STORI'24: Improvements of time-of-flight detector utilizing a thin foil and crossed static electric and magnetic fields

Mass is the fundamental physical quantity of atomic nuclei, in particular, the masses of neutron-rich nuclei far from stability are important to understand the path of the nucleosynthesis for heavy elements. The Rare-RI Ring (R3) constructed at the RIKEN RI beam factory is a storage ring dedicated for precise mass measurements of short-lived rare RIs. The details of the R3 are described in Refs [1, 2]. A principle of the mass measurements at the R3 is based on isochronous mass spectrometry. The revolution time (T) of a nucleus of interest in the R3 should be corrected by the velocity (β) of nucleus because the isochronous condition of the R3 is tuned to a reference nucleus. To obtain β and T, three time-of-flight (TOF) detectors are used for mass measurements at the R3; the first and second TOF detectors are installed upstream of the R3, and the third one is downstream. The second one is used as both the stop detector of the TOF measurement for β and the start one of the TOF measurement for T. To achieve a mass precision of $ 10^{-6} $, the time resolution of TOF detectors must be less than 100 ps, and the detection efficiency is preferably close to 100%. In addition, the second TOF detector requires the thinnest possible material thickness to minimize changes in ion velocity.

To satisfy the above requirements, we developed the TOF detector utilizing a thin foil and crossed static electric and magnetic fields, called the BE-MCP [3, 4]. In the previous study [3], a reduction of the detection efficiency around the left and right edges on the foil was reported. Also, the operation of the previous BE-MCP was unstable due to the electric discharge. This issue should be solved for a long-term operation during mass measurements. Figure 1 (a) shows a schematic view of the BE-MCP. Of the three electrodes, the inner electrode is fitted with an aluminum-coated Mylar (1 μm thick) as the thin foil, and the two outer electrodes are equipped with holes for entrance and exit as the ions pass through. The polarity of the potentials for the inner and outer electrodes is negative and positive, respectively. The magnetic field is produced by arranging several pieces of neodymium permanent magnets and the strength can be changed by varying the number of magnet pieces. Secondary electrons are generated as the ions pass through the thin foil. They are isochronously transported to two microchannel plate (MCP) detectors by crossed static electric and magnetic fields. TOF detectors of the same type as the BE-MCP are installed in the experimental cooler-storage ring (CSRe) at IMP and the experimental storage ring (ESR) at GSI [5−7]. These TOF detectors measure the turn-by-turn TOF of ions circulating in these storage rings. In the present study, we have modified the BE-MCP to increase the good efficiency region on the foil and to prevent discharge.

Figure 1.  Schematic view of (a) BE-MCP and (b) BE-MCP90. E and B indicate static electric and magnetic fields, respectively. MCP and $ \rm{e^{-}} $ represent a microchannel plate detector and secondary electrons, respectively.

We have also developed a position-sensitive detector, called the BE-MCP90, based on the BE-MCP. Figure 1 (b) shows a schematic view of the BE-MCP90. Unlike the BE-MCP, two MCP detectors of the BE-MCP90 are located in the gap of electrodes, and the effective area of the MCP is orthogonal to the electrodes. By this arrangement, a difference in the transport time of electrons emitted forward and backward from the foil is sensitive to the horizontal position of the ions passing through the foil. In the previous study, this position sensitivity of the BE-MCP90 was experimentally confirmed [4]. In the present study, we have enlarged the vertical acceptance by using large MCP detectors compared to those of the prototype.

In the present study, we modified electrodes of the BE-MCP. The inner electrode of the BE-MCP has two holes where the MCP detectors are mounted electrically floating. The diameter of these holes was changed from 78 mm to 82 mm to prevent electric discharges between the MCP and the electrode. According to Ref. [3], these holes has almost no effect on the electric field along the path of secondary electrons. The gap between the inner and outer electrodes was changed from 34 mm to 38 mm to transport secondary electrons by using a more homogeneous electric field. As the gap between the electrodes increases, the uniformity of the electric field is enhanced [3]. Reducing the electric field due to the voltages applied to the inner ($ V_{\rm i} $) and outer ($ V_{\rm o} $) electrodes and the magnetic field is also effective in preventing electric discharges. In the present study, we reduced the electric field from 547 V/mm ($ V_{\rm i} = -4.0 $ kV, $ V_{\rm o} = +14.6 $ kV) to 342 V/mm ($ V_{\rm i} = -6.0 $ kV, $ V_{\rm o} = +7.0 $ kV) and the magnetic one from 0.0150 T to 0.0117 T. Increasing the negative potential causes secondary electrons to focus along the horizontal axis. This improves the detection efficiency. Except for these changes, the design of the BE-MCP is almost the same as that of the previous one [3].

An online test of the current BE-MCP was performed using $ ^{84} $Kr ion beams of 200 MeV/nucleon provided by the Heavy Ion Medical Accelerator in Chiba (HIMAC). In this case, a relative change of velocity as ions pass through the thin foil (Mylar, 1 μm thick) is calculated to be $ \delta\beta/\beta \sim -2 \times 10^{\rm {-5}} $, and this change is acceptable for mass measurements at the R3. The typical beam intensity was $ \sim1\times 10^3 $ particles per second. Several defocused beams shifted along the horizontal and vertical axes were used to irradiate the entire area of the foil. An experimental setup shown in Fig. 2 was almost the same as the previous one [3]. The plastic scintillation counter was used as a trigger detector for the data acquisition and the TOF start. Two parallel plate avalanche counters (PPACs) [8], which are position-sensitive gas detectors, were placed upstream and downstream of the vacuum chamber for a beam tracking. The BE-MCP was installed in the vacuum chamber with entrance and exit windows (Mylar, 100 μm thick), and the signals of the two MCP detectors were used for the TOF stop. A thallium-doped sodium iodide (NaI(Tl)) scintillation counter was used for the event selection that ions passed through the foil. The distances from the foil of the BE-MCP to the plastic scintillation counter and the entrance of the NaI(Tl) were $ \sim $1000 mm and $ \sim $560 mm, respectively.

Figure 2.  Schematic view of the experimental setup. PPAC and NaI(Tl) represent parallel plate avalanche and thallium-doped sodium iodide scintillation counters, respectively. The BE-MCP in the vacuum chamber was replaced by the BE-MCP90 for the latter's online test.

Figure 3 (a) shows a typical time spectrum of the BE-MCP, which is an average of the TOFs obtained by the forward and backward MCP detectors. A time resolution of 45 ps in standard deviation (σ) was obtained by a Gaussian fit to the peak as shown in Fig. 3 (a). The intrinsic time resolution was obtained as $ \sigma\sim $40 ps by subtracting those of the trigger timing and the electronics. This resolution is sufficient for mass measurements at the R3 as well as the previous result ($ \sigma=38.6(2) $ ps [3]). The present modifications do not significantly affect the time resolution of the BE-MCP. Figure 3 (b) shows the position dependence of the detection efficiency on the foil. The detection efficiency was defined as the ratio of the number of coincidences between the forward and backward detections divided by the number of ions passing through the foil. As shown in Fig. 3 (b), a good efficiency was obtained over the entire foil, and the average of the efficiency was 92.5(2)% within a diameter of 40 mm on the foil. For the present BE-MCP, a stable operation for more than a week was accomplished without electric discharge. Therefore, we finalized the design and specification of the BE-MCP. So far, the BE-MCP was installed upstream of the R3 as the second TOF detector and was used for mass measurements. In Ref. [9], we successfully confirmed the extraction of five nuclides, including $ ^{74} $Ni, from the R3. Data analysis is ongoing. Note that the data from Ref. [9] are preliminary; final results will be published in a peer-reviewed journal.

Figure 3.  (color online) (a) Typical time spectrum of the BE-MCP for ions passing through the entire 45-mm-diameter foil. The solid line indicates the best fit using a Gaussian function. A time resolution of the BE-MCP in standard deviation (σ) was obtained as σ $ \sim $45 ps. (b) The position dependence of the detection efficiency on the foil. X and Y indicate the horizontal and vertical positions on the foil, respectively. The detection efficiency was 92.5(2)% within the 40-mm-diameter area surrounded by the solid line.

The vertical acceptance of the prototype BE-MCP90 was limited to within about $ \pm7.5 $ mm by the effective area of the MCP detectors ($ \phi15 $ mm). In the present study, to improve the vertical acceptance, we used large MCP detectors with the effective area of $ \phi42 $ mm which is almost the same as that of the foil. The inner electrode was modified to mount large MCP detectors, as shown in Fig. 4, and the gap between the inner and outer electrodes was increased to 73 mm. An online test of the current BE-MCP90 was performed by using $ ^{84} $Kr ions of 200 MeV/nucleon at HIMAC. The experimental setup is shown in Fig. 2, but the BE-MCP in the vacuum chamber was replaced by the BE-MCP90. Figure 5 shows a position dependence of the detection efficiency on the foil in the different magnetic fields. The definition of the detection efficiency is the same as that of Fig. 3 (b). For each magnetic field, the potential of the inner electrode was fixed to -6 kV, and that of the outer electrodes was optimized so as to maximize the efficient region. As shown in Fig. 5, a good efficiency was obtained over the entire foil at the magnetic field of 0.072 T. Increasing the electric and magnetic fields reduces the positional scattering of the secondary electrons on the vertical axis. The difference of the detection time for secondary electrons between forward and backward MCP detectors is sensitive to the horizontal position on the foil as shown in Fig. 6 (a). By fitting this correlation with a linear function, the time difference of the MCP detectors can be converted to the position ($ \rm{X}_{\rm{MCP}} $) as shown in Fig. 6 (b). Figure 7 (a) shows the position resolution (σ) obtained by a Gaussian fit to the peak of the $ \rm{X}_{\rm{MCP}} $ axis with events selected every 1 mm along the $ \rm{X}_{\rm{PPAC}} $. The average position resolution is $ \sigma\sim $1.5 mm, but the resolution around the center is better than that of the outsides. This position dependence of the resolution is likely due to the deviation of the stop time for the MCP detector at the far side, because the deviation of the transport time of the secondary electrons increases with the transport length of these electrons. The position dependence was not significantly seen along the vertical axis. The tracking uncertainty of the PPACs is estimated to be 0.6 mm [8], but this value does not significantly affect the position resolution of the BE-MCP90. As shown in Fig. 7 (b), the average position resolution on the horizontal axis is almost constant even though the magnetic field is varied. In the present study, the magnetic field of 0.072 T is desirable for the operation of the BE-MCP90 from the point of view of the detection efficiency. An energy loss of $ ^{84} $Kr ions at 200 MeV/nucleon is calculated to be about 200 MeV for the PPAC. In contrast, the energy loss at the BE-MCP90 is estimated to be about 1 MeV, which is less than $ 10^{-4} $ of the total energy of the $ ^{84} $Kr ions, since the foil material is only a 1-μm-thick Mylar. Therefore, in comparison with the PPAC, we successfully developed the position-sensitive detector with a very low material thickness.

Figure 4.  The inner electrode attached with microchannel plate (MCP) detectors for the current BE-MCP90. In the present study, the inner electrode was modified to attach large MCPs. A thin foil for the production of secondary electrons is attached to the center hole of the inner electrode. X and Y indicate the horizontal and vertical axes, respectively.

Figure 5.  (color online) Position dependence of the detection efficiency of the BE-MCP90 for the magnetic field of (a) 0.049 T, (b) 0.060 T, and (c) 0.072 T. The efficiency is color coded. $ \rm{X}_{\rm {PPAC}} $ and $ \rm{Y}_{\rm {PPAC}} $ were obtained from the tracking of two PPACs. The detection efficiency on the upper and lower sides was improved by increasing the electric and magnetic fields.

Figure 6.  (color online) (a) Time difference between forward and backward MCP detectors ($ \rm{TOF_{\rm F}} $-$ \rm{TOF_{\rm B}} $) as a function of the horizontal position on the foil ($ \rm{X}_{\rm{PPAC}} $), obtained from the tracking of PPACs. A clear correlation is observed between $ \rm{X}_{\rm{PPAC}} $ and $ \rm{TOF_{\rm F}} $-$ \rm{TOF_{\rm B}} $. (b)Horizontal position obtained from the BE-MCP90 ($ \rm{X}_{\rm{MCP}} $) as a function of $ \rm{X}_{\rm{PPAC}} $. $ \rm{X}_{\rm{MCP}} $ was converted from the correlation between $ \rm{X}_{\rm{PPAC}} $ and $ \rm{TOF_{\rm F}} $-$ \rm{TOF_{\rm B}} $.

Figure 7.  (color online) (a) Position resolution (σ) of the $ \rm{X}_{\rm{MCP}} $ along the horizontal axis. The solid line and the band indicate the average and standard deviation, respectively. The average position resolution is $ \sigma\sim $1.5 mm in the magnetic field of 0.072 T. (b) Average position resolution of the $ \rm{X}_{\rm{MCP}} $ as a function of the magnetic field. The position resolution of $ \rm{X}_{\rm{MCP}} $ is almost constant even though the magnetic field is varied.

We developed the time-of-flight detector utilizing a thin foil with crossed static electric and magnetic fields, called the BE-MCP, for the mass measurements at the Rare-RI Ring (R3). Secondary electrons emitted forward and backward from the foil where ions pass through are detected by two microchannel plate (MCP) detectors. By the present improvements, the intrinsic time resolution was typically $ \sim $40 ps for the entire foil from the performance test using $ ^{\rm 84} $Kr ion beams, and a long-term operation more than a week was achieved without the electric discharge. We finalized the design and specification of the BE-MCP, and it was used for the mass measurement at the R3. In addition, as an application of the BE-MCP, we developed a position-sensitive detector called the BE-MCP90. The BE-MCP90 is designed so that the difference in transport time of secondary electrons emitted forward and backward to MCP detectors depends on the horizontal position on the foil. Compared to the prototype, the vertical acceptance was improved by using large MCP detectors. From the online test using $ ^{\rm 84} $Kr ions, a position resolution of $ \sim $1.5 mm in standard deviation was achieved with the full acceptance on the foil at the magnetic field of 0.072 T. We demonstrated that the BE-MCP90 can be used as a position-sensitive detector with very low material thickness. Using the present detectors, isochronous mass spectrometry at the R3 is expected to achieve a relative mass precision of $ 10^{-6} $ for short-lived rare RIs.

We thank the AEC staff of HIMAC for steady operation of the accelerators and appreciate their technical support.