Parallel-plate avalanche counters for heavy-ion beam tracking: History and mysteries

Despite being an old detector concept, position-sensitive parallel-plate avalanche counters (PPACs) remain widely used today for heavy-ion position and timing measurements. In modern rare isotope beam facilities and large-acceptance fragment separators, PPACs are used to characterize beam properties (diagnostics), facilitate beam delivery (tuning), and provide event-by-event beam tracking for particle identification (PID). Most popular particle localization methods in PPAC detectors are based on strip electrodes electrically connected to resistive-chain circuits or delay lines. More exotic systems include optical readouts based on recording electroluminescence light with high-granularity photodetector arrays or high-resolution resistive anode electrodes. PPACs with conventional resistive-chain or delay-line readouts achieve typical position and time resolutions of below 1 mm ($\sigma$) and around 150 ps ($\sigma$), respectively. In addition, delay-line systems are capable of counting rates above several hundred kHz for beam areas of a few millimeters square, and around 1 MHz for larger beam sizes. Resistive-chain readouts have limited rates of a few tens of kHz. A review of the operation principles and performance of PPAC detectors is presented in this paper. We will discuss decades-long experience building and operating PPACs developed at the National Superconducting Cyclotron Laboratory, and then refined at the Facility for Rare Isotope Beams (FRIB), mostly focusing on the delay-line readout technique (DLPPAC). We will also discuss problems that arise due to electric discharges at high rates and briefly describe ongoing developments at FRIB.

Figure 1

Comparison of calculated (left) and measured (right) signals induced on the PPAC anode electrode. The snapshot depicted on the right panel was obtained by irradiating a PPAC with 5.9 MeV $\alpha$-particles from an Am-241 source. The PPAC (3.5 mm wide) was operated in isobutane at 7 Torr and the signal was processed by a fast-current preamplifier.

Figure 2

Fast electric current profile induced on the anode by electrons moving across a 3 mm thick PPAC, computed according to Eq. (2). Parameters used for the calculation are given in the figure.

Figure 3

Graphical representation of the weighting field lines computed for the central strip of a segmented anode.

Figure 4

Illustrations of different charge densities induced by a charge ($q$) located at different distances from a segmented anode. For each position, the area under the distributions corresponds to the total charge accumulated in the central strip. When the charge ($q$) moves from one position to another, a current is induced to compensate for the variation in the integral charge accumulated on the strip.

Figure 5

Schematic representation of the transition from proportional avalanche mode to streamer due to the fast breakdown mechanism. Figure taken from Ref. [26].

Figure 6

Relation between the total avalanche charge and the irradiation density (rate), for different gaseous detector technology. The forbidden zone is dominated by fast breakdown mechanisms (spark discharge). The allowed zone is the region of operation of gas detectors without a spark discharge, featuring a progressive reduction of the maximum as a function of the rate. Figure taken from Ref. [29].

Figure 7

Schematic design of a two-dimensional position-sensitive PPAC: A common cathode foil is sandwiched between two segmented anode electrodes. The latter, coupled with a suitable readout, provides the $(X,Y)$ particles localization.

Figure 8

Traditional localization encoding techniques used in low-pressure PPAC are based on charge-division circuitry (left) or delay line (right).

Figure 9

Resistive foil technique with localization encoding processed by analyzing signals from the four corners of the cathode. The resulting image is affected by pincushion distortion.

Figure 10

Resistive foil technique with localization encoding processed by analyzing signals from the four sides of the cathode. The resulting image is affected by barrel distortion.

Figure 11

Conceptual design of a PPAC with optical readout. Particle localization is accomplished by calculating the center of gravity of light collected by the photosensor arrays.

Figure 12

Photograph of the two-dimensional OPPAC prototype (on the left side) and an image of a calibration mask obtained by irradiating the detector with 5.9 MeV $\alpha$ particles (on the right side).

Figure 13

Overview of the design of the Advanced Rare Isotope Separator (ARIS) at FRIB.

Figure 14

Exploded view of the mechanical model of a $10\times 10{\mathrm{cm}}^2$ FRIB DLPPAC. The top portion of the figure shows a photograph of the delay line and miniaturized fast-current preamplifiers used to process the delay-line signals.

Figure 15

Photograph of the interior of a $10\times 10{\mathrm{cm}}^2$ DLPPAC vessel. Four screws are used to fasten the PCB that supports the segmented anode electrodes and the cathode to the vessel’s corners. The PCBs also host fast-current preamplifiers. The vessel is equipped with proper interfaces for gas flow and anode voltage bias.

Figure 16

Arrangement for the test for the DLPPAC performance evaluation (left) and image of the pinholes brass mask (right), obtained by irradiating the detector with 5.9 MeV $\alpha$ particles.

Figure 17

Edge spread function analysis of the calibration mask image for the estimation of the position resolution achieved by a $10\times 10{\mathrm{cm}}^2$ DLPPAC. As a result of the intrinsic resolution of the system being better than delay-line space segmentation (1.27 mm pitch), the image is subdivided into many points corresponding to the intersection between the $x$ coordinate strips and the $y$ coordinated strips.

Figure 18

Histogram analysis (on the right) of an empty field image (on the left) for evaluating the uniformity of the detector response throughout the entire effective area.

Figure 19

Integral nonlinearity (INL) analysis of the calibration mask image—variation of the INL is below 0.3%.

Figure 20

Setup for the timing performance evaluation of the DLPPAC (top diagram). The measured time resolution provided by the DLPPAC is about 250 psec (bottom left graph). Results of time resolution and detection efficiency are also shown on the bottom right graph.

Figure 21

Transverse phase space of ${}^{64}\mathrm{Fe}$ fragment beam measured by DLPPACs installed in diagnostics station DB1, after transverse matching. The left figure shows the horizontal phase space and the right figure shows the vertical phase space. Graph taken from [58].