- Email: [email protected]
ISAC time-of-flight system with laser-based calibration
Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 Q1 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
ISAC time-of-flight system with laser-based calibration$ V.A. Verzilov n TRIUMF, 4004 Wesbrook Mall, Vancouver B.C., Canada V6T 2A3
art ic l e i nf o
a b s t r a c t
Article history: Received 17 December 2014 Received in revised form 21 February 2015 Accepted 1 March 2015
The time-of-flight (TOF) system is available at the ISAC rare isotope facility to measure the energy of stable and radioactive ion beams in the range of 0.5–22 Mev/u. The system, comprised of three secondary electron emission based monitors, is operated with practically all available beam intensities starting from as low as 103 ions per second. Recently the system was equipped with the calibration setup based on a 266 nm ultraviolet laser. Laser light interacting with the TOF monitors generates secondary electrons due to the photoelectric effect and acts as a reference beam traveling at a well- known velocity. After calibration, accuracy of energy measurements improved to be better than 0.1%. & 2015 Published by Elsevier B.V.
Keywords: Accelerator Radioactive ion beam Energy measurement Laser based calibration
1. Introduction ISAC is a facility for the production and post-acceleration of rareisotope beams (RIBs) that has been in operation at TRIUMF Laboratory since 1998. The RIBs are produced using the Isotope Separation On Line (ISOL) method by bombarding a thick target with a 100 μA proton beam extracted from the 500 MeV TRIUMF cyclotron. The facility is comprised of the production target station, the precision mass separator and a complex of several heavy ion accelerators delivering ion beams with energies between 2 keV/u and 22 MeV/u to experimental areas [1,2]. The science program at ISAC is devoted to studies of nuclear structure, nuclear astrophysics and fundamental symmetries. Accurate measurements of the energy of produced beams are essential for nuclear physics experiments. At ISAC the beam energy is measured at several locations using either the timeof-flight (TOF) system or magnetic energy analyzers. For low energy (v/c«1) accelerators, the TOF technique offers some advantages compared to classic magnet based systems. TOF systems represent a space and cost efficient solution when it is directly accommodated in primary beam transport lines. TOF can be implemented to be fully non-intercepting, so beam energy is continuously monitored while delivering beams to experiments [3,4]. However, non-intercepting TOF devices make use of electromagnetic pickups and require beam currents which are typically orders of magnitude higher than intensities of radioactive beams produced at ISAC.
☆ TRIUMF receives federal funding via a contribution agreement through the National Research Council of Canada. n Tel.: þ 1 6042227657; fax: þ 1 6042221074. E-mail address: [email protected]
The ISAC TOF system was installed downstream of the superconducting linear accelerator and has been in continuous operation since the commissioning in 2006 [5]. To provide measurable signals with low intensity exotic beams the TOF monitors intercept a small portion of the beam. Recently, the system underwent a major upgrade that included substantial change to the mechanics as well as installation of a UV- laser for the system calibration. This paper deals with the details of the design and implementation of the ISAC TOF system. Special consideration is given to the calibration setup and associated improvements in energy measurements.
2. Design considerations Typical intensities of RIBs presently available at ISAC are well below the sensitivity limit of non-intercepting devices. For this and other reasons, in spite of obvious advantages, the non-intercepting approach was not considered the optimum choice for the ISAC TOF monitors. In order to be useful for both radioactive beams and stable pilot beams with currents of tens of nA, the desired monitor had to possess a dynamic range in excess of seven orders of magnitude. The three main parameters which determined the system design were:
Energy range: 0.5–20 MeV/u Energy resolution: ΔE/E o0.2% Beam Intensity: 103–1012 ions per second Micro-channel plate (MCP) based secondary electron emission monitors seemed to be most promising in achieving both the required resolution and dynamic range. Such monitors are commonly used for ion beam diagnostics and many different designs are available [6,7].
http://dx.doi.org/10.1016/j.nima.2015.03.010 0168-9002/& 2015 Published by Elsevier B.V.
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103
V.A. Verzilov / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
One of them [6], developed at Argonne National Laboratory and particularly suitable for high resolution timing applications, was adopted for the ISAC TOF system. The principle of operation is explained in Fig. 1. When the incoming ion beam crosses a metal cylinder, some ions impinge on a thin wire placed at the axis. Secondary electrons extracted from the wire by the incident particles are accelerated in the radial electrical field, created by applying a negative potential to the wire, towards grounded walls of the cylinder. A fraction of them passes through a small aperture in the cylinder and is collected by an MCP producing a short sub-ns electric pulse at the MCP anode. Electical pulses from two or more monitors, separated by a sufficient distance, are processed by fast electronics and the time intervals between the pulses, corresponding to the beam travel time between the monitors, are measured and then converted to the beam velocity and, thus, to its energy. 2.1. Dynamic range The gain of a standard MCP assembly starts rapidly decreasing as the count rate exceeds approximately 106 counts per second under nominal applied voltage. An ion beam with a current of 10 nA may produce fluxes of as much as 1011 secondary electrons per second. Therefore, a reduction of the secondary electron flux by up to 5 orders of magnitude on the MCP surface is required. Some attenuation of the flux can be obtained by choosing the monitor geometry appropriately. The ratio of the wire diameter to the transverse beam size determines the fraction of the beam that is intercepted by the wire. Due to the axial symmetry, only a small part of the secondary electrons, swept by the electrical field towards the cylinder wall, is selected with the aperture and reaches the MCP. A fine mesh in front of the MCP provides a small additional attenuation. A combined attenuation of 103 is achieved with the geometrical factors. The attenuation of the secondary electron flux due to geometry is still not sufficient for the MCP to operate without overloading at the upper end of the range of available beam intensities. It seems that operating the MCP with a lower voltage improves channel recovery times and permits slightly higher count rates to be measured. Also, a very wide MCP Pulse Height Distribution (PHD) insures that even with the moderate gain drop due to overload, a reasonable amount of pulses with amplitudes above the detection threshold can still be processed. The geometrical factors directly affect the lower end of the dynamic range. With the MCP dark count rate of a few counts per second, the minimum beam intensities that can be measured are in the range 103–104 ions per second. There are also practical considerations for the maximum and minimum count rates. The MCP life time and electronics dead time impact the higher values. All micro-channel plates show degradation
of performance due to aging effects after accumulating a critical amount of charge and have to be replaced. On the lower end, the minimum count rate is controlled by the maximum acquisition times permissible by operation requirements. 2.2. Accuracy and resolution The absolute accuracy of the TOF system depends on precise alignment of the system components in the beam line, the system time resolution, precision and stability of electronics and cables. In majority of cases the systematic portion of errors can be eliminated by the system calibration. As for the time resolution, it is determined by a number of factors: the transit time spread (TTS) of the secondary electrons from the wire-emitter to the MCP, the time spread of development and propagation of the electron avalanche across the MCP, the signal noise and the timing jitter of electronics. The beam time spread provides a substantial contribution to the resolution. The electron motion between the wire and the MCP consists of acceleration inside the metal cylinder and uniform motion outside of the cylinder. For acceleration in the cylindrical configuration, when the potential U is applied between the wire and the cylinder, with radii a and b respectively, the transient time can be expressed in the following form Z b dx pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ta ¼ ð1Þ ð2W=mÞ þ ð2eU=mÞððln x ln aÞ=ðln b ln aÞÞ a The duration of the uniform motion depends on a distance h between the cylinder and the MCP and an angle α relative to the normal to the MCP surface at which an electron travels after leaving the cylinder through the aperture. h= cos α t u ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2W=mÞ þ ð2eU=mÞ
ð2Þ
A part from geometrical parameters and potential applied to the wire the transit time depends on the electron initial kinetic energy W. Secondary electrons are emitted from the surface with an initial energy spread that causes dispersion of their transit time from the wire to the MCP. The initial energy of secondary electrons peaks at 0.5–2 eV with the FWHM of the distribution of 5 eV. The tail of the distribution, however, extends to 20 eV and beyond. The negative effect of the spread in initial parameters on the transit time dispersion is strongly reduced by the rapid acceleration of secondary electrons in the applied high gradient electric field. The stability of the electric potential is determined by ripples of the high voltage power supply which is around 20 mV, typically, and has a negligible effect on TTS. Cumulative contribution from geometrical factors and initial energy spread to TTS is less than 15 ps for our design parameters. The time spread of the signal propagation through the MCP is determined in the first place by the pore diameter, MCP thickness and applied voltage. For 10–12 mm pore diameter the time resolution is expected to be in the range of 60–80 ps from Monte-Carlo simulations [8]. The overall timing jitter of the selected electronics is dominated by the walk time of the constant fraction discriminator (CFD) and was estimated to be less than 60 ps. The overall time resolution is, therefore, around or better than 100 ps (FWHM). It can be further improved by selecting an MCP with a smaller pore diameter. 3. Design implementation
Fig. 1. Schematic view of the TOF monitor.
Although strictly speaking, two monitors are sufficient to measure the beam energy by the time-of- flight technique, the actual ISAC TOF
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
V.A. Verzilov / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
setup incorporates three identical monitors to enhance the system reliability. Monitor locations were dictated by availability of the space in the beam transfer line. After installation the inter-monitor distances were measured with a laser tracker to sub-mm accuracy. These distances are 2.193 m between the first and the second monitors and 9.071 m between the second and the third ones. The distances are used in the beam energy calculations. The relative energy error for a system of two monitors separated by a distance L for a beam traveling at a speed βc can be approximated as ΔE=E
2βc Δt L
with respect to vertical and horizontal directions and as such allows scanning across the beam to find the beam center (Fig. 3). The assembled monitors were tested in the laboratory with a short pulse UV laser. The laser beam was first sent to the TOF monitor and then to a fast UV silicon photodiode. The pulse shape of the photodiode signal was measured with a 6 GHz Tektronix TDS820 sampling scope. The photodiode and the TOF data were in very good agreement. Both measured a laser pulse width of 500 ps.
3.2. Data acquisition and processing ð3Þ
In fact, as it follows from Eq. (3), to provide the design energy error of 0.2% for a 20 MeV/u ion beam and the time resolution Δt of 100 ps, a distance of several meters is required for the monitor separation. For the energies within the specified energy range of 0.5–20 MeV/u, ions will travel between monitors for a time of 150–1000 ns in the case of 9 m separation. All accelerated ion beams at ISAC consist of short, typically subnanosecond, bunches spaced in time by an RF period of T 0 ¼ 84.75 ns meaning that up to 14 bunches can be traveling simultaneously between the first and third ISAC TOF monitors. It is true at the same time that not all bunches are populated at low beam intensities. Since individual bunches are indistinguishable for the monitors, the TOF system does not actually measure the time of flight T but rather its excess ΔT o T 0 over the nearest integer number n of periods T 0 ΔT ¼ T n T 0
3
The data acquisition system built on the basis of commercial NIM and VME modules is schematically shown in Fig. 4. The signal from each TOF monitor is first amplified by 20 dB with a 2.9 GHz
ð4Þ
where n depends on the distance between the monitors and beam energy. Three monitors in the system provide three independent ΔT measurements for the same beam energy which allows unambiguous finding of corresponding n value and, thus, the actual time of flight for each measurement. One should also note that a good initial estimate of the beam energy is available from the settings of the accelerators.
Fig. 2. Cross-sectional view of the monitor head comprising beam intercepting wire on the axis of the grounded cylinder and the MCP detector assembly.
3.1. Time-of-flight monitors The cross-section of the monitor head is present in Fig. 2. It consists of the metal cylinder and the MCP assembly. The cylinder has a diameter of 76 mm and height of 86 mm and is made of aluminum. A 50 mm diameter gold plated tungsten wire is stretched on the axis of the cylinder by means of a spring contact that is used to apply a negative voltage of about 2 kV with respect to the grounded cylinder. The wire typically intercepts less than 5% of the beam. The beam crosses the cylinder walls through 20 mm diameter beam ports, while the secondary electrons escape through an 11 mm aperture towards the micro-channel plate. The MCP is the Hamamatsu F4655-12 assembly comprising two plates in the chevron configuration and the anode coupled through a capacitor directly to the BNC connector. The assembly is particularly suited for TOF measurements with the pulse rise time of less than 400 ps when detecting electrons. The MCP is mounted inside the housing with a protection mesh of 20 lines per mm on the input aperture. The MCP housing is attached to the cylinder to form a rigid head construction. A positive voltage in the range of 1.4–2.0 kV is applied to the MCP. The head assembly is driven by a stepper motorized linear actuator with a step size of 0.1 mm within a range of 84 mm. The motor step size and speed can be adjusted according to the need. In addition to counting stepper motor steps, the absolute position of the monitor head is read back by means of a linear potentiometer. Two limit switches are available to indicate extremes of the traveling range. Each monitor is mounted on the diagnostic vacuum box at 451
Fig. 3. Positioning of the retractable and adjustable TOF monitor in the beam line.
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
V.A. Verzilov / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
amplifier. The amplifier monitors the average input current and provides a TTL level signal if the current value exceeds 0.1 uA. This signal is used to switch off the MCP bias as a protective measure in the case of overload conditions. The amplifier is mounted in the immediate proximity to the monitor and connected to the rest of electronics with 30 m long cables. The low loss, phase stable signal cables for the TOF monitors were cut to have equal electrical lengths within a few tens of ps. Raw analog signals are processed by a low jitter (o 50 ps) CFD and converted to logical NIM pulses which are sent to a 25 ps resolution multi-hit time-to-digital converter (TDC), CAEN model V1290, operating in the trigger mode. The 11.8 MHz reference clock is applied to another input of the TDC. The CFD logical signals are also fed to an OR-gate module to generate the common trigger for all three monitors. The trigger starts the TDC acquisition window and is used to reduce the event load on the TDC. The TDC measures the time difference between the signals from the TOF monitors and the common reference clock within the acquisition window. Both positive and negative voltages required for TOF monitors are provided by ISEG High Voltage supplies, model V205. Electronic modules are controlled via a VME CPU running Linux. The same CPU provides the interface to the EPICS based ISAC control system. The fast data acquisition is performed by a set of applications communicating with the EPICS soft IO channels by means of the OS supported shared memory. The on-line data processing is done by a number of MATLAB applications running on a remote host. Peak fitting, peak position and widths, energy computations are done in this way. The system is highly automated and transparent to the users.
4. System calibration The high accuracy of beam energy measurements demands that all three monitors are nearly identical and the delays in cables and electronics are precisely known. In practice this is difficult to achieve. Small variances in construction, applied voltages and signal propagation through cables and electronics, component parameter drifts due to temperature and aging are difficult to control to the acceptable level. A more practical approach is to calibrate the system using a reference beam with a precisely known energy. For the ISAC TOF system we proposed and built a very efficient calibration setup using
a pulsed UV laser with the photon energy sufficiently high to generate photo electron emission from the TOF monitor wires. In this way the laser light acts as a “reference beam” traveling at a wellknown speed. The idea has been in consideration since the conceptual stage of the system design but actual implementation took a few years. Another important application of the UV laser for the TOF system is that all functionality checks do not necessarily require an ion beam and can be performed during shutdown or maintenance periods. Calibration and checks can be done conveniently on-line or off-line as required. 4.1. Calibration laser An inexpensive passively Q-switched MicroChip Nd:YAG UV laser was acquired from Teem Photonic, model SNU-02P-100, to be used in the calibration setup. The laser generates o500 ps pulses at a repetition rate of about 9 kHz. The average output power is 2 mW at a wavelength of 266 nm. The laser and the laser light shaping optics were mounted on an optical table approximately 30 m upstream of the TOF monitors (Fig. 5).The laser beam transport was complicated by the fact that the laser horizontal divergence of 11 mrad is about ten times larger than the vertical one. A short focal length concave lens was used to strongly expand the laser beam in both planes and collimate it with an aperture. In this way a fraction of the light with similar angular divergences in both planes was selected and then focused by a 500 mm focal length bi-convex lens towards the center of the TOF system. The laser beam was brought into the accelerator vacuum space through a UV grade fused silica window. Approximately half of the laser light was deflected towards a fast silicon photodiode using a 50% splitter located in front of the laser head. Alignment of the laser light through the 30 m long vacuum pipe of 50 mm in diameter was not trivial and was accomplished by means of two mirrors on fine adjustment mirror mounts. The final tuning was done with the help of a plastic scintillator installed in the vacuum space and viewed with a GigE CCD camera. The scintillator was supplied with orthogonal and polar grids to measure the laser spot size and position (Fig. 6). The laser spot size varies along the extent of the TOF system by about 1 mm (FWHM). A size of 0.5 mm was measured at the waist by moving the TOF monitor transversely and recording the count rate (Fig. 7). The time of flight of the laser light between the TOF monitors was measured using the data acquisition setup described above
Fig. 4. TOF system signal data acquisition setup.
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
V.A. Verzilov / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
5
Fig. 5. Calibration laser schematic setup. Here M1–M3 are adjustable mirrors, L1 and L2 lenses, IR diaphragm, PD photodiode, BS 50% beam splitter, VS scintillating view screen and CCD stands for a CCD camera.
Fig. 6. Image of the laser beam on the scintillating view screen. Horizontal and vertical lines are spaced by 5 mm, concentric circles point to the center of the beam pipe.
Fig. 8. Time spectra of laser pulse measured with the TOF system. The travel time of the laser beam from the first to the last TOF monitors is 37.5 ns.
after replacing the 11.8 MHz reference clock with the signal from the fast photodiode on the optical table. A typical timing spectrum measured with the laser light is presented in Fig. 8. Each peak had a nearly Gaussian shape with a FWHM width of about 250 ps and its center position was found from the fit. Comparing measured times of flight of the laser light with the calculated ones, using known distances between the monitors, all necessary corrections were determined. The results of the first calibration revealed that in spite of all measures taken to insure equity of three TOF monitors, the discrepancy between them turned out to be surprisingly large. Taking the first monitor as a reference, correction offsets of 600 740 ps and -2307 60 ps were found for the second and third monitor data, correspondingly. A few following calibrations have not changed these numbers significantly. 4.2. Calibration verification Fig. 7. Scan of the laser beam near to its waist with the middle TOF monitor. The measured FWHM width of the beam is about 0.5 mm.
For verification purpose, the corrections obtained from the laser calibration were applied to two different energy measurements. In
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
V.A. Verzilov / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
preparation for each measurement, TOF monitors were individually positioned in the center of the beam by scanning across until the maximum count rate was achieved. Then the time spectra were recorded. An example of the spectrum for the 25Mg10 þ beam with the current of about 1 enA is given in Fig. 9a. A similar spectrum obtained with the 0.5 enA beam of 22Ne4 þ ions is shown in Fig. 9b. It should be noted that depending on the accelerator tuning, the shape
Fig. 9. Time spectra of 25Mg10 þ (a) and 22Ne4þ (b) ion beams measured with the TOF system. Time from each monitor is measured with respect to a common 11.8 MHz reference clock.
of time peaks can vary substantially. Upon completion of the tuning process the peak widths (FWHM) were about 400 ps and 750 ps for magnesium and neon beams, correspondingly. For each beam the time peak width was about the same for all three monitors. In the performed measurements, including the calibration, recorded peak widths were substantially larger than the intrinsic resolution of the TOF monitors of 100 ps. This is expected to hold true for practically any accelerated beam at ISAC meaning that statistical errors in the energy measurements are dominated by the beam time spread. These errors are calculated by means of nonlinear regression analysis in the process of fitting the data with a suitable parameterized model. In particular, an asymmetric Lorentz fit was applied to the measured data in Fig. 9. The key fit parameter relevant to the TOF data analysis is the position of the time peak center and its error. Standard errors of the peak position calculated by fitting algorithms are typically substantially smaller than the peak width. For reported measurements they did not exceed several TDC channels. Fit parameter errors were calculated with the confidence level of 0.999 and used to compute the energy uncertainties given below. Using peak positions, two sets of time-of-flight values were found; calibration data was applied to one of them and ignored for the other. For each set, three corresponding values of the beam energy E21, E32 and E31 and their errors were calculated. The Indexes refer to the monitors used to calculate the beam energy. The errors for the data corrected using calibration offsets include both statistical and residual systematic errors due to calibration and, thus, are slightly higher. In the opposite case the errors are purely statistical. In addition, the unweighted average energies, standard deviations and relative errors were also computed. Both uncorrected and corrected data are summarized in Table 1. The table shows that although the average energy values change by as little as 0.2–0.4% between uncorrected and corrected data, applying the calibration corrections dramatically improves the agreement between the data recorded with all three monitor configurations. Standard deviations (from the average) σ E of uncorrected data, exceeding individual errors by more than ten times, indicate the presence of large unaccounted systematic errors. Once the systematic errors are essentially eliminated after correction, the associated standard deviations are reduced by a factor of 25–30 and approach the level of individual errors. It can be noted that E32 and E31 are systematically closer to each other than to E21. This is easy to understand taking into account that the distance between the first and second TOF monitors is substantially smaller than the spacing between these monitors and the third one. In this case, according to Eq. (3), E21 is expected to be more sensitive to statistical and systematic errors in time measurements. If E21 is excluded from calculations, the numbers in the last two columns of the Table 1 are reduced by another factor of three. However, it is more consistent to add weights to individual measurements when computing the average value. The effect of E21 on the average is then naturally reduced because of its higher error. Eq. (3) also suggests that relative errors increase for a faster beam proportionally to the beam velocity, which quantitatively agrees with obtained data. Based on these results, we can conservatively conclude that due to the implemented calibration, the systematic errors were drastically
Table 1 Results of applying calibration data to ion beam energy measurements. Ion beam
E21. MeV/u
E32. MeV/u
E31. MeV/u
Eave. MeV/u
σE MeV/u
σE/Eave %
Mg 10 þ , uncorrected þ Mg 10 corrected , 22 Ne 4 þ , uncorrected 22 4þ Ne , corrected
9.502470.0146 9.2830 70.0201 2.3014 70.0018 2.2752 70.0024
9.1995 7 0.0034 9.27127 0.0071 2.2653 7 0.0004 2.27407 0.0009
9.2573 70.0029 9.2735 70.0040 2.2723 70.0003 2.2742 70.0005
9.3197 9.2759 2.2797 2.2745
0.1608 0.0062 0.0192 0.0007
1.72 0.07 0.84 0.03
25 25
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
V.A. Verzilov / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
reduced and the absolute accuracy of energy measurements with the ISAC TOF system improved to better than 0.1%. Acknowledgments The author thanks D.Cameron, B.Minato, J.Holek for assembly and installation of the TOF monitors and J.Lassen for the help with the calibration laser setup. References [1] P. Bicault, M. Dombsky, P.W. Schmor, G. Stanford, Nuclear Instruments and Methods in Physics Research Section B 126 (1997) 231.
7
[2] P. Bricault, European Physical Journal-Special Topics 150 (2007) 227. [3] B. Feinberg, D. Meaney, R. Thatcher, C. Timossi, Nuclear Instruments and Methods in Physics Research Section A 270 (1988) 1. [4] R. Pardo, B.E. Clifft, P. Denhartog, D. Kovar, W. Kutschera, K.E. Rehm, Nuclear Instruments and Methods in Physics Research Section A 270 (1988) 226. [5] R.E. Laxdal, W. Andersson, K. Fong, M. Marchetto, A.K. Mitra, W.R. Rawnsley, I. Sekachev, G.Stanford, V.Verzilov, V.Zviagentsev, Proceedings of EPAC 2006, Edinburgh, Scotland, 2006, p. 1556. [6] P.K. DenHartog, J.M. Bogaty, L.M. Bollinger, B.E. Clifft, R.C. Pardo, K.W. Shepard, “The beam bunching and transport system of the Argon positive ion injector”, in: Proceedings of PAC 1989, Chicago, IL, March 1989, p. 545. [7] N.E. Vinigradov, P.N. Ostroumov, P.C. Pardo, S.I. Sharamentov, G.P. Zinkann, Nuclear Instruments and Methods in Physics Research Section A 526 (2004) 206. [8] M. Wu, C.A. Kruschwitz, D.V. Morgan, J. Morgan, Review of Scientific Instruments 79 (2008) 073104.
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
16 17 18 19 20 21 22 23 24 25 26 27 28 29
1 2 3 4 5 6 7 8 9 10 11 12 13 Q1 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
ISAC time-of-flight system with laser-based calibration$ V.A. Verzilov n TRIUMF, 4004 Wesbrook Mall, Vancouver B.C., Canada V6T 2A3
art ic l e i nf o
a b s t r a c t
Article history: Received 17 December 2014 Received in revised form 21 February 2015 Accepted 1 March 2015
The time-of-flight (TOF) system is available at the ISAC rare isotope facility to measure the energy of stable and radioactive ion beams in the range of 0.5–22 Mev/u. The system, comprised of three secondary electron emission based monitors, is operated with practically all available beam intensities starting from as low as 103 ions per second. Recently the system was equipped with the calibration setup based on a 266 nm ultraviolet laser. Laser light interacting with the TOF monitors generates secondary electrons due to the photoelectric effect and acts as a reference beam traveling at a well- known velocity. After calibration, accuracy of energy measurements improved to be better than 0.1%. & 2015 Published by Elsevier B.V.
Keywords: Accelerator Radioactive ion beam Energy measurement Laser based calibration
1. Introduction ISAC is a facility for the production and post-acceleration of rareisotope beams (RIBs) that has been in operation at TRIUMF Laboratory since 1998. The RIBs are produced using the Isotope Separation On Line (ISOL) method by bombarding a thick target with a 100 μA proton beam extracted from the 500 MeV TRIUMF cyclotron. The facility is comprised of the production target station, the precision mass separator and a complex of several heavy ion accelerators delivering ion beams with energies between 2 keV/u and 22 MeV/u to experimental areas [1,2]. The science program at ISAC is devoted to studies of nuclear structure, nuclear astrophysics and fundamental symmetries. Accurate measurements of the energy of produced beams are essential for nuclear physics experiments. At ISAC the beam energy is measured at several locations using either the timeof-flight (TOF) system or magnetic energy analyzers. For low energy (v/c«1) accelerators, the TOF technique offers some advantages compared to classic magnet based systems. TOF systems represent a space and cost efficient solution when it is directly accommodated in primary beam transport lines. TOF can be implemented to be fully non-intercepting, so beam energy is continuously monitored while delivering beams to experiments [3,4]. However, non-intercepting TOF devices make use of electromagnetic pickups and require beam currents which are typically orders of magnitude higher than intensities of radioactive beams produced at ISAC.
☆ TRIUMF receives federal funding via a contribution agreement through the National Research Council of Canada. n Tel.: þ 1 6042227657; fax: þ 1 6042221074. E-mail address: [email protected]
The ISAC TOF system was installed downstream of the superconducting linear accelerator and has been in continuous operation since the commissioning in 2006 [5]. To provide measurable signals with low intensity exotic beams the TOF monitors intercept a small portion of the beam. Recently, the system underwent a major upgrade that included substantial change to the mechanics as well as installation of a UV- laser for the system calibration. This paper deals with the details of the design and implementation of the ISAC TOF system. Special consideration is given to the calibration setup and associated improvements in energy measurements.
2. Design considerations Typical intensities of RIBs presently available at ISAC are well below the sensitivity limit of non-intercepting devices. For this and other reasons, in spite of obvious advantages, the non-intercepting approach was not considered the optimum choice for the ISAC TOF monitors. In order to be useful for both radioactive beams and stable pilot beams with currents of tens of nA, the desired monitor had to possess a dynamic range in excess of seven orders of magnitude. The three main parameters which determined the system design were:
Energy range: 0.5–20 MeV/u Energy resolution: ΔE/E o0.2% Beam Intensity: 103–1012 ions per second Micro-channel plate (MCP) based secondary electron emission monitors seemed to be most promising in achieving both the required resolution and dynamic range. Such monitors are commonly used for ion beam diagnostics and many different designs are available [6,7].
http://dx.doi.org/10.1016/j.nima.2015.03.010 0168-9002/& 2015 Published by Elsevier B.V.
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103
V.A. Verzilov / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
One of them [6], developed at Argonne National Laboratory and particularly suitable for high resolution timing applications, was adopted for the ISAC TOF system. The principle of operation is explained in Fig. 1. When the incoming ion beam crosses a metal cylinder, some ions impinge on a thin wire placed at the axis. Secondary electrons extracted from the wire by the incident particles are accelerated in the radial electrical field, created by applying a negative potential to the wire, towards grounded walls of the cylinder. A fraction of them passes through a small aperture in the cylinder and is collected by an MCP producing a short sub-ns electric pulse at the MCP anode. Electical pulses from two or more monitors, separated by a sufficient distance, are processed by fast electronics and the time intervals between the pulses, corresponding to the beam travel time between the monitors, are measured and then converted to the beam velocity and, thus, to its energy. 2.1. Dynamic range The gain of a standard MCP assembly starts rapidly decreasing as the count rate exceeds approximately 106 counts per second under nominal applied voltage. An ion beam with a current of 10 nA may produce fluxes of as much as 1011 secondary electrons per second. Therefore, a reduction of the secondary electron flux by up to 5 orders of magnitude on the MCP surface is required. Some attenuation of the flux can be obtained by choosing the monitor geometry appropriately. The ratio of the wire diameter to the transverse beam size determines the fraction of the beam that is intercepted by the wire. Due to the axial symmetry, only a small part of the secondary electrons, swept by the electrical field towards the cylinder wall, is selected with the aperture and reaches the MCP. A fine mesh in front of the MCP provides a small additional attenuation. A combined attenuation of 103 is achieved with the geometrical factors. The attenuation of the secondary electron flux due to geometry is still not sufficient for the MCP to operate without overloading at the upper end of the range of available beam intensities. It seems that operating the MCP with a lower voltage improves channel recovery times and permits slightly higher count rates to be measured. Also, a very wide MCP Pulse Height Distribution (PHD) insures that even with the moderate gain drop due to overload, a reasonable amount of pulses with amplitudes above the detection threshold can still be processed. The geometrical factors directly affect the lower end of the dynamic range. With the MCP dark count rate of a few counts per second, the minimum beam intensities that can be measured are in the range 103–104 ions per second. There are also practical considerations for the maximum and minimum count rates. The MCP life time and electronics dead time impact the higher values. All micro-channel plates show degradation
of performance due to aging effects after accumulating a critical amount of charge and have to be replaced. On the lower end, the minimum count rate is controlled by the maximum acquisition times permissible by operation requirements. 2.2. Accuracy and resolution The absolute accuracy of the TOF system depends on precise alignment of the system components in the beam line, the system time resolution, precision and stability of electronics and cables. In majority of cases the systematic portion of errors can be eliminated by the system calibration. As for the time resolution, it is determined by a number of factors: the transit time spread (TTS) of the secondary electrons from the wire-emitter to the MCP, the time spread of development and propagation of the electron avalanche across the MCP, the signal noise and the timing jitter of electronics. The beam time spread provides a substantial contribution to the resolution. The electron motion between the wire and the MCP consists of acceleration inside the metal cylinder and uniform motion outside of the cylinder. For acceleration in the cylindrical configuration, when the potential U is applied between the wire and the cylinder, with radii a and b respectively, the transient time can be expressed in the following form Z b dx pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ta ¼ ð1Þ ð2W=mÞ þ ð2eU=mÞððln x ln aÞ=ðln b ln aÞÞ a The duration of the uniform motion depends on a distance h between the cylinder and the MCP and an angle α relative to the normal to the MCP surface at which an electron travels after leaving the cylinder through the aperture. h= cos α t u ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð2W=mÞ þ ð2eU=mÞ
ð2Þ
A part from geometrical parameters and potential applied to the wire the transit time depends on the electron initial kinetic energy W. Secondary electrons are emitted from the surface with an initial energy spread that causes dispersion of their transit time from the wire to the MCP. The initial energy of secondary electrons peaks at 0.5–2 eV with the FWHM of the distribution of 5 eV. The tail of the distribution, however, extends to 20 eV and beyond. The negative effect of the spread in initial parameters on the transit time dispersion is strongly reduced by the rapid acceleration of secondary electrons in the applied high gradient electric field. The stability of the electric potential is determined by ripples of the high voltage power supply which is around 20 mV, typically, and has a negligible effect on TTS. Cumulative contribution from geometrical factors and initial energy spread to TTS is less than 15 ps for our design parameters. The time spread of the signal propagation through the MCP is determined in the first place by the pore diameter, MCP thickness and applied voltage. For 10–12 mm pore diameter the time resolution is expected to be in the range of 60–80 ps from Monte-Carlo simulations [8]. The overall timing jitter of the selected electronics is dominated by the walk time of the constant fraction discriminator (CFD) and was estimated to be less than 60 ps. The overall time resolution is, therefore, around or better than 100 ps (FWHM). It can be further improved by selecting an MCP with a smaller pore diameter. 3. Design implementation
Fig. 1. Schematic view of the TOF monitor.
Although strictly speaking, two monitors are sufficient to measure the beam energy by the time-of- flight technique, the actual ISAC TOF
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
V.A. Verzilov / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
setup incorporates three identical monitors to enhance the system reliability. Monitor locations were dictated by availability of the space in the beam transfer line. After installation the inter-monitor distances were measured with a laser tracker to sub-mm accuracy. These distances are 2.193 m between the first and the second monitors and 9.071 m between the second and the third ones. The distances are used in the beam energy calculations. The relative energy error for a system of two monitors separated by a distance L for a beam traveling at a speed βc can be approximated as ΔE=E
2βc Δt L
with respect to vertical and horizontal directions and as such allows scanning across the beam to find the beam center (Fig. 3). The assembled monitors were tested in the laboratory with a short pulse UV laser. The laser beam was first sent to the TOF monitor and then to a fast UV silicon photodiode. The pulse shape of the photodiode signal was measured with a 6 GHz Tektronix TDS820 sampling scope. The photodiode and the TOF data were in very good agreement. Both measured a laser pulse width of 500 ps.
3.2. Data acquisition and processing ð3Þ
In fact, as it follows from Eq. (3), to provide the design energy error of 0.2% for a 20 MeV/u ion beam and the time resolution Δt of 100 ps, a distance of several meters is required for the monitor separation. For the energies within the specified energy range of 0.5–20 MeV/u, ions will travel between monitors for a time of 150–1000 ns in the case of 9 m separation. All accelerated ion beams at ISAC consist of short, typically subnanosecond, bunches spaced in time by an RF period of T 0 ¼ 84.75 ns meaning that up to 14 bunches can be traveling simultaneously between the first and third ISAC TOF monitors. It is true at the same time that not all bunches are populated at low beam intensities. Since individual bunches are indistinguishable for the monitors, the TOF system does not actually measure the time of flight T but rather its excess ΔT o T 0 over the nearest integer number n of periods T 0 ΔT ¼ T n T 0
3
The data acquisition system built on the basis of commercial NIM and VME modules is schematically shown in Fig. 4. The signal from each TOF monitor is first amplified by 20 dB with a 2.9 GHz
ð4Þ
where n depends on the distance between the monitors and beam energy. Three monitors in the system provide three independent ΔT measurements for the same beam energy which allows unambiguous finding of corresponding n value and, thus, the actual time of flight for each measurement. One should also note that a good initial estimate of the beam energy is available from the settings of the accelerators.
Fig. 2. Cross-sectional view of the monitor head comprising beam intercepting wire on the axis of the grounded cylinder and the MCP detector assembly.
3.1. Time-of-flight monitors The cross-section of the monitor head is present in Fig. 2. It consists of the metal cylinder and the MCP assembly. The cylinder has a diameter of 76 mm and height of 86 mm and is made of aluminum. A 50 mm diameter gold plated tungsten wire is stretched on the axis of the cylinder by means of a spring contact that is used to apply a negative voltage of about 2 kV with respect to the grounded cylinder. The wire typically intercepts less than 5% of the beam. The beam crosses the cylinder walls through 20 mm diameter beam ports, while the secondary electrons escape through an 11 mm aperture towards the micro-channel plate. The MCP is the Hamamatsu F4655-12 assembly comprising two plates in the chevron configuration and the anode coupled through a capacitor directly to the BNC connector. The assembly is particularly suited for TOF measurements with the pulse rise time of less than 400 ps when detecting electrons. The MCP is mounted inside the housing with a protection mesh of 20 lines per mm on the input aperture. The MCP housing is attached to the cylinder to form a rigid head construction. A positive voltage in the range of 1.4–2.0 kV is applied to the MCP. The head assembly is driven by a stepper motorized linear actuator with a step size of 0.1 mm within a range of 84 mm. The motor step size and speed can be adjusted according to the need. In addition to counting stepper motor steps, the absolute position of the monitor head is read back by means of a linear potentiometer. Two limit switches are available to indicate extremes of the traveling range. Each monitor is mounted on the diagnostic vacuum box at 451
Fig. 3. Positioning of the retractable and adjustable TOF monitor in the beam line.
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
V.A. Verzilov / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
amplifier. The amplifier monitors the average input current and provides a TTL level signal if the current value exceeds 0.1 uA. This signal is used to switch off the MCP bias as a protective measure in the case of overload conditions. The amplifier is mounted in the immediate proximity to the monitor and connected to the rest of electronics with 30 m long cables. The low loss, phase stable signal cables for the TOF monitors were cut to have equal electrical lengths within a few tens of ps. Raw analog signals are processed by a low jitter (o 50 ps) CFD and converted to logical NIM pulses which are sent to a 25 ps resolution multi-hit time-to-digital converter (TDC), CAEN model V1290, operating in the trigger mode. The 11.8 MHz reference clock is applied to another input of the TDC. The CFD logical signals are also fed to an OR-gate module to generate the common trigger for all three monitors. The trigger starts the TDC acquisition window and is used to reduce the event load on the TDC. The TDC measures the time difference between the signals from the TOF monitors and the common reference clock within the acquisition window. Both positive and negative voltages required for TOF monitors are provided by ISEG High Voltage supplies, model V205. Electronic modules are controlled via a VME CPU running Linux. The same CPU provides the interface to the EPICS based ISAC control system. The fast data acquisition is performed by a set of applications communicating with the EPICS soft IO channels by means of the OS supported shared memory. The on-line data processing is done by a number of MATLAB applications running on a remote host. Peak fitting, peak position and widths, energy computations are done in this way. The system is highly automated and transparent to the users.
4. System calibration The high accuracy of beam energy measurements demands that all three monitors are nearly identical and the delays in cables and electronics are precisely known. In practice this is difficult to achieve. Small variances in construction, applied voltages and signal propagation through cables and electronics, component parameter drifts due to temperature and aging are difficult to control to the acceptable level. A more practical approach is to calibrate the system using a reference beam with a precisely known energy. For the ISAC TOF system we proposed and built a very efficient calibration setup using
a pulsed UV laser with the photon energy sufficiently high to generate photo electron emission from the TOF monitor wires. In this way the laser light acts as a “reference beam” traveling at a wellknown speed. The idea has been in consideration since the conceptual stage of the system design but actual implementation took a few years. Another important application of the UV laser for the TOF system is that all functionality checks do not necessarily require an ion beam and can be performed during shutdown or maintenance periods. Calibration and checks can be done conveniently on-line or off-line as required. 4.1. Calibration laser An inexpensive passively Q-switched MicroChip Nd:YAG UV laser was acquired from Teem Photonic, model SNU-02P-100, to be used in the calibration setup. The laser generates o500 ps pulses at a repetition rate of about 9 kHz. The average output power is 2 mW at a wavelength of 266 nm. The laser and the laser light shaping optics were mounted on an optical table approximately 30 m upstream of the TOF monitors (Fig. 5).The laser beam transport was complicated by the fact that the laser horizontal divergence of 11 mrad is about ten times larger than the vertical one. A short focal length concave lens was used to strongly expand the laser beam in both planes and collimate it with an aperture. In this way a fraction of the light with similar angular divergences in both planes was selected and then focused by a 500 mm focal length bi-convex lens towards the center of the TOF system. The laser beam was brought into the accelerator vacuum space through a UV grade fused silica window. Approximately half of the laser light was deflected towards a fast silicon photodiode using a 50% splitter located in front of the laser head. Alignment of the laser light through the 30 m long vacuum pipe of 50 mm in diameter was not trivial and was accomplished by means of two mirrors on fine adjustment mirror mounts. The final tuning was done with the help of a plastic scintillator installed in the vacuum space and viewed with a GigE CCD camera. The scintillator was supplied with orthogonal and polar grids to measure the laser spot size and position (Fig. 6). The laser spot size varies along the extent of the TOF system by about 1 mm (FWHM). A size of 0.5 mm was measured at the waist by moving the TOF monitor transversely and recording the count rate (Fig. 7). The time of flight of the laser light between the TOF monitors was measured using the data acquisition setup described above
Fig. 4. TOF system signal data acquisition setup.
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
V.A. Verzilov / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
5
Fig. 5. Calibration laser schematic setup. Here M1–M3 are adjustable mirrors, L1 and L2 lenses, IR diaphragm, PD photodiode, BS 50% beam splitter, VS scintillating view screen and CCD stands for a CCD camera.
Fig. 6. Image of the laser beam on the scintillating view screen. Horizontal and vertical lines are spaced by 5 mm, concentric circles point to the center of the beam pipe.
Fig. 8. Time spectra of laser pulse measured with the TOF system. The travel time of the laser beam from the first to the last TOF monitors is 37.5 ns.
after replacing the 11.8 MHz reference clock with the signal from the fast photodiode on the optical table. A typical timing spectrum measured with the laser light is presented in Fig. 8. Each peak had a nearly Gaussian shape with a FWHM width of about 250 ps and its center position was found from the fit. Comparing measured times of flight of the laser light with the calculated ones, using known distances between the monitors, all necessary corrections were determined. The results of the first calibration revealed that in spite of all measures taken to insure equity of three TOF monitors, the discrepancy between them turned out to be surprisingly large. Taking the first monitor as a reference, correction offsets of 600 740 ps and -2307 60 ps were found for the second and third monitor data, correspondingly. A few following calibrations have not changed these numbers significantly. 4.2. Calibration verification Fig. 7. Scan of the laser beam near to its waist with the middle TOF monitor. The measured FWHM width of the beam is about 0.5 mm.
For verification purpose, the corrections obtained from the laser calibration were applied to two different energy measurements. In
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
V.A. Verzilov / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
6
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
preparation for each measurement, TOF monitors were individually positioned in the center of the beam by scanning across until the maximum count rate was achieved. Then the time spectra were recorded. An example of the spectrum for the 25Mg10 þ beam with the current of about 1 enA is given in Fig. 9a. A similar spectrum obtained with the 0.5 enA beam of 22Ne4 þ ions is shown in Fig. 9b. It should be noted that depending on the accelerator tuning, the shape
Fig. 9. Time spectra of 25Mg10 þ (a) and 22Ne4þ (b) ion beams measured with the TOF system. Time from each monitor is measured with respect to a common 11.8 MHz reference clock.
of time peaks can vary substantially. Upon completion of the tuning process the peak widths (FWHM) were about 400 ps and 750 ps for magnesium and neon beams, correspondingly. For each beam the time peak width was about the same for all three monitors. In the performed measurements, including the calibration, recorded peak widths were substantially larger than the intrinsic resolution of the TOF monitors of 100 ps. This is expected to hold true for practically any accelerated beam at ISAC meaning that statistical errors in the energy measurements are dominated by the beam time spread. These errors are calculated by means of nonlinear regression analysis in the process of fitting the data with a suitable parameterized model. In particular, an asymmetric Lorentz fit was applied to the measured data in Fig. 9. The key fit parameter relevant to the TOF data analysis is the position of the time peak center and its error. Standard errors of the peak position calculated by fitting algorithms are typically substantially smaller than the peak width. For reported measurements they did not exceed several TDC channels. Fit parameter errors were calculated with the confidence level of 0.999 and used to compute the energy uncertainties given below. Using peak positions, two sets of time-of-flight values were found; calibration data was applied to one of them and ignored for the other. For each set, three corresponding values of the beam energy E21, E32 and E31 and their errors were calculated. The Indexes refer to the monitors used to calculate the beam energy. The errors for the data corrected using calibration offsets include both statistical and residual systematic errors due to calibration and, thus, are slightly higher. In the opposite case the errors are purely statistical. In addition, the unweighted average energies, standard deviations and relative errors were also computed. Both uncorrected and corrected data are summarized in Table 1. The table shows that although the average energy values change by as little as 0.2–0.4% between uncorrected and corrected data, applying the calibration corrections dramatically improves the agreement between the data recorded with all three monitor configurations. Standard deviations (from the average) σ E of uncorrected data, exceeding individual errors by more than ten times, indicate the presence of large unaccounted systematic errors. Once the systematic errors are essentially eliminated after correction, the associated standard deviations are reduced by a factor of 25–30 and approach the level of individual errors. It can be noted that E32 and E31 are systematically closer to each other than to E21. This is easy to understand taking into account that the distance between the first and second TOF monitors is substantially smaller than the spacing between these monitors and the third one. In this case, according to Eq. (3), E21 is expected to be more sensitive to statistical and systematic errors in time measurements. If E21 is excluded from calculations, the numbers in the last two columns of the Table 1 are reduced by another factor of three. However, it is more consistent to add weights to individual measurements when computing the average value. The effect of E21 on the average is then naturally reduced because of its higher error. Eq. (3) also suggests that relative errors increase for a faster beam proportionally to the beam velocity, which quantitatively agrees with obtained data. Based on these results, we can conservatively conclude that due to the implemented calibration, the systematic errors were drastically
Table 1 Results of applying calibration data to ion beam energy measurements. Ion beam
E21. MeV/u
E32. MeV/u
E31. MeV/u
Eave. MeV/u
σE MeV/u
σE/Eave %
Mg 10 þ , uncorrected þ Mg 10 corrected , 22 Ne 4 þ , uncorrected 22 4þ Ne , corrected
9.502470.0146 9.2830 70.0201 2.3014 70.0018 2.2752 70.0024
9.1995 7 0.0034 9.27127 0.0071 2.2653 7 0.0004 2.27407 0.0009
9.2573 70.0029 9.2735 70.0040 2.2723 70.0003 2.2742 70.0005
9.3197 9.2759 2.2797 2.2745
0.1608 0.0062 0.0192 0.0007
1.72 0.07 0.84 0.03
25 25
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
V.A. Verzilov / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
reduced and the absolute accuracy of energy measurements with the ISAC TOF system improved to better than 0.1%. Acknowledgments The author thanks D.Cameron, B.Minato, J.Holek for assembly and installation of the TOF monitors and J.Lassen for the help with the calibration laser setup. References [1] P. Bicault, M. Dombsky, P.W. Schmor, G. Stanford, Nuclear Instruments and Methods in Physics Research Section B 126 (1997) 231.
7
[2] P. Bricault, European Physical Journal-Special Topics 150 (2007) 227. [3] B. Feinberg, D. Meaney, R. Thatcher, C. Timossi, Nuclear Instruments and Methods in Physics Research Section A 270 (1988) 1. [4] R. Pardo, B.E. Clifft, P. Denhartog, D. Kovar, W. Kutschera, K.E. Rehm, Nuclear Instruments and Methods in Physics Research Section A 270 (1988) 226. [5] R.E. Laxdal, W. Andersson, K. Fong, M. Marchetto, A.K. Mitra, W.R. Rawnsley, I. Sekachev, G.Stanford, V.Verzilov, V.Zviagentsev, Proceedings of EPAC 2006, Edinburgh, Scotland, 2006, p. 1556. [6] P.K. DenHartog, J.M. Bogaty, L.M. Bollinger, B.E. Clifft, R.C. Pardo, K.W. Shepard, “The beam bunching and transport system of the Argon positive ion injector”, in: Proceedings of PAC 1989, Chicago, IL, March 1989, p. 545. [7] N.E. Vinigradov, P.N. Ostroumov, P.C. Pardo, S.I. Sharamentov, G.P. Zinkann, Nuclear Instruments and Methods in Physics Research Section A 526 (2004) 206. [8] M. Wu, C.A. Kruschwitz, D.V. Morgan, J. Morgan, Review of Scientific Instruments 79 (2008) 073104.
Please cite this article as: V.A. Verzilov, Nuclear Instruments & Methods in Physics Research A (2015), http://dx.doi.org/10.1016/j. nima.2015.03.010i
16 17 18 19 20 21 22 23 24 25 26 27 28 29