Development of a compact E×B microchannel plate detector for beam imaging

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Abstract

A beam imaging detector was developed by coupling a multi-strip anode with delay line readout to an E×B microchannel plate (MCP) detector. This detector is capable of measuring the incident position of the beam particles in one-dimension. To assess the spatial resolution, the detector was illuminated by an α-source with an intervening mask that consists of a series of precisely-machined slits. The measured spatial resolution was 520μm FWHM, which was improved to 413μm FWHM by performing an FFT of the signals, rejecting spurious signals on the delay line, and requiring a minimum signal amplitude. This measured spatial resolution of 413μm FWHM corresponds to an intrinsic resolution of 334μm FWHM when the effect of the finite slit width is de-convoluted. To understand the measured resolution, the performance of the detector is simulated with the ion-trajectory code SIMION.

Introduction

A new generation of radioactive beam facilities provide unique opportunities to investigate nuclei far from β-stability. However, the beam intensity of the most N/Z exotic nuclei is typically less than 1000 ions/s posing significant challenges in imaging these beams. In the case of low energy beams, it is particularly important that the imaging detector introduce the least amount of material into the beam path in order to minimally distort the beam. In addition, as most accelerator facilities are pulsed it is beneficial if the imaging detector has good timing characteristics. Due to their high gain, fast temporal response, sensitivity to a single electron, and compact size, microchannel plates (MCPs) are often used as an electron amplifier for these imaging detectors [1].

There are several methods for providing position sensitivity with an MCP detector including: multi-strip anode [2], helical delay line [3], [4], cross-strip anode [5], induced signal [6], [7], resistive anode [8], [9], [10], and Timepix CMOS readout [11]. To realize a beam imaging detector requires transport of electrons produced at a secondary-emission foil onto the surface of the position sensitive MCP detector situated away from the beam axis. In one approach, a clever magnetic field arrangement provided transport of the electrons on helical trajectories onto the surface of a MCP detector [12], [13], [14]. This technique resulted in a spatial resolution of 588μm FWHM [13]. The most serious limitation of this approach is the large space occupied by this detector making its use prohibitive in many experiments.

A beam timing detector which is compact and introduces a minimal amount of material into the beam path is an E×B detector [15], [16], [17], [18]. Such a detector has been used to measure the time-of-flight of beam particles and reaction products in nuclear reaction studies [19], [20], [21]. To make the MCP in an E×B detector position-sensitive we employed a multi-strip anode with delay line readout, which is a particularly appealing because of its simplicity and low cost. Moreover, due to the fast time response of the detector it is capable of resolving two particles that arrive simultaneously but are spatially separated. Two principal factors influence one’s ability to accurately image the beam: the impact of electron transport from the electron-emission foil to the MCP and the inherent spatial resolution of the position-sensitive element. In this article, we describe the design, development, and performance of an E×B position-sensitive detector suitable for imaging low-intensity radioactive beams. We explore the impact of the electron transport for this detector geometry on the measured resolution using the ion trajectory code SIMION [22].

Section snippets

Experimental setup

Presented in Fig. 1 (a) is a schematic drawing of the experimental setup used to determine the spatial resolution of the position-sensitive E×B MCP detector. Electrons, ejected from the 0.5μm thick aluminized mylar foil by the passage of ionizing radiation, are accelerated and bent onto the surface of a 40 mm diameter MCP. The MCP used was a standard chevron stack (APD 2 MA 40/12/10/12 60:1 NR) with 10μm diameter microchannels provided by Photonis USA [23]. The MCP amplifies the incident

Measuring the spatial resolution of the E×B MCP detector

To test the performance of the detector, it was placed in a vacuum chamber that was evacuated to a pressure of 4×107 torr and illuminated by a 1.5μ Ci 241Am α-source. Between the α-source and the secondary-emission foil was a 0.8 mm thick aluminum plate with 355μm wide slits that are 6.4 mm long. The 13 slits in the mask have a center-to-center spacing of 2 mm. Alpha-particles passing through the mask and foil were detected using a silicon surface barrier detector (SBD) as shown in Fig. 1 (a).

Intrinsic spatial resolution of E×B MCP detector

The spatial resolution measured corresponds to the convolution of the intrinsic spatial resolution of the detector with the finite slit width. The measured resolution M(Y) is given by: M(Y)=G(Y)I(Y)dYwhere G(Y) is taken as a step function with a width of 355μm to represent the slit, and I(Y) is a Gaussian with the intrinsic width, σintrinsic. For a given intrinsic width the measured resolution can be calculated. By varying the intrinsic width, the relationship between intrinsic

Simulating the detector resolution

The significantly larger spatial resolution of 520μm obtained with the E×B detector as compared to the 94μm [24] associated with the simple electrostatic arrangement [6], [9] indicates that the electron transport from the foil to the MCP dictates the measured resolution. To understand the electron transport in the crossed electric and magnetic fields between the secondary-emission foil and the front surface of the MCP detector we simulated the electron trajectories using the ion trajectory code

Conclusion

An E×B MCP detector with position-sensitivity in 1-dimension has been realized. Position-sensitivity was achieved by utilizing a MCP coupled to a multi-strip anode with delay line readout. Signals arriving at either end of the delay line were digitized by high speed digitizers and subsequently analyzed. To measure the position-sensitivity, a mask was inserted and the detector was exposed to α-particles from an 241Am source. While the simplest analysis provided a measured spatial resolution of

Acknowledgments

We gratefully acknowledge the technical support provided by the personnel in the Mechanical Instrument Services and Electronic Instrument Services (EIS) at the Department of Chemistry, Indiana University. In particular, we gratefully acknowledge A. Alexander of EIS for the design of the multi-strip anode and delay printed circuit boards. We thank Mr. Luis Morales (University of Notre Dame) for providing additional magnetic field calculations which aided our understanding of the detector

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      Citation Excerpt :

      MCPs are particularly useful as they are sensitive to a variety of particles including: electrons, ions, UV radiation, X-rays, slow neutrons, and fast neutrons [1–3]. Since their inception in the late 1950’s [4], position-sensitive MCP detectors have been used in several applications including but not limited to: distance-of-flight mass spectrometry [5], neutron radiography [2,3], time-of-flight positron emission tomography [6], and beam imaging at radioactive beam facilities [7,8]. Different methods exist to extract the position of an incident particle via measurement of the electron cloud emanating from an MCP including: multi-strip anode [9], helical delay line [10,11], cross-strip anode [12], sense wires [13,14], resistive anode [15–17], and Medipix2/Timepix CMOS detector [3,18].

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