 μs. In order to maximize the mass resolving power of the multi-reflection time-of-flight mass spectrometer (MR-ToF MS), to be installed soon at the IGISOL, this bunch length needs to be compressed. The design goal of the modified cooler is to reach a bunch length of 100 ns, while keeping the energy spread of such short bunches below 40 eV. For collinear laser spectroscopy shorter bunch lengths are also advantageous, but since an increase in the energy spread of the ion beam results in a loss in resolution and efficiency, the trade-off between energy spread and bunch length needs to be investigated carefully. In Section 4, the first in a series of such systematic energy spread measurements will be presented. 2. Collinear laser spectroscopy at the IGISOL In the IGISOL laboratory, ion beams can be produced either by offline sources or by the IGISOL itself. Typically, beams are mass-separated using a dipole magnet with a mass resolving power of 400, except for one of the surface ion sources and the laser-ablation ion source which can be installed after the magnet prior to the RFQ coolerbuncher. In all cases, beam is injected into the RFQ cooler-buncher, which is discussed in more detail in Section 4. Ions are extracted from
Corresponding author. E-mail address: [email protected] (R.P. de Groote).
https://doi.org/10.1016/j.nimb.2019.04.028 Received 14 January 2019; Received in revised form 21 March 2019; Accepted 10 April 2019 0168-583X/ © 2019 Published by Elsevier B.V.
Please cite this article as: R.P. de Groote, et al., Nuclear Inst. and Methods in Physics Research B, https://doi.org/10.1016/j.nimb.2019.04.028
Nuclear Inst. and Methods in Physics Research B xxx (xxxx) xxx–xxx
R.P. de Groote, et al.
Fig. 1. Schematic view of the collinear beamline, laser systems, ion optics and diagnostic elements. The ion beam produced by the IGISOL enters the beamline from the right side.
the RFQ cooler-buncher at an energy of 800 eV. After passing through two quadrupole benders, the beam is then accelerated to 30 keV and injected into the collinear beamline. A schematic overview of this beamline is shown in Fig. 1. Electrostatic deflectors guide the ions into the charge exchange cell (CEC) and the light collection region (LCR). A quadrupole triplet is used to bring the beam to a focus in front of the (segmented) Photo-Multiplier Tube (PMT). Ion and laser beams are overlapped by tuning both through a removable 0.9 mm aperture. The beamline furthermore has two Faraday cups and a silicon detector which are used to optimize the transport efficiency. An off-axis Micro-Channel Plate (MCP) detector and a new MagneTOF mini-detector mounted on a linear actuator are installed after the CEC and LCR, which are used for both beam tuning and monitoring the neutralization efficiency. Several knife-edge baffles are placed near the LCR to reduce the amount of scattered light that reaches the PMT. All experiments at the IGISOL so far have used light produced by a Spectra Physics 380 dye laser, which uses intra-cavity doubling to generate up to 1 mW of UV laser light. This laser is pumped by a 5 W Coherent Verdi 532 nm pump laser. The dye laser is locked to an external cavity, which in turn can be locked to an iodine absorption line, or to a HighFinesse WSU/10 wavemeter to ensure long-term wavelength stability. The UV light is focused into a 1 mm spot in front of the PMT using a telescope system. Background rates due to scattered laser light are typically 100 Hz per 100 μW of laser light.
Python and EPICS-based control software for the whole IGISOL. Synchronization of cooler release, the stepwise voltage scan and the TDC card is achieved using a PulseBlaster PB24-100-32k-PCI Programmable TTL Pulse Generator. This pulse generator triggers the TDC card, cooler release, and the digital multimeter. Other information which does not require this precise synchronization, such as the cooler voltage or laser wavelength readout, is obtained from EPICS variables that are periodically published on the internal IGISOL network. The wavemeter lock of the dye laser also relies on the EPICS-published wavemeter readout, since the wavemeter is connected to a different computer in another part of the laboratory. A software PID-loop stabilizes the wavelength of the laser by supplying a ±5 V signal to the external stabilization cavity of the laser. This voltage is generated by a Measurement Computing USB-230 data acquisition board. The laser can be scanned around modehop-free over a range of 1 cm−1. Larger wavelength changes require manual intervention. 3.2. Charge exchange cell A charge exchange cell was added to the beamline at the start of 2018. This charge exchange cell, and the vacuum chamber that houses it, were originally designed for the collinear laser spectroscopy beamline at the TRIGA facility in Mainz, Germany [12]. The cell consists of a cylinder which can be filled with a neutralizing element of choice (e.g. K or Na). The cell is mounted on a platform which is electrically insulated from the chamber, so that it can be floated to ± 10 kV. This is used to Doppler-tune the atoms onto resonance with the laser. It is electrically heated using a variable AC power supply, while the ends of the cylinder are kept at a fixed temperature using an Lauda RA-8 oil circulator. This circulator is filled with GALDEN HT-230 heat transfer fluid, chosen for its good thermal conductivity, low electrical conductivity and low flammability. Ions not neutralized in the CEC are deflected from the atom beam using a pair of electrostatic deflector plates. The first tests with the CEC at the IGISOL were performed using a stable beam of 58Ni and using K as the neutralizer. Nickel was studied in detail using the BECOLA beamline at NSCL [13], making it an ideal first test case. In this work it was found that the two meta-stable levels, the 3d9(2D) 4s 3D (J = 3) at 204.787 cm−1 and 3d9(2D) 4s 3D (J = 2) 879.816 cm−1 are populated approximately equally. From these two levels, there are two strong optical transitions, at conveniently close wavelengths of 300.363 nm and 300.249 nm. Since the CEC and the light collection region of the BECOLA setup differ from the setup at the IGISOL, efficiencies and signal-to-background ratios are expected differ, but similar atomic populations after charge exchange can be expected. The temperature of the cell, measured by a thermocouple attached to the top of the cell (a few cm away from the reservoir itself), was 130 degrees during these measurements, while the ends of the cell were cooled to 70 degrees. The potassium reservoir is likely hotter, since it is wrapped tightly with heating wires. Fig. 2 shows typical resonances obtained using these two transitions. These plots show the photon rate, normalized to the beam intensity (measured with an unsuppressed Faraday cup) and UV laser power. Very similar signal rates and signalto-background ratios are obtained for both transitions. These
3. Recent changes and upgrades 3.1. New data acquisition system Signals from the PMT segments are amplified by a Fast Timing Amplifier (ORTEC FTA820A) and discriminated by a constant-fraction discriminator (ORTEC model 935). These signals are then time-stamped by a Cronologic TimeTagger 4-2G Time-to-Digital-Converter (TDC). This board has four channels, each with a 500 ps single-shot resolution. The hyperfine structure of an isotope is scanned by applying an acceleration or deceleration potential to the ion beam. Ideally, this potential is changed with precise reproducibility and with a short settling time. To achieve this, a voltage between −4 and 4 V, generated by one of the 16-bit analog outputs of a Measurement Computing USB3102 data acquisition board, is amplified using a ×1000 TREK 609E-6 high-voltage amplifier. The large slew rate (specified as 150 V/ µ s by the manufacturer) and fast settle time of both devices makes fast voltage sweeps possible. The settle time for a 5 kV voltage change was measured to be less than 200 μs, well within experimental requirements. After every scan, a calibration of the voltage scanning range is performed by a Keysight 34465 digital multimeter which reads out the scanning voltage after a 1:1000 voltage divider. The cooler platform voltage can be similarly monitored using either a Solartron 7071 voltmeter or a second Keysight 34465 multimeter combined with a 1:10000 voltage divider. Calibration measurements of this voltage divider stack are presented in Section 3.3. Control software was written in Python and integrated into the 2
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well with a previous (unpublished) calibration performed with the same voltage divider stack, where an offset of 29 V was required for a beam energy of 50 kV. Table 1 summarizes the corrected isotope shifts and hyperfine parameters, which are shown to be in good agreement with literature. 4. RFQ cooler-buncher modifications The installation of the RFQ cooler-buncher in 1998 has given access to higher quality beams for both the mass spectroscopy and collinear laser spectroscopy programs [5]. This device is positioned downstream of the magnet separator on a high voltage (HV) platform that is typically 50 V below the 30 kV of the IGISOL HV platform. The device consists of a gas-filled rf-quadrupole, longitudinally divided into 16 segments. The voltage applied on opposite pairs of rods is V = Vrf cos t + Udci where Udci is the dc voltage on the segment i. The characteristic radius of the quadrupole is r0 = 10 mm. A miniature quadrupole (also named mini-RFQ) with a characteristic radius of rmini = 1.5 mm is installed at the extraction side with an identical rf-field to prevent a radial spread of the ion beam until it is transported into high vacuum. The commissioning of this original configuration of the RFQ coolerbuncher was reported in 2001 by Nieminen [5]. The transmission efficiency and the energy spread were measured to be 60% 70% and below 1 eV, respectively, in a non-bunching mode. The characterization of the bunching mode was done with an ion beam of Hf+, at an intensity of 10 4 ions/s and an accumulation time of 1 s. The observed bunch width was 18 22 μs with an energy spread on the order of 1 eV and a transverse emittance estimated at 3 mm mrad. A more detailed description can be found in [5,6]. In order to enhance the capabilities of the JYFLTRAP Penning trap at IGISOL, an MR-ToF MS is currently being developed. The maximum mass resolving power of the MR-ToF MS is determined by the minimum width of an ion bunch delivered by the RFQ cooler-buncher. Therefore, to operate the MR-ToF MS at its full potential ion bunches should have a maximum time dispersion of 100 ns. As decreasing the time dispersion of the ion bunches comes at the cost of increasing the energy spread, crucial for the collinear laser spectroscopy work, two different extraction modes are thus needed at IGISOL; the original one providing a low energy-spread for collinear laser spectroscopy experiments and the new one with a short ToF-spread for mass measurements. The easiest way to implement a new bunching mode in the RFQ cooler-buncher, while keeping the original one, is to add a new bunching region after the existing RFQ cooler-buncher. A schematic of the modifications of the cooler-buncher extraction is shown in Fig. 4. The new bunching section, named mini buncher, takes place right after the mini-RFQ. It consists of two small axial sets of quadrupoles, with a total length of 5 mm, narrowing down the effective trapping region from 15 mm to 2–3 mm. In order to optimize the design parameters, simulations were performed using SIMION. A system of push/pull DC plates has also been installed before and after the mini buncher, respectively, to increase the extraction-field strength thus constraining the ion bunch width below 100 ns. A full re-commissioning of the RFQ cooler-buncher is currently ongoing using an ion beam of 107Ag+ provided by the IGISOL spark ion source [18]. During the measurements presented in the following, the transmission efficiency in non-bunching mode was 30% for a beam of 4–5 nA intensity. Since then, other experiments, both offline and online, have reached the usual transmission efficiency of > 60%, indicating the new bunching region does not negatively impact the performance of the main cooler. The shortest bunch width obtained from the mini buncher is 50 ns. However, this mode of operation decreased the transmission by several orders of magnitude. The source of this transmission loss is currently under investigation. By performing laser spectroscopy on the ion beam, extracted from the cooler under different conditions, its energy spread can be
Fig. 2. Two resonances of 58Ni, obtained with the 300.363 nm (left) and 300.249 nm (right) transitions. Photon counts were scaled with the beam intensity (13 and 10 pA of neutral beam current on an unsuppressed FC respectively) and laser power (0.5 and 0.45 mW respectively).
observations are in good agreement with the experimental and theoretical results in [13]. No obvious lineshape asymmetry, often observed in non-resonance charge exchange, is observed in this work. Higher CEC temperatures may be required to make the asymmetry more apparent in these measurements. 3.3. Calibration of the beam energy In order to extract accurate isotope shifts and hyperfine splittings from the measurements, the beam energy needs to be known precisely and accurately. Calibration of the beam energy can be achieved through measurements of the structure of a series of isotopes with well-known hyperfine structure and isotope shifts - only if the correct beam energy is used in the analysis will the hyperfine splittings agree with literature. To this end, the structure of the stable 168,170-174,176Yb isotopes was measured. There are two odd-A isotopes, which have a large and very precisely known hyperfine splitting [14,15]. The isotope shifts were also measured in [16] and more recently in [17]. The stable Yb isotopes were produced using an offline discharge ion source. The measured cooler platform voltage was nominally 29915 V. In Fig. 3, the difference of the fitted hyperfine splitting of 171,173Yb and the literature value is plotted as function of the offset on beam energy that was assumed in the analysis. The band shows the combined statistical uncertainty obtained from the fits. Best agreement with literature for both 171,173Yb isotopes is obtained for an offset of 15.1(14) V from the nominal voltage (i.e., a total beam energy of 29930 eV), where the uncertainty represents one standard deviation. This value agrees
Fig. 3. Difference of the measured hyperfine separation and literature, as function of cooler voltage. 3
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Table 1 Isotope shifts and hyperfine parameters compared to literature [14–17]. For the isotope shifts, the weighted mean of [16,17] was used. All values are in MHz. Mass
IS
ISlit
Alower
Alit lower
Aupper
lit Aupper
B upper
Blit upper
168 170 171 172 173 174 176
3004.2(25) 1452.8(15) 919.7(18) 0 −517.1(16) −1151.4(16) −2250.2(28)
3007.8(30) 1458.6(17) 920.7(13) 0 −535(38) −1153.4(9) −2257.2(21)
– – 12643.0(4) – −3497.2(1) – –
– – 12642.8121184682(4) – −3497.24007985(3) – –
– – 875.5(2) – −241.64(4) – –
– – 875.8(8) – −245(9) – –
– – – – 1456.1(2) – –
– – – – 1460(50) – –
400 ns. This increase in linewidth is due to an increase in the energy spread of the ion beam. For this particular mass and laser wavelength, this represents an increase from 85 to 130 MHz (to be compared to the natural linewidth of 14 MHz). In addition to a line broadening effect, the new extraction method may also introduce asymmetries and line shifts. Understanding and, if possible, mitigating these effects is crucial for optical spectroscopy. Measurements have already begun at the IGISOL to explore these effects, and yet more tests are planned to better understand the trade-offs and to define optimal running parameters. 5. Future outlook The developments presented in this paper pave the way for new measurements at the IGISOL. The new data acquisition and control systems have readied the experiments for the coming years. In the coming year, the 380 dye laser will be replaced with a new Sirah Matisse Dye DS, and first experiments with the Sirah Matisse Ti:Sapphire laser will be performed. Together with the newly installed charge exchange cell, many elements that have so far been out of reach are now accessible. This paper presented offline measurements on stable nickel as a first example of the expanded capabilities of the collinear line. Since these offline tests, measurements on beams of radioactive silver have already been successfully performed. The short temporal length of the bunches extracted from the modified RFQ cooler-buncher will permit an improved suppression of the background due to scattered laser light. The beam-related background mostly due to isobaric contaminants in the beam is not affected by this reduced bunch length, but could in the future be reduced by extracting cleaned beams from the new MR-ToF MS. These two factors combined could greatly enhance the experimental sensitivity and will allow measurements at the IGISOL to push towards more exotic and challenging cases.
Fig. 4. Schematic illustration of the new electrode configurations on the extraction side of the cooler-buncher.
Acknowledgement We gratefully acknowledge W. Nörtershäuser for the use of the charge exchange cell. References Fig. 5. Optical (top panels) and time-of-flight spectrum (lower panels) using the original (left) and modified (right) cooler-buncher obtained with a beam of 107 Ag. The optical resonance the F = 0 to F = 1 singlet transition.
[1] L. Vormawah, M. Vilén, et al., Phys. Rev. A 97 (2018) 042504 . [2] B. Cheal, D. Forest, Three decades of research using IGISOL technique at the University of Jyväskylä, Springer, 2012, pp. 83–91. [3] P. Campbell, I. Moore, M. Pearson, Prog. Part. Nucl. Phys. 86 (2016) 127. [4] R. Neugart, et al., J. Phys. G: Nucl. Part. Phys. 44 (2017) 064002 . [5] A. Nieminen, et al., Nucl. Instrum. Meth. A 469 (2001) 244. [6] A. Nieminen, et al., Phys. Rev. Lett. 88 (2002) 094801 . [7] P. Campbell, et al., Phys. Rev. Lett. 89 (2002) 082501 . [8] B. Cheal, et al., Phys. Rev. Lett. 102 (2009) 222501 . [9] K. Baczynska, et al., J. Phys. G: Nucl. Part. Phys. 37 (2010) 105103 . [10] C. Babcock, et al., Phys. Lett. B 760 (2016) 387. [11] S. Kelly, et al., Hyperfine Interact. 238 (2017) 42. [12] J. Ketelaer, J. Krämer, et al., Nucl. Instrum. Methods Phys. Res. Section A 594 (2008) 162. [13] C. Ryder, K. Minamisono, et al., Spectrochimica Acta Part B: At. Spectrosc. 113 (2015) 16. [14] P. Phoonthong, M. Mizuno, K. Kido, N. Shiga, Appl. Phys. B 117 (2014) 673. [15] A. Münch, M. Berkler, C. Gerz, D. Wilsdorf, G. Werth, Phys. Rev. A 35 (1987) 4147. [16] R. Berends, L. Maleki, JOSA B 9 (1992) 332. [17] T. Feldker, et al., Phys. Rev. A 97 (2018) 032511 . [18] S. Rahaman, et al., Phys. Lett. B 662 (2008) 111.
estimated. Fig. 5 shows examples of such measurements, performed on an atom beam of stable 107Ag, using the 328.0679 nm transition from the 4d105s 2S (J = 1/2) state at 0 cm−1 to the 4d105p 2P° (J = 3/2) level at 30472.66516 cm−1. By fitting the optical resonances with a Gaussian function, their width can be approximately determined. Note that the resonances obtained for the shortest bunch lengths can display some asymmetry, caused by a corresponding asymmetry in the energy distribution of the ions, which is not taken into account in this way. From the old bunching region, a typical time distribution measured at 10 μs yields resonance linewidths of about 7 V. Using the new bunching region, a total linewidth of 11 V is obtained for a bunch length of 4