Received: 6 December 2018 Revised: 7 April 2019 Accepted: 16 April 2019

S P E C I A L I S S U E ART I C L E

DOI: 10.1002/xrs.3073

Optical detection system for laser cooling and precisionlaser spectroscopy of relativistic highly charged ions at theCSRe

Hanbing Wang1 | Weiqiang Wen1 | Zhongkui Huang1 | Dacheng Zhang2 |

Dongyang Chen2 | Dongmei Zhao1 | Xiaolong Zhu1 | Danyal Winters3 |

Michael Bussmann4 | Xinwen Ma1,5

1 Institute of Modern Physics, ChineseAcademy of Sciences, Lanzhou, China2School of Physics and OptoelectronicEngineering, Xidian University, Xi'an,China3GSI Helmholtzzentrum fuerSchwerionenforschung GmbH,Darmstadt, Germany4Helmholtz‐Zentrum Dresden‐Rossendorf, Dresden, Germany5ExtreMe Matter Institute EMMI, GSIHelmholtzzentrum fuerSchwerionenforschung GmbH,Darmstadt, Germany

CorrespondenceX. Ma, Institute of Modern Physics,Chinese Academy of Sciences, 730000Lanzhou, China.Email: x.ma@impcas.ac.cn

Funding informationYouth Innovation Promotion AssociationCAS; National Postdoctoral Program forInnovative Talents; National Natural Sci-ence Foundation of China, Grant/AwardNumbers: 11504388 and U1732141

X‐Ray Spectrometry. 2019;1–5.

Laser cooling and precision laser spectroscopy experiments of relativistic

highly charged ions are being prepared at the heavy‐ion experimental cooler

storage ring (CSRe). Optical detection of fluorescence photons, emitted from

the laser‐excited ions, is extremely important for both powerful methods. In

this paper, we briefly report on the current status of the existing optical detec-

tors and also on their performance during laser cooling of relativistic Li‐like16O5+ ion beams at the CSRe. In addition, we introduce the designs for our

new optical detection systems, which have much higher photon detection effi-

ciencies and can cover a much broader wavelength range. These detector sys-

tems will be used for the upcoming laser spectroscopy experiment of Li‐like16O5+ ions, as well as for future laser spectroscopy experiments with other

highly charged ions.

1 | INTRODUCTION

Laser cooling experiments of relativistic ion beams havebeen performed at the storage rings experimental storagering (ESR)[1–3] in Germany and the experimental coolerstorage ring (CSRe) in Lanzhou, China. In addition, laserspectroscopy experiments have been performed with rela-tivistic heavy ions at storage rings to test strong‐fieldQED effects,[4–6] as well as special relativity.[7,8] In all theseexperiments, optical detection systems were employed to

wileyonlinelibrary.com/jou

observe the fluorescence photons from the laser‐excitedions.[9] For laser cooling experiments, the optical detectionmethod can,[10] because of its sensitivity, be used to diag-nose ultracold ion beams and could even be used toobserve phase transitions.[11,12] This is, for instance, notpossible with a resonant Schottky noise diagnostic sys-tem.[13–15] Also, in laser spectroscopy experiments, opticaldetection of fluorescence photons plays a key role, as hasrecently been demonstrated by measurements of the hyper-fine splitting in H‐ and Li‐like bismuth at the ESR.[5,16]

© 2019 John Wiley & Sons, Ltd.rnal/xrs 1

2 WANG ET AL.

Very recently, laser cooling of lithium‐like 16O5+ ionbeams with a relativistic energy of 275.7 MeV/u wasachieved for the first time by using a CW laser with200 nm at the heavy‐ion CSRe in Lanzhou, China. Itshould be noted that the laser cooling effects were clearlyobserved by the so called Schottky noise pick‐up system;however, the current optical detectors installed at theCSRe do not have high photon detection efficiencies espe-cially for the experiments with ultrahigh relativistichighly charged ions (HCIs) and therefore could not pro-vide a strong indication of laser cooling effect by observ-ing the fluorescent signals in the experiment. Moredetailed data analysis is in progress, and a very prelimi-nary result on laser cooling of 16O5+ ion beams at theCSRe is recently accepted to be published as a proceed-ings of TCP2018.[17] On the basis of this successful exper-iment, we are planning to perform a precision laserspectroscopy experiment to measure the wavelengths ofthe 2s1/2 → 2p1/2 and 2s1/2 → 2p3/2 transitions in Li‐like16O5+ ions. In addition, laser spectroscopy of relativisticH‐like Pb and Bi, Be‐like Ni, and also B‐like Fe are alsounder consideration at the CSRe. Therefore, new opticaldetectors that have high detection efficiency for theseforward‐emitted photons are needed, which will alsoserve as preparations for similar experiments at the futurefacility HIAF.[18] Based on the success of the opticaldetector systems at the storage ring ESR,[9,19–21] twonew optical detection systems are being designed andare planned to be installed at the CSRe.

2 | THE PRESENT OPTICALDETECTION SYSTEM AT THE CSRe

A schematic view of the experimental setup for lasercooling and precision laser spectroscopy of relativistic

HCI beams at the CSRe, at the Institute of Modern Phys-ics (IMP) in Lanzhou, China, is shown in Figure 1a. Lasercooling of lithium‐like 16O5+ ion beams with a relativisticenergy of 275.7 MeV/u was achieved for the first time atthe storage ring CSRe. In the experiment, a CW laser sys-tem with a wavelength of 220 nm was used to interactwith the closed 2s1/2 → 2p1/2 optical transition at a restwavelength of 103.76 nm,[22] and the interaction of theCW laser with a coasting 16O5+ ion beam are shown inFigure 1b,c. The optical detection system is composed ofa UV‐sensitive (50–200 nm) channeltron photomultiplier(CPM, PHOTONIS CEM 4869) and two UV‐sensitive(110–230 nm) photomultiplier tubes (PMTs, ET 9403B).A detailed description of the CPM detector can be foundin reference.[23]

Figure 2a,b shows the count rates of the CPM andPMT detectors as a function of time after one injectionof the 16O5+ ion beam at the CSRe. The laser was turnedon at about 75 s and turned off at 150 s after this injec-tion. It has been found that the CPM is not sensitive tothe stray light from laser beam, but the PMT is. Thebackground signal detected by the CPM mainly camefrom the ionized residual gas molecules. The counts ofthe CPM detector are proportional to the number ofions according to Ncounts = σρηN, where N is the numberof ions, σ the ionization cross section, ρ the area densityof the residual gas molecules, and η the detection effi-ciency of the CPM detector. The fitted beam lifetime is40 ± 1 s, which agrees with the beam current measure-ment. Finally, the present CPM and PMT detectorscould not provide a strong indication of laser coolingeffect by observing the fluorescent signals in the experi-ment. The more detailed data analysis is in progress.Therefore, new optical detectors that have high detec-tion efficiency for the forward‐emitted fluorescencephotons are needed.

FIGURE 1 (a) A schematic view of the

experimental setup for laser cooling and

precision laser spectroscopy of relativistic

highly charged ion beams at the

experimental cooler storage ring (CSRe).

The locations of the laser system, the RF

buncher, the optical detection system, the

Schottky resonator, and the electron

cooler are indicated. The two photographs

in the center show the CPM detector

without and with shield. (b) Schottky

spectrum as a function of storage time for

a 16O5+ ion beam at an energy of

275.7 MeV/u with a fixed UV laser. (c) The

projection of the slice (1) from (b), clearly

showing the interaction of the laser with a

coasting ion beam

FIGURE 2 (a) Beam lifetime measured by the channeltron

photomultiplier (CPM), the exponential fitted lifetime is 40 ± 1 s.

(b) The photon signals measured by the photomultiplier tube (PMT)

show the status of the laser system

WANG ET AL. 3

The wavelength of the fluorescence photons emittedby the relativistic ions, for example, 16O5+ ions, appearsDoppler shifted in the lab frame as

λphoton ¼ λtrans⋅γ 1 − β⋅ cosθð Þ; (1)

where λtrans is the transition wavelength in the rest frame,λphoton the photon wavelength detected at the observationangle θ, β the relativistic unit‐less speed, and γ the relativ-istic Lorentz factor. The angular distribution of the rela-tive flux of the fluorescence photons is given by[9]:

ϕion θionð Þϕ θð Þ ¼

ffiffiffiffiffiffiffiffiffiffiffi1−β2

p� �3

1−β⋅ cosθð Þ3 ¼λtransλphoton

� �3

: (2)

In Figure 3a,c, the wavelength and the angular distribu-tion of the relative photon flux as a function of the obser-vation angles for the laser‐excited relativistic 16O5+ ionsare shown, respectively. It can be found that most of thefluorescence is emitted in a small cone in the direction

FIGURE 3 The emitted photon

wavelength in the lab frame (a) and the

angular distribution of the relative flux (b)

as a function of the lab observation angles

for laser‐excited 16O5+ ions. The blue bar

represents the photons detected around

90°, which could be measured by the

current optical detectors at the CSRe

of the ion beams and that the wavelength of the photonsemitted in this cone are much shorter than the transitionwavelength in the rest frame. As indicated in Figure 3b,the detection efficiency of the present optical detectors(CPM and PMT) at the CSRe is limited, because theyare mounted around 90° with respect to the ion beam.

For laser cooling of 16O5+ ions at the CSRe, if weassume only one ion stored inside the storage ring, andthis ion always sees the laser at every revolution, thenumber of emitted photons per second is

No ¼ 14⋅Loverlapβc

⋅1

2γ⋅τ0⋅f rev; (3)

where τ0 is the lifetime of 2p state and f rev the revolu-tion frequency. However, we must take into accountthe real experimental conditions to estimate thedetected photon counts:

Ncounts ¼ No⋅Nions⋅ηcollect⋅ηdetect; (4)

where Nions is the number of stored ions at the storagering and ηcollect and ηdetect are the photon collection effi-ciency and the photon detection efficiency of the detec-tor depending on beam energy, photon wavelength, andobservation angle, respectively. For the real experimentof 16O5+ ion beams at the CSRe, the estimated countrate of the CPM detector is about 10 Hz after consider-ation of the laser power, the spatial overlap betweenlaser beam and ion beam, and the photon collectionand detection efficiency.

3 | NEW OPTICAL DETECTIONSYSTEMS

As already discussed above, our new optical detectionsystems used for laser cooling and laser spectroscopyexperiments need to have a high sensitivity over a broadwavelength range and have a high detection efficiency.These kinds of detectors[9,19–21] have already beeninstalled at the ESR and tested in the experiments.[5,16]

FIGURE 4 The designs of the new

optical detection systems. The plate can be

moved in and out of the ion beam by a

high‐speed stepper motor

4 WANG ET AL.

On the basis of these detectors, we designed two newoptical detectors for the future laser cooling and laserspectroscopy experiment. The design of the first detectorsystem is shown in Figure 4a, which aims for detectingthe fluorescence photons at the wavelength range from200 to 1000 nm. This detector will be used for hyperfinesplitting experiments with HCI.[24] A fluorescence collec-tion plate with a slit in it to let the ion beam pass throughwill be installed on the beamline to collect the photons.[9]

The plate will be coated with copper to reflect theforward‐emitted photons to the PMT detector which canbe replaced for different experiments. This detector willbe used to detect the photons with wavelengths longerthan 200 nm because of the limitation of reflectivity ofthe copper. The design of the second optical detection sys-tem is shown in Figure 4b, which aims to detect fluores-cence photons at wavelengths below 200 nm inside ofthe vacuum. The fluorescence collection plate will becoated with CsI. The photons emitted in small angle willhit the CsI and produce photoelectrons. These photoelec-trons will be collected by the guiding electric field anddetected by a channeltron electron multiplier (CEM)detector. Because the production efficiency of the photo-electrons is related to the energy of the photons, thisdetector is suitable for detecting the photons with wave-lengths shorter than 200 nm and can be used for XUVand X‐ray photons.

The wavelength of the forward‐emitted photons from2 s → 2p transitions in Li‐like 16O5+ ions is much shorterthan 100 nm in the experiment, and the correspondingdetection cone covered by the newly designed detectionsystems shown by the green shaded region in Figure 3a,b. According to Equation (4), the calculated count rateis about 106 Hz, which is much higher than the currentCPM detector. It should be noted that in the precisionlaser spectroscopy experiment with HCI at heavy ion stor-age rings, the transition wavelength could be obtained byprecisely measuring the laser wavelength and the beamenergy. In principle, the laser wavelength could be mea-sured with an ultrahigh precision; therefore, the accuracyof the precision laser spectroscopy experiment is mainlylimited by the measurement of the ion beam energy thatis determined by the calibration of the high voltage of the

electron cooler at the CSRe. The calibration of the CSRe'se‐cooler up to 200 kV is under preparation, and theexpected precision is about 2 × 10−5, which correspondsto the uncertainty of 7 × 10−6 in the laser spectroscopymeasurement. By taking into account of the other uncer-tainties such as the space charge effect and the anglebetween the laser and the ion beams, the accuracy ofthe laser spectroscopy experiment could reach around1 × 10−5.

4 | CONCLUSIONS

In this paper, we briefly present the status of the currentoptical detectors at the CSRe. The photon energy and theangular distribution of the fluorescence photons emittedby the relativistic ions are calculated. The count rateof the CPM detector in the laser cooling experimentof 16O5+ ions is estimated. We have also introduced ourtwo new optical detection systems for forward‐emittedphotons, which will be installed at the CSRe. The countrate for our fluorescence photons of the newly designedoptical detection systems was calculated and shows amuch higher efficiency than the current CPM detector.Based on the successful laser cooling experiment of 16O5+

ion beams at the CSRe, the next experiment is aiming toprecisely measure the wavelengths of the 2s1/2 → 2p1/2and 2s1/2 → 2p1/2 transitions in Li‐like 16O5+ ions. Thenew optical detection systems will be used in this experi-ment, and the accuracy of the measured transition wave-length could be about 10−5.

ACKNOWLEDGEMENTS

This work is supported by the National Natural ScienceFoundation of China (U1732141 and 11504388), NationalPostdoctoral Program for Innovative Talents, and theYouth Innovation Promotion Association CAS.

ORCID

Hanbing Wang https://orcid.org/0000-0002-7940-0415Weiqiang Wen https://orcid.org/0000-0001-5266-3058

WANG ET AL. 5

Zhongkui Huang https://orcid.org/0000-0002-5486-7844Xiaolong Zhu https://orcid.org/0000-0002-4301-8724Xinwen Ma https://orcid.org/0000-0001-9831-0565

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How to cite this article: Wang H, Wen W,Huang Z, et al. Optical detection system for lasercooling and precision laser spectroscopy ofrelativistic highly charged ions at the CSRe. X‐RaySpectrometry. 2019. https://doi.org/10.1002/xrs.3073