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Nonlinear response of phosphor screen used in velocity map imaging spectrometry
International Journal of Mass Spectrometry 442 (2019) 23e28
Contents lists available at ScienceDirect
International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms
Nonlinear response of phosphor screen used in velocity map imaging spectrometry Yu-Fan Li a, b, Dong-Mei Zhao a, Wen-Chang Zhou a, b, Dong-Bin Qian a, *, Jie Yang a, **, Shao-Feng Zhang a, Xiao-Long Zhu a, Xin-Wen Ma a a b
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, 730000, China University of Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 January 2019 Received in revised form 25 February 2019 Accepted 9 April 2019 Available online 6 May 2019
We present an approach to experimentally investigate the nonlinear response of phosphor screen (PS) module applied in velocity-map-imaging spectrometry under the condition of microchannel plate (MCP) module working in an appropriate amplifier voltage to ensure a constant unsaturation gain. Our approach, taking the Photek's VID240 detector system as an example, is based on the measurements of the position spectra of 87Rbþ photoion pulses with well-determined numbers of ions and duration times at various voltage differences (Uf) between MCP and PS modules. The measured spectra allow us to obtain the relationship between the two-dimensional distribution of luminescence (the luminescence intensity in the kth pixel) and the total number of ions in one pulse at a given Uf value. The obtained relationship leads to the luminescence efficiency and the acceptable limit of input ion flux density (the maximal number of ions arriving at MCP per unit time per unit area to avoid the nonlinear response) for a particular PS to be determined. © 2019 Elsevier B.V. All rights reserved.
Keywords: Velocity map imaging Photoion imaging Photoelectron imaging Linearity of detector
1. Introduction This type of detector system, a microchannel plate (MCP) module followed by a phosphor screen (PS) module, has played a central role in many physics and chemistry experiments for measuring the charged particles such as photoions and photoelectrons based on the well-known velocity map imaging (VMI) technique (see, for example, Refs. [1e5]). The linear responses of the detector system to the input particle flux, mainly including the MCP module and the PS module, are usually required in these VMI experiments because they are the basis of the quantitative analyses. For the MCP module, the nonlinear response (NLR, i.e. “gain saturation”) has been frequently investigated (see, for example, Refs. [6e12]) using various calibration methods. The most striking results are that the detection efficiency, the acceptable limit of input particle flux, and the appropriate operation parameters have been estimated for a particular MCP module [11,12]; the NLR
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D.-B. Qian), [email protected] (J. Yang). https://doi.org/10.1016/j.ijms.2019.04.007 1387-3806/© 2019 Elsevier B.V. All rights reserved.
behavior has been attributed to the space-charge effect when the secondary electron flux amplified during the propagation in the MCP module becomes too strong [13]. However, up to now, the PS's NLR, also called “saturation of luminescence efficiency” in previous publishes [14,15], has not been paid great attention in VMI experiments because almost all these experiments are considered intuitively as a low light image. This causes that a linearity assumption for the PS module, instead of complicated calibration procedures, is used frequently in previous data analysis. In fact, in those experiments where the image brightness is adjusted in a wide range, the PS's NLR is not entirely avoidable, indicating the necessity for the PS's calibration with carefulness. In an VMI experiment, when ensuring the MCP module working in a constant unsaturation gain regime, the PS's NLR should be complicated by at least two aspects. One is the increase of input particle flux, leading to a higher light image. This is related to the experimental phenomenon that more and more ions arrive simultaneously at the same position of the input surface of the MCP module, probably inducing the PS's NLR. This is due to the fact that the number of secondary electrons, coming from the output surface of the MCP module and subsequently striking the PS surface, becomes more and more strong. Actually, a common feature of VMI experiments is that the production of quite a few charged particles
24
Y.-F. Li et al. / International Journal of Mass Spectrometry 442 (2019) 23e28
in a small focal volume of the laser radiation and a short duration (usually in ns scale or shorter) prefers to induce the phenomenon occurrence, especially when the VMI spectrometry is designed to minimize the effect of the initial geometrical dispersion of the charged particles following the imaging principle proposed by Eppink and Parker [16]. The other aspect is the increase of voltage difference (Uf) between MCP and PS modules, leading to a higher light image. This also probably induces the PS's NLR owing to the secondary electrons with higher energy striking the PS surface. To our knowledge, the influence of the two aspects on the PS's NLR has never been well investigated by experiments and thus, a reliable calibration method for estimating the acceptable input-particle limit of the PS module remains to be established. In this paper, taking the Photek's VID240 detector system as an example, we propose an approach to experimentally investigate the PS's NLR behavior. The approach is based on a novel experimental design, including the achievement of the MCP module having a small efficiency (constant unsaturation gain) to generate (amplify) initial secondary electrons, the preparation of various photoion pulses with well-determined number of ions and pulse durations, and the measurement of two-dimensional (2D) distribution of luminescence induced by each photoion pulse as a function of Uf. Such an experimental design leads us to observe the PS's response transition from linearity to nonlinearity and to determine its luminescence efficiency and acceptable input-ion limit in given operation parameters, and consequently provides a calibration method for a particular PS involved in VMI experiments.
2. Experiment and data analysis Fig. 1 shows a schematic diagram of the experimental setup used in this work, including a magneto-optical trap (MOT) system coupling with an ionization laser and an VMI spectrometer. The MOT system has been described in a recent paper [17]. Briefly, an 87 Rb atom cloud with a temperature around 500 mK is firstly prepared using the laser cooling and trapping technique by pumping 87 Rb atoms to the 52P3/2(F’ ¼ 3) state. The number of atoms and the diameter of the atom cloud are estimated by an optical absorption imaging method [18] to be around 1 107 atoms and 1.4 mm,
respectively. Cold 87Rb atoms in the 52P3/2 state are then ionized by single-photon process using an additional focused (spot diameter, about 500 mm) laser beam (5 ns; 480 nm; 10 Hz) to produce a pulse of 87Rbþ ions. During the experiment, the ionization laser beam has a constant pulse energy of 2.0 mJ and passes through a group of neutral density filters located in the front of the MOT system. By changing the transmittance of the optical filters, the pulse energy in the ionization zone is controlled and scanned over a wide range from 0.3 to 20 mJ. Combining the photoionization cross section of 87 Rb atoms in 5P (14.8 ± 2.2 Mb) reported in Ref. [19] one can deduce that our experiment is operated in the unsaturation ionization regime. A well-calibrated photomultiplier tube (PMT) is used to monitor the fluorescence from the atomic cooling circulation in the MOT to estimate the number of atoms in 5P state in real time. As a result, the total ion number in one pulse duration (Nit) can be determined by considering the difference of fluorescence intensities between before and after ionization-laser irradiation and adjusted in the range of 100e7000 ions/pulse by changing the laser intensity in the ionization zone. Following the previous experiment [20], the uncertainty of Nit is estimated to be about 10% which mainly includes the instabilities of the ionization laser and the MOT system. The VMI spectrometer is designed following the principle proposed by Eppink and Parker [16], where a specific electric field ratio of 3.4:6.6 is used to ensure it working in the velocity measurement mode. The Photek's VID240 detector system with a PS attached to a dual MCPs has been used frequently in previous experiments (see, for example, Refs. [21,22]). For the MCPs, the effective diameter is 40 mm and the ratio of microchannel length to diameter is 50:1. The input surface of the MCPs is grounded and the bias voltage between input and output surfaces is kept constant (Um ¼ 1.74 kV). The kinetic energy that 87Rbþ ions impinging on the MCPs is derived from the voltages imposed on the acceleration electrodes to be Ek ¼ 1.3 keV and kept invariable. At this energy, initial secondary electrons are generated in accordance with a constant efficiency (a) for the 87Rbþ ion detection [12] and amplified during the propagation in the MCPs. The amplified electrons reaching the output surface of the MCPs are further accelerated by the voltage difference between the output surface of the MCPs and the PS (Uf,
Fig. 1. Schematic diagram of experimental setup.
Y.-F. Li et al. / International Journal of Mass Spectrometry 442 (2019) 23e28
varied from 3.6 to 5.0 kV) and finally strike the PS, which causes luminescence. The phosphor used for the screen is P46, and the luminescence lasts for approximately 300 ns. Regarding the record for one ion pulse, the signals from the MCPs are first converted to digital signals by a fast A/D converter and then recorded by a PC, generating a time of flight (TOF) spectrum. The luminescence from the PS is captured by a CCD camera (Basler A312F) whose exposure time was fixed 50 ms (much longer than the luminescence lifetime), generating a 2D image. In Fig. 2, taking the case of Nit ¼ 4.1 103 ions/pulse and Uf ¼ 5.0 kV as an example, we present a typical TOF spectrum (Fig. 2(a)) and corresponding raw 2D image (Fig. 2(b)). A strong peak can be clearly visible in the TOF spectrum. The total number of secondary electrons (Net) reaching the output surface of the MCPs can be scaled by integrating the peak along the TOF. Assuming that the MCPs are operated in a linear regime, Net values are expected to present a linear dependency on Nit. In addition, the duration (tw) of the ion pulse arriving at the MCPs can be extracted from the TOF spectrum by using the Gaussian function to fit the peak shape and is quantized by the full width at half maximum of the peak (see Fig. 2(a)). In Fig. 2(b), the color used for describing the 2D image serves as the brightness index, representing a distribution of luminescence (Lk, defined as the luminescence intensity in the kth pixel). Because each Lk was captured, the total luminescence intensity (Lt) can be obtained by integrating all Lk values in a selected region covering the full image (see the region selected by a white solid square in Fig. 2(b)). If we further assume that the PS is also operated in a linear regime, Lk is expected to present a linear dependency on the number of those ions arriving at a small surface of the MCPs, where the small surface could be well positioned by the kth pixel. It implies that, under the two assumptions made above, the 2D image provides detailed information on the temporal-spatial distribution of the pulsed ion cloud reaching the detector. It needs to be explained that, with Nit increasing in the range limited here from 100 to 7000 ions/pulse, the duration (tw) and the imaging sizes (diameter) for these pulsed ion clouds when arriving at the input surface of the MCP are various from 16 to 80 ns and from 3 to 15 mm, respectively. This is because increasing Nit leads to a higher initial charge density in the ionization zone and thus to a stronger collective ion-ion Coulomb repulsion during the flight path from the ionization zone to the detector. Furthermore, as shown in Fig. 2, the shapes of all these spectra measured in time and position have a slight asymmetry. By detailed analysis we deduce that their asymmetries should be attributed to the effect of complex coupling between the ion-ion Coulomb repulsion during the flight path and the limited installation orientation precisions for the acceleration electrodes used in the present experiment.
Fig. 2. (a) TOF spectrum of
87
25
3. Results and discussions Firstly, we examine the linearity of the relationship between the total number of secondary electrons (Net) reaching the output surface of the MCPs and the total number of ions (Nit) in one pulse arriving at the input surface of the MCPs. The measured Net values were plotted in Fig. 3 as a function of Nit. The uncertainty of Net is fairly low (˂ 4%) because each spectrum measured here was accumulated up to more than 500 laser shots. One can see that there is an excellent linear relationship between them within the error range although the change of Nit extends to almost two order of magnitude (from 100 to 7000 ions/pulse). This provides direct evidence for the MCPs working in a constant unsaturation gain regime in the whole Nit range investigated here. The observed wide-range linearity should thank to the present experimental design that low-energy ions impinging on the MCPs, corresponding to a small efficiency (a) to generate initial secondary electrons [9,23,24]. Such a design brings a unique merit to ensure the linearity of the MCPs in a wide Nit range because smaller a values lead to higher acceptable limits of input ion flux for a particular MCP module when Um keeps invariable [12]. After we confirm the MCPs working in a constant unsaturation gain regime, the PS's calibration could be achieved by analyzing the dependence of the Lt (Nit) relationship on the Uf value in a wide range of Nit. Fig. 4 shows the dependence of Lt on Nit at three different Uf values. Here, the error bars of Lt are defined as the square root of the total luminescence counts. According to the
Fig. 3. Dependency of Net on Nit, fitting linearly by considering error weight (red solid line).
Rbþ at Nit ¼ 4.1 103 ions/pulse and Uf ¼ 5.0 kV; (b) the corresponding raw 2D image.
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Fig. 5. Dependencies of ε on Uf. Error bars represent the linear-fit deviation.
5.0 kV, the trends of the Lt versus Nit are divided into 2 different phases by a critical Nc it: a linear trend is evident up to the Nc it value; beyond the critical value, the Lt (Nit) relationship deviates from the linearity. This deviation is directly related to the reduction of the luminescence efficiency ε with increasing Nit. It should be attributed to local saturation of luminescence occurring in the center of the image due to the fact that increasing Nit leads to more secondary electrons striking the same position of the PS, and consequently to the saturation excitation (i.e. ground-state depletion) of those fluorescent molecules adhering to the PS surface and related to the image center. In order to display visually the saturation phenomenon, the 1D distributions of luminescence (Lr) along a line across the brightest pixel on the image (see the white
Fig. 4. Dependencies of Lt on Nit at different Uf values. Red dash lines are the linear fits to data points in the “linear” region designated by the Cartesian criterium.
cartesian criterium and considering the errors, one can find that in the case where Uf ¼ 3.6 kV, the total luminescence intensities increase linearly in the whole Nit range investigated here. In the cases of Uf ¼ 4.5 and 5.0 kV, the obtained Lt (Nit) relationships also obey the linear behavior when Nit up to the values of 410 ions/pulse (the fourth data point) and 260 ions/pulse (the third data point), respectively. For each Uf value, based on these points in the “linear” region designated by the Cartesian criterium, the luminescence efficiency (ε ¼ Lt/Nit, defined as the average luminescence intensity per ion.) can be estimated by fitting these points using a linear function (see Fig. 4). The plotted black squares in Fig. 5 show the dependencies of ε on Uf in the present experiment operation. As observed, the response of ε to Uf is near linear between 3.6 and 5.0 kV within the error range, indicating that the luminescence efficiency defined here depends on the energy of secondary electrons striking the PS. This is not surprising because higher electron energy corresponds to a higher conversion factor of the PS from electron to photon. Once one experiment avoids the NLRs of MCPs and PS and keeps a well-defined ε value, the spatial distribution of the ions reaching the detector can be extracted directly from the 2D distribution of luminescence. In fact, one can obviously notice that, in the cases of Uf ¼ 4.5 and
Fig. 6. Typical profiles of Lr related to three different Nit values at Uf ¼ 5.0 kV.
Y.-F. Li et al. / International Journal of Mass Spectrometry 442 (2019) 23e28
horizontal line in Fig. 2(b)) are plotted in Fig. 6 for three different Nit cases at Uf ¼ 5.0 kV. One can see that, with the increase of Nit, the Lr distribution shows a flat top profile gradually growing around the center. This provides a visual evidence for the occurrence of the saturation phenomenon of luminescence. And thus, the necessity for the PS's calibration in VMI experiments is once again emphasized. One of the main tasks in the calibration procedure is that estimating the acceptable input-ion limit to avoid the NLR of a particular PS at a given experiment operation. From previous discussions one can see that parameter Nc it should be called as the acceptable limit of input ion flux because it represents the maximal number of ions arriving at MCP in one pulse duration to avoid the NLR occurrence. But here, we introduce a parameter rc, which is named as the acceptable limit of input ion flux density and defined as the acceptable maximal number of ions impinging on MCP per unit time and per unit area to avoid the PS's NLR. Compared to parameter Nc it, rc has more great universal applicability because it is independent from the duration and size of the ion pulse. For each Uf value under investigation, as mentioned above, the measured linear relationship of Lt (Nit) implies that each recorded Lk can be expected to scale as the number of ions arriving at a small surface of the MCP by Nik ¼ (Lk/Lt)✕Nit, where the position and size of the small surface can be well defined by the kth pixel with an area of Sk ¼ 0.005 mm2. In this case of Uf ¼ 4.5 kV, when Nit 410 ions/ pulse, the saturation of Lk has never occurred at any one pixel. Here, assuming that Nc it is equal to 410 ions/pulse for Uf ¼ 4.5 kV, this leads to the estimation of the acceptable limit of input ion flow density by rc ¼ (Lmax k/Lt(Nc it))✕[Nc it/(Sk✕tw)] to be about 120 ± 12 ions/(ns$mm2), where Lmax k is the luminescence intensity of the brightest pixel in the image. Using a similar way and assuming that Nc it is equal to 260 ions/pulse for Uf ¼ 5.0 kV, the acceptable limit is obtained to be about 70 ± 7 ions/(ns$mm2). Noteworthy is that the actual values of Nc it should be determined by fitting the trends of Lt versus Nit in different phases (see Fig. 4), but here, they are replaced by the last measured points in the linear regions by cartesian criterium. As a result, the acceptable limit estimated here should be slightly underestimated, especially in the case of Uf ¼ 5.0 kV. Furthermore, the main factors affecting the acceptable limit are most likely to be the luminescence duration of PS and the specific experimental operation parameters such as Ek, Um, and Uf. Therefore, the same type PS with different experiment operations or different types of PS with the same experiment operations will behave very differently. However, it should be emphasized that, although using the same type detector system and the same experiment operations, such a PS's NLR must be investigated for each species of charged particles to be detected because detection efficiencies of MCP depends on particle species [25]. 4. Summary A phosphor screen (PS) attached to a Microchannel plate (MCP) module, as a typical detector system, has been applied frequently in the velocity map imaging experiments. The linear responses of the detector system to input particle flux are the basis of the quantitative analyses. We have reported an approach to experimentally investigate the response transition of a particular PS from linearity to nonlinearity using various photoion pulses with welldetermined numbers of ions and pulse durations under the condition of MCP(s) working in a linear regime. The PS's response is studied by changing the input ion flux (Nit) and the voltage difference (Uf) between MCP(s) and PS. When Uf exceeds a limit value, the luminescence efficiency begins to saturate with the increase of Nit. To avoid the PS's nonlinearity, a calibration method for deriving
27
the acceptable limit of the input ion flux density, which is a new parameter proposed by us and defined as the maximal number of ions arriving at MCP(s) per unit time per unit area to avoid the PS's nonlinearity, is presented by considering the relationship between the temporal-spatial distribution of ions arriving at MCP(s) and the luminescence distribution. Although the calibration method proposed here is based on a particular ion source prepared with the help of a MOT system, it should be of interest to anyone using VMI spectrometry because a simple unsaturated single-photon ionization source could serve for calibration. Acknowledgments This work was supported by the National Key R&D Program (No. 2017YFA0402300) and the NSFC program (Grant Nos. U1232122 and U1632143). References [1] G.A. Garcia, L. Nahon, C.J. Harding, E.A. Mikajlo, I. Powis, A refocusing modified velocity map imaging electron/ion spectrometer adapted to synchrotron radiation studies, Rev. Sci. Instrum. 76 (2005) 053302. €decke, A.I. Chichinin, M. Veckenstedt, C. Maul, K.-H. Gericke, [2] S. Kauczok, N. Go Three-dimensional velocity map imaging: setup and resolution improvement compared to three-dimensional ion imaging, Rev. Sci. Instrum. 80 (2009) 083301. pine, [3] M.-A. Lebeault, B. Baguenard, B. Concina, F. Calvo, B. Climen, F. Le C. Bordas, Decay of C60 by delayed ionization and C2 emission: experiment and statistical modeling of kinetic energy release, J. Chem. Phys. 137 (2012) 054312. e, B. Redlich, A.F.G. van der Meer, [4] C. Cauchy, J.M. Bakker, Y. Huismans, A. Rouze pine, Single-size thermometric measurements C. Bordas, M.J.J. Vrakking, F. Le on a size distribution of neutral fullences, Phys. Rev. Lett. 110 (2013) 193401. n, Z. Yang, H.T. Liu, L.S. Wang, The design and construction of a high[5] I. Leo resolution velocity-map imaging apparatus for photoelectron spectroscopy studies of size-selected clusters, Rev. Sci. Instrum. 85 (2014) 083106. n, H. Norde n, Performance of a micro[6] M. Hellsing, L. Karlsson, H.-O. Andre channel plate ion detector in the energy range 3-25 keV, J. Phys. E Sci. Instrum. 18 (1985) 920e925. [7] P.G. Friedman, K.J. Bertsche, M.C. Michel, D.E. Morris, R.A. Muller, P.P. Tans, Low background rate detector for 40keV ions using a conversion dynode and a microchannel electron multiplier to reject low energy ions, electrons, and photons, Rev. Sci. Instrum. 59 (1988) 98e111. [8] Y. Kikuchi, Y. Kiwamoto, T. Takahashi, T. Saito, Y. Tatematsu, H. Abe, K. Kajiwara, N. Yamaguchi, T. Tamano, In situ calibration of microchannelplate-based x-ray pinhole camera for observation magnetically trapped plasma, Rev. Sci. Instrum. 68 (1997) 3421e3425. [9] I.S. Gilmore, M.P. Seah, Ion detection efficiency in SIMS: dependencies on energy, mass and composition for microchannel plates used in mass spectrometry, Int. J. Mass Spectrom. 202 (2000) 217e229. , V.V. Golovko, M. Herbane, A. Lindroth, [10] S. Coeck, M. Beck, B. Delaure S. Kopecky, V.Y. Kozlov, I.S. Kraev, T. Phalet, N. Severijns, Microchannel plate response to high-intensity ion bunches, Nucl. Instrum. Methods Phys. Res. 557 (2006) 516e522. [11] K. Ito, K. Nakayama, S. Ohtsubo, H. Higaki, H. Okamoto, Determination of transverse distributions of ion plasmas confined in a linear Paul trap by imaging diagostics, Jpn. J. Appl. Phys. 47 (2008) 8017e8025. [12] H. Himura, S. Nakata, A. Sanpei, Applicability of micro-channel plate followed by phosphor screen to charged particles, Rev. Sci. Instrum. 87 (2016) 063306. [13] S. Matsuura, S. Umebayashi, C. Okuyama, K. Oba, Current status of the micro channel plate, IEEE Trans. Nucl. Sci. 31 (1984) 399e403. [14] A.A. Setlur, J.J. Shiang, U. Happek, Eu2þ - Mn2þ phosphor saturation in 5 mm light emitting diode lamps, Appl. Phys. Lett. 92 (2008) 081104. nez-Rey, B. Zurro, G. García, A. Baciero, L. Rodríguez-Barquero, [15] D. Jime M. García-Munoz, Ionoluminescent response of several phosphor screens to keV ions of different masses, J. Appl. Phys. 104 (2008) 064911. [16] A.T.J.B. Eppink, D.H. Parker, Velocity map imaging of ions and electrons using electrostatic lenses: application in photoelectron and photofragment ion imaging of molecular oxygen, Rev. Sci. Instrum. 68 (1997) 3477e3484. [17] Y.F. Li, S. Zaheeruddin, D.M. Zhao, X. Ma, J. Yang, Ionization spectroscopic measurement of nP Rydberg levels of 87Rb cold atoms, J. Phys. Soc. Jpn. 87 (2018) 054301. [18] M. Walhout, U. Sterr, C. Orzel, M. Hoogerland, S.L. Rolston, Optical control of ultracold collisions in metastable xenon, Phys. Rev. Lett. 74 (1995) 506e509. [19] C. Gabbanini, S. Gozzini, A. Lucchesini, Photoionization cross section measurement in a Rb vapor cell trap, Optic Commun. 141 (1997) 25e28. [20] A. Walz-Flannigan, J.R. Guest, J.-H. Choi, G. Raithel, Cold-Rydberg-gas dynamics, Phys. Rev. 69 (2004) 063405. [21] D.B. Qian, X.J. Zhang, D.C. Zhang, S.F. Zhang, J. Yang, R. Cheng, X.L. Zhu, X. Ma,
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Regime change and transitional regime behavior of C60 molecular beams, Chem. Phys. Lett. 691 (2018) 110e113. [22] D.B. Qian, F.D. Shi, L. Chen, S. Martin, J. Bernard, J. Yang, S.F. Zhang, Z.Q. Chen, X.L. Zhu, X. Ma, Time-resolved photoion imaging spectroscopy: determining energy distribution in multiphoton absorption experiments, J. Chem. Phys. 148 (2018) 134303. [23] B. Brehm, J. Grosser, T. Ruscheinski, M. Zimmer, Absolute detection efficiences of a microchannel plate detector for ions, Meas. Sci. Technol. 6 (1995)
953e958. [24] S. Hosokawa, N. Takahashi, M. Saito, Y. Haruyama, Absolute detection efficiences of a microchannel plate detector for 0.5-5 keV neutrals, Rev. Sci. Instrum. 81 (2010) 063301. [25] J. Oberheide, P. Wilhelms, M. Zimmer, New results on the absolute ion detection efficiences of a microchannel plate, Meas. Sci. Technol. 8 (1997) 351e354.
Contents lists available at ScienceDirect
International Journal of Mass Spectrometry journal homepage: www.elsevier.com/locate/ijms
Nonlinear response of phosphor screen used in velocity map imaging spectrometry Yu-Fan Li a, b, Dong-Mei Zhao a, Wen-Chang Zhou a, b, Dong-Bin Qian a, *, Jie Yang a, **, Shao-Feng Zhang a, Xiao-Long Zhu a, Xin-Wen Ma a a b
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, 730000, China University of Chinese Academy of Sciences, Beijing, 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 January 2019 Received in revised form 25 February 2019 Accepted 9 April 2019 Available online 6 May 2019
We present an approach to experimentally investigate the nonlinear response of phosphor screen (PS) module applied in velocity-map-imaging spectrometry under the condition of microchannel plate (MCP) module working in an appropriate amplifier voltage to ensure a constant unsaturation gain. Our approach, taking the Photek's VID240 detector system as an example, is based on the measurements of the position spectra of 87Rbþ photoion pulses with well-determined numbers of ions and duration times at various voltage differences (Uf) between MCP and PS modules. The measured spectra allow us to obtain the relationship between the two-dimensional distribution of luminescence (the luminescence intensity in the kth pixel) and the total number of ions in one pulse at a given Uf value. The obtained relationship leads to the luminescence efficiency and the acceptable limit of input ion flux density (the maximal number of ions arriving at MCP per unit time per unit area to avoid the nonlinear response) for a particular PS to be determined. © 2019 Elsevier B.V. All rights reserved.
Keywords: Velocity map imaging Photoion imaging Photoelectron imaging Linearity of detector
1. Introduction This type of detector system, a microchannel plate (MCP) module followed by a phosphor screen (PS) module, has played a central role in many physics and chemistry experiments for measuring the charged particles such as photoions and photoelectrons based on the well-known velocity map imaging (VMI) technique (see, for example, Refs. [1e5]). The linear responses of the detector system to the input particle flux, mainly including the MCP module and the PS module, are usually required in these VMI experiments because they are the basis of the quantitative analyses. For the MCP module, the nonlinear response (NLR, i.e. “gain saturation”) has been frequently investigated (see, for example, Refs. [6e12]) using various calibration methods. The most striking results are that the detection efficiency, the acceptable limit of input particle flux, and the appropriate operation parameters have been estimated for a particular MCP module [11,12]; the NLR
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D.-B. Qian), [email protected] (J. Yang). https://doi.org/10.1016/j.ijms.2019.04.007 1387-3806/© 2019 Elsevier B.V. All rights reserved.
behavior has been attributed to the space-charge effect when the secondary electron flux amplified during the propagation in the MCP module becomes too strong [13]. However, up to now, the PS's NLR, also called “saturation of luminescence efficiency” in previous publishes [14,15], has not been paid great attention in VMI experiments because almost all these experiments are considered intuitively as a low light image. This causes that a linearity assumption for the PS module, instead of complicated calibration procedures, is used frequently in previous data analysis. In fact, in those experiments where the image brightness is adjusted in a wide range, the PS's NLR is not entirely avoidable, indicating the necessity for the PS's calibration with carefulness. In an VMI experiment, when ensuring the MCP module working in a constant unsaturation gain regime, the PS's NLR should be complicated by at least two aspects. One is the increase of input particle flux, leading to a higher light image. This is related to the experimental phenomenon that more and more ions arrive simultaneously at the same position of the input surface of the MCP module, probably inducing the PS's NLR. This is due to the fact that the number of secondary electrons, coming from the output surface of the MCP module and subsequently striking the PS surface, becomes more and more strong. Actually, a common feature of VMI experiments is that the production of quite a few charged particles
24
Y.-F. Li et al. / International Journal of Mass Spectrometry 442 (2019) 23e28
in a small focal volume of the laser radiation and a short duration (usually in ns scale or shorter) prefers to induce the phenomenon occurrence, especially when the VMI spectrometry is designed to minimize the effect of the initial geometrical dispersion of the charged particles following the imaging principle proposed by Eppink and Parker [16]. The other aspect is the increase of voltage difference (Uf) between MCP and PS modules, leading to a higher light image. This also probably induces the PS's NLR owing to the secondary electrons with higher energy striking the PS surface. To our knowledge, the influence of the two aspects on the PS's NLR has never been well investigated by experiments and thus, a reliable calibration method for estimating the acceptable input-particle limit of the PS module remains to be established. In this paper, taking the Photek's VID240 detector system as an example, we propose an approach to experimentally investigate the PS's NLR behavior. The approach is based on a novel experimental design, including the achievement of the MCP module having a small efficiency (constant unsaturation gain) to generate (amplify) initial secondary electrons, the preparation of various photoion pulses with well-determined number of ions and pulse durations, and the measurement of two-dimensional (2D) distribution of luminescence induced by each photoion pulse as a function of Uf. Such an experimental design leads us to observe the PS's response transition from linearity to nonlinearity and to determine its luminescence efficiency and acceptable input-ion limit in given operation parameters, and consequently provides a calibration method for a particular PS involved in VMI experiments.
2. Experiment and data analysis Fig. 1 shows a schematic diagram of the experimental setup used in this work, including a magneto-optical trap (MOT) system coupling with an ionization laser and an VMI spectrometer. The MOT system has been described in a recent paper [17]. Briefly, an 87 Rb atom cloud with a temperature around 500 mK is firstly prepared using the laser cooling and trapping technique by pumping 87 Rb atoms to the 52P3/2(F’ ¼ 3) state. The number of atoms and the diameter of the atom cloud are estimated by an optical absorption imaging method [18] to be around 1 107 atoms and 1.4 mm,
respectively. Cold 87Rb atoms in the 52P3/2 state are then ionized by single-photon process using an additional focused (spot diameter, about 500 mm) laser beam (5 ns; 480 nm; 10 Hz) to produce a pulse of 87Rbþ ions. During the experiment, the ionization laser beam has a constant pulse energy of 2.0 mJ and passes through a group of neutral density filters located in the front of the MOT system. By changing the transmittance of the optical filters, the pulse energy in the ionization zone is controlled and scanned over a wide range from 0.3 to 20 mJ. Combining the photoionization cross section of 87 Rb atoms in 5P (14.8 ± 2.2 Mb) reported in Ref. [19] one can deduce that our experiment is operated in the unsaturation ionization regime. A well-calibrated photomultiplier tube (PMT) is used to monitor the fluorescence from the atomic cooling circulation in the MOT to estimate the number of atoms in 5P state in real time. As a result, the total ion number in one pulse duration (Nit) can be determined by considering the difference of fluorescence intensities between before and after ionization-laser irradiation and adjusted in the range of 100e7000 ions/pulse by changing the laser intensity in the ionization zone. Following the previous experiment [20], the uncertainty of Nit is estimated to be about 10% which mainly includes the instabilities of the ionization laser and the MOT system. The VMI spectrometer is designed following the principle proposed by Eppink and Parker [16], where a specific electric field ratio of 3.4:6.6 is used to ensure it working in the velocity measurement mode. The Photek's VID240 detector system with a PS attached to a dual MCPs has been used frequently in previous experiments (see, for example, Refs. [21,22]). For the MCPs, the effective diameter is 40 mm and the ratio of microchannel length to diameter is 50:1. The input surface of the MCPs is grounded and the bias voltage between input and output surfaces is kept constant (Um ¼ 1.74 kV). The kinetic energy that 87Rbþ ions impinging on the MCPs is derived from the voltages imposed on the acceleration electrodes to be Ek ¼ 1.3 keV and kept invariable. At this energy, initial secondary electrons are generated in accordance with a constant efficiency (a) for the 87Rbþ ion detection [12] and amplified during the propagation in the MCPs. The amplified electrons reaching the output surface of the MCPs are further accelerated by the voltage difference between the output surface of the MCPs and the PS (Uf,
Fig. 1. Schematic diagram of experimental setup.
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varied from 3.6 to 5.0 kV) and finally strike the PS, which causes luminescence. The phosphor used for the screen is P46, and the luminescence lasts for approximately 300 ns. Regarding the record for one ion pulse, the signals from the MCPs are first converted to digital signals by a fast A/D converter and then recorded by a PC, generating a time of flight (TOF) spectrum. The luminescence from the PS is captured by a CCD camera (Basler A312F) whose exposure time was fixed 50 ms (much longer than the luminescence lifetime), generating a 2D image. In Fig. 2, taking the case of Nit ¼ 4.1 103 ions/pulse and Uf ¼ 5.0 kV as an example, we present a typical TOF spectrum (Fig. 2(a)) and corresponding raw 2D image (Fig. 2(b)). A strong peak can be clearly visible in the TOF spectrum. The total number of secondary electrons (Net) reaching the output surface of the MCPs can be scaled by integrating the peak along the TOF. Assuming that the MCPs are operated in a linear regime, Net values are expected to present a linear dependency on Nit. In addition, the duration (tw) of the ion pulse arriving at the MCPs can be extracted from the TOF spectrum by using the Gaussian function to fit the peak shape and is quantized by the full width at half maximum of the peak (see Fig. 2(a)). In Fig. 2(b), the color used for describing the 2D image serves as the brightness index, representing a distribution of luminescence (Lk, defined as the luminescence intensity in the kth pixel). Because each Lk was captured, the total luminescence intensity (Lt) can be obtained by integrating all Lk values in a selected region covering the full image (see the region selected by a white solid square in Fig. 2(b)). If we further assume that the PS is also operated in a linear regime, Lk is expected to present a linear dependency on the number of those ions arriving at a small surface of the MCPs, where the small surface could be well positioned by the kth pixel. It implies that, under the two assumptions made above, the 2D image provides detailed information on the temporal-spatial distribution of the pulsed ion cloud reaching the detector. It needs to be explained that, with Nit increasing in the range limited here from 100 to 7000 ions/pulse, the duration (tw) and the imaging sizes (diameter) for these pulsed ion clouds when arriving at the input surface of the MCP are various from 16 to 80 ns and from 3 to 15 mm, respectively. This is because increasing Nit leads to a higher initial charge density in the ionization zone and thus to a stronger collective ion-ion Coulomb repulsion during the flight path from the ionization zone to the detector. Furthermore, as shown in Fig. 2, the shapes of all these spectra measured in time and position have a slight asymmetry. By detailed analysis we deduce that their asymmetries should be attributed to the effect of complex coupling between the ion-ion Coulomb repulsion during the flight path and the limited installation orientation precisions for the acceleration electrodes used in the present experiment.
Fig. 2. (a) TOF spectrum of
87
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3. Results and discussions Firstly, we examine the linearity of the relationship between the total number of secondary electrons (Net) reaching the output surface of the MCPs and the total number of ions (Nit) in one pulse arriving at the input surface of the MCPs. The measured Net values were plotted in Fig. 3 as a function of Nit. The uncertainty of Net is fairly low (˂ 4%) because each spectrum measured here was accumulated up to more than 500 laser shots. One can see that there is an excellent linear relationship between them within the error range although the change of Nit extends to almost two order of magnitude (from 100 to 7000 ions/pulse). This provides direct evidence for the MCPs working in a constant unsaturation gain regime in the whole Nit range investigated here. The observed wide-range linearity should thank to the present experimental design that low-energy ions impinging on the MCPs, corresponding to a small efficiency (a) to generate initial secondary electrons [9,23,24]. Such a design brings a unique merit to ensure the linearity of the MCPs in a wide Nit range because smaller a values lead to higher acceptable limits of input ion flux for a particular MCP module when Um keeps invariable [12]. After we confirm the MCPs working in a constant unsaturation gain regime, the PS's calibration could be achieved by analyzing the dependence of the Lt (Nit) relationship on the Uf value in a wide range of Nit. Fig. 4 shows the dependence of Lt on Nit at three different Uf values. Here, the error bars of Lt are defined as the square root of the total luminescence counts. According to the
Fig. 3. Dependency of Net on Nit, fitting linearly by considering error weight (red solid line).
Rbþ at Nit ¼ 4.1 103 ions/pulse and Uf ¼ 5.0 kV; (b) the corresponding raw 2D image.
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Fig. 5. Dependencies of ε on Uf. Error bars represent the linear-fit deviation.
5.0 kV, the trends of the Lt versus Nit are divided into 2 different phases by a critical Nc it: a linear trend is evident up to the Nc it value; beyond the critical value, the Lt (Nit) relationship deviates from the linearity. This deviation is directly related to the reduction of the luminescence efficiency ε with increasing Nit. It should be attributed to local saturation of luminescence occurring in the center of the image due to the fact that increasing Nit leads to more secondary electrons striking the same position of the PS, and consequently to the saturation excitation (i.e. ground-state depletion) of those fluorescent molecules adhering to the PS surface and related to the image center. In order to display visually the saturation phenomenon, the 1D distributions of luminescence (Lr) along a line across the brightest pixel on the image (see the white
Fig. 4. Dependencies of Lt on Nit at different Uf values. Red dash lines are the linear fits to data points in the “linear” region designated by the Cartesian criterium.
cartesian criterium and considering the errors, one can find that in the case where Uf ¼ 3.6 kV, the total luminescence intensities increase linearly in the whole Nit range investigated here. In the cases of Uf ¼ 4.5 and 5.0 kV, the obtained Lt (Nit) relationships also obey the linear behavior when Nit up to the values of 410 ions/pulse (the fourth data point) and 260 ions/pulse (the third data point), respectively. For each Uf value, based on these points in the “linear” region designated by the Cartesian criterium, the luminescence efficiency (ε ¼ Lt/Nit, defined as the average luminescence intensity per ion.) can be estimated by fitting these points using a linear function (see Fig. 4). The plotted black squares in Fig. 5 show the dependencies of ε on Uf in the present experiment operation. As observed, the response of ε to Uf is near linear between 3.6 and 5.0 kV within the error range, indicating that the luminescence efficiency defined here depends on the energy of secondary electrons striking the PS. This is not surprising because higher electron energy corresponds to a higher conversion factor of the PS from electron to photon. Once one experiment avoids the NLRs of MCPs and PS and keeps a well-defined ε value, the spatial distribution of the ions reaching the detector can be extracted directly from the 2D distribution of luminescence. In fact, one can obviously notice that, in the cases of Uf ¼ 4.5 and
Fig. 6. Typical profiles of Lr related to three different Nit values at Uf ¼ 5.0 kV.
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horizontal line in Fig. 2(b)) are plotted in Fig. 6 for three different Nit cases at Uf ¼ 5.0 kV. One can see that, with the increase of Nit, the Lr distribution shows a flat top profile gradually growing around the center. This provides a visual evidence for the occurrence of the saturation phenomenon of luminescence. And thus, the necessity for the PS's calibration in VMI experiments is once again emphasized. One of the main tasks in the calibration procedure is that estimating the acceptable input-ion limit to avoid the NLR of a particular PS at a given experiment operation. From previous discussions one can see that parameter Nc it should be called as the acceptable limit of input ion flux because it represents the maximal number of ions arriving at MCP in one pulse duration to avoid the NLR occurrence. But here, we introduce a parameter rc, which is named as the acceptable limit of input ion flux density and defined as the acceptable maximal number of ions impinging on MCP per unit time and per unit area to avoid the PS's NLR. Compared to parameter Nc it, rc has more great universal applicability because it is independent from the duration and size of the ion pulse. For each Uf value under investigation, as mentioned above, the measured linear relationship of Lt (Nit) implies that each recorded Lk can be expected to scale as the number of ions arriving at a small surface of the MCP by Nik ¼ (Lk/Lt)✕Nit, where the position and size of the small surface can be well defined by the kth pixel with an area of Sk ¼ 0.005 mm2. In this case of Uf ¼ 4.5 kV, when Nit 410 ions/ pulse, the saturation of Lk has never occurred at any one pixel. Here, assuming that Nc it is equal to 410 ions/pulse for Uf ¼ 4.5 kV, this leads to the estimation of the acceptable limit of input ion flow density by rc ¼ (Lmax k/Lt(Nc it))✕[Nc it/(Sk✕tw)] to be about 120 ± 12 ions/(ns$mm2), where Lmax k is the luminescence intensity of the brightest pixel in the image. Using a similar way and assuming that Nc it is equal to 260 ions/pulse for Uf ¼ 5.0 kV, the acceptable limit is obtained to be about 70 ± 7 ions/(ns$mm2). Noteworthy is that the actual values of Nc it should be determined by fitting the trends of Lt versus Nit in different phases (see Fig. 4), but here, they are replaced by the last measured points in the linear regions by cartesian criterium. As a result, the acceptable limit estimated here should be slightly underestimated, especially in the case of Uf ¼ 5.0 kV. Furthermore, the main factors affecting the acceptable limit are most likely to be the luminescence duration of PS and the specific experimental operation parameters such as Ek, Um, and Uf. Therefore, the same type PS with different experiment operations or different types of PS with the same experiment operations will behave very differently. However, it should be emphasized that, although using the same type detector system and the same experiment operations, such a PS's NLR must be investigated for each species of charged particles to be detected because detection efficiencies of MCP depends on particle species [25]. 4. Summary A phosphor screen (PS) attached to a Microchannel plate (MCP) module, as a typical detector system, has been applied frequently in the velocity map imaging experiments. The linear responses of the detector system to input particle flux are the basis of the quantitative analyses. We have reported an approach to experimentally investigate the response transition of a particular PS from linearity to nonlinearity using various photoion pulses with welldetermined numbers of ions and pulse durations under the condition of MCP(s) working in a linear regime. The PS's response is studied by changing the input ion flux (Nit) and the voltage difference (Uf) between MCP(s) and PS. When Uf exceeds a limit value, the luminescence efficiency begins to saturate with the increase of Nit. To avoid the PS's nonlinearity, a calibration method for deriving
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the acceptable limit of the input ion flux density, which is a new parameter proposed by us and defined as the maximal number of ions arriving at MCP(s) per unit time per unit area to avoid the PS's nonlinearity, is presented by considering the relationship between the temporal-spatial distribution of ions arriving at MCP(s) and the luminescence distribution. Although the calibration method proposed here is based on a particular ion source prepared with the help of a MOT system, it should be of interest to anyone using VMI spectrometry because a simple unsaturated single-photon ionization source could serve for calibration. Acknowledgments This work was supported by the National Key R&D Program (No. 2017YFA0402300) and the NSFC program (Grant Nos. U1232122 and U1632143). References [1] G.A. Garcia, L. Nahon, C.J. Harding, E.A. Mikajlo, I. Powis, A refocusing modified velocity map imaging electron/ion spectrometer adapted to synchrotron radiation studies, Rev. Sci. Instrum. 76 (2005) 053302. €decke, A.I. Chichinin, M. Veckenstedt, C. Maul, K.-H. Gericke, [2] S. Kauczok, N. Go Three-dimensional velocity map imaging: setup and resolution improvement compared to three-dimensional ion imaging, Rev. Sci. Instrum. 80 (2009) 083301. pine, [3] M.-A. Lebeault, B. Baguenard, B. Concina, F. Calvo, B. Climen, F. Le C. Bordas, Decay of C60 by delayed ionization and C2 emission: experiment and statistical modeling of kinetic energy release, J. Chem. Phys. 137 (2012) 054312. e, B. Redlich, A.F.G. van der Meer, [4] C. Cauchy, J.M. Bakker, Y. Huismans, A. Rouze pine, Single-size thermometric measurements C. Bordas, M.J.J. Vrakking, F. Le on a size distribution of neutral fullences, Phys. Rev. Lett. 110 (2013) 193401. n, Z. Yang, H.T. Liu, L.S. Wang, The design and construction of a high[5] I. Leo resolution velocity-map imaging apparatus for photoelectron spectroscopy studies of size-selected clusters, Rev. Sci. Instrum. 85 (2014) 083106. n, H. Norde n, Performance of a micro[6] M. Hellsing, L. Karlsson, H.-O. Andre channel plate ion detector in the energy range 3-25 keV, J. Phys. E Sci. Instrum. 18 (1985) 920e925. [7] P.G. Friedman, K.J. Bertsche, M.C. Michel, D.E. Morris, R.A. Muller, P.P. Tans, Low background rate detector for 40keV ions using a conversion dynode and a microchannel electron multiplier to reject low energy ions, electrons, and photons, Rev. Sci. Instrum. 59 (1988) 98e111. [8] Y. Kikuchi, Y. Kiwamoto, T. Takahashi, T. Saito, Y. Tatematsu, H. Abe, K. Kajiwara, N. Yamaguchi, T. Tamano, In situ calibration of microchannelplate-based x-ray pinhole camera for observation magnetically trapped plasma, Rev. Sci. Instrum. 68 (1997) 3421e3425. [9] I.S. Gilmore, M.P. Seah, Ion detection efficiency in SIMS: dependencies on energy, mass and composition for microchannel plates used in mass spectrometry, Int. J. Mass Spectrom. 202 (2000) 217e229. , V.V. Golovko, M. Herbane, A. Lindroth, [10] S. Coeck, M. Beck, B. Delaure S. Kopecky, V.Y. Kozlov, I.S. Kraev, T. Phalet, N. Severijns, Microchannel plate response to high-intensity ion bunches, Nucl. Instrum. Methods Phys. Res. 557 (2006) 516e522. [11] K. Ito, K. Nakayama, S. Ohtsubo, H. Higaki, H. Okamoto, Determination of transverse distributions of ion plasmas confined in a linear Paul trap by imaging diagostics, Jpn. J. Appl. Phys. 47 (2008) 8017e8025. [12] H. Himura, S. Nakata, A. Sanpei, Applicability of micro-channel plate followed by phosphor screen to charged particles, Rev. Sci. Instrum. 87 (2016) 063306. [13] S. Matsuura, S. Umebayashi, C. Okuyama, K. Oba, Current status of the micro channel plate, IEEE Trans. Nucl. Sci. 31 (1984) 399e403. [14] A.A. Setlur, J.J. Shiang, U. Happek, Eu2þ - Mn2þ phosphor saturation in 5 mm light emitting diode lamps, Appl. Phys. Lett. 92 (2008) 081104. nez-Rey, B. Zurro, G. García, A. Baciero, L. Rodríguez-Barquero, [15] D. Jime M. García-Munoz, Ionoluminescent response of several phosphor screens to keV ions of different masses, J. Appl. Phys. 104 (2008) 064911. [16] A.T.J.B. Eppink, D.H. Parker, Velocity map imaging of ions and electrons using electrostatic lenses: application in photoelectron and photofragment ion imaging of molecular oxygen, Rev. Sci. Instrum. 68 (1997) 3477e3484. [17] Y.F. Li, S. Zaheeruddin, D.M. Zhao, X. Ma, J. Yang, Ionization spectroscopic measurement of nP Rydberg levels of 87Rb cold atoms, J. Phys. Soc. Jpn. 87 (2018) 054301. [18] M. Walhout, U. Sterr, C. Orzel, M. Hoogerland, S.L. Rolston, Optical control of ultracold collisions in metastable xenon, Phys. Rev. Lett. 74 (1995) 506e509. [19] C. Gabbanini, S. Gozzini, A. Lucchesini, Photoionization cross section measurement in a Rb vapor cell trap, Optic Commun. 141 (1997) 25e28. [20] A. Walz-Flannigan, J.R. Guest, J.-H. Choi, G. Raithel, Cold-Rydberg-gas dynamics, Phys. Rev. 69 (2004) 063405. [21] D.B. Qian, X.J. Zhang, D.C. Zhang, S.F. Zhang, J. Yang, R. Cheng, X.L. Zhu, X. Ma,
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