Laboratorio di Tecniche Nucleari per i Beni Culturali - Firenze
Labec - the external scanning microbeam


The external scanning microbeam line at LABEC (view from the end-station)

of "Overview"

The figure above shows a general view of the external scanning microbeam facility, the design of which is detailed in the figure hereafter. A magnetic lens, constituted by a doublet of quadrupoles, focuses the beam to a size which can range from some hundreds of microns down to 8 microns (for protons) on target.

Overview of the microbeam line: 1 - switcher magnet; 2 - gate valve; 3 - fast acting valve; 4 - horizontal and vertical beam steerers; 5 - pump station; 6 - object slits; 7 - beam profile monitor; 8 - quartz and Faraday cup station; 9 - horizontal and vertical beam steerers; 10 - Gate Valve; 11 - collimation slits; 12 - beam profile monitor; 13 - scanning coils; 14 - quadrupole doublet.
Targets are placed out of vacuum; in this way, no damage at all is procured to the object under investigation. External analyses are performed by extracting the beam out of vacuum typically through a 100 nm thick, 1 x 1 mm2 window (the orange square in the picture aside). These windows withstand the pressure difference between the beam line in vacuum and the external environment, minimising the energy loss and divergence of the particles traversing it.
Besides the use of the extremely thin windows Si3N4 windows, the extent of beam spread and halo due to the external set-up can be reduced by saturating the path traversed by the beam with helium and reducing as much as possible the distance between the exit window and sample (in our set-up, down to about 2-3 mm).
The combined use of:
• the 100 nm thick Si3N4 window,
• the helium atmosphere,
• the very short path (2-3 mm) traversed by the beam
is the key feature to reach the lowest dimensions.

With 1x1 mm2, 100 nm exit windows, for a 3 MeV proton beam we have less than ~10 mm FWHM on target, with ~ 1nA current; even higher beam currents (~10 nA) can be reached with some minor deterioration of the spatial resolution (~15 mm FWHM).

Test map collected scanning over the reference copper grid (10 mm bar, 125 mm pitch)

• two detectors for X rays (PIXE technique), yellow outline;

• one detector for gamma rays (PIGE technique), dark-orange outline at the bottom of the picture;

• one detector for particles backscattered from the sample under investigation (BS technique), pink outline,
with good overall energy resolution (~15 keV) notwithstanding the out-of-vacuum configuration;

• a system for the detection of visible luminescence induced by the beam in the sample (IBIL technique), light red outline.

Beam scanning is provided by a coils system immediately upstream of the quadrupole doublet, coupled to a multiple-input list-mode acquisition, by means of which signals from the detectors + beam coordinates on target are recorded sequentially (scanning and acquisition systems as well as quadrupole doublet are supplied by Oxford Microbeams Ltd.).

Elemental maps of the scanned area can thus be created from any space-resolved information (such as X rays, gamma rays, particles backscattered and light) obtained from the detectors active at the measurement point.

The system is typically used with 3 MeV protons, but we have worked with external microbeams of protons from 1 to 5 MeV and of alpha particles from 3 to 8 MeV. Proton microbeam current on target can be up to 1 nA still keeping the minimum spot size. It can be finely tuned down to ultra-low intensities, of the order of only hundreds of particles per second. The latter setting is useful to perform e.g. space resolved Ion Beam Induced Charge Collection on detectors, directly exposed to the scanning beam to test their response as a function of position, or to perform Scanning Transmission Ion Microscopy of thin samples, by placing a particle detector behind them.

Altogether, this facility is providing a unique tool for analysis in many fields, from Cultural Heritage to geology, electronics, chemistry and biology, as shortly presented in the following sections.

1) L. Giuntini, M. Massi, S. Calusi, The external scanning proton microprobe of Firenze: A comprehensive description, Nucl.Instr.&Meth.A 266-273 (2007), 576, Issue 2-3.

2) M. Massi, L. Giuntini, M. Chiari, N. Gelli, P.A. Manḍ, The external beam microprobe facility in Florence: set-up and performance, Nucl.Instr.&Meth.B 190 (2002), 276.

3) S. Calusi, E. Colombo, L. Giuntini, A. Lo Giudice, C. Manfredotti, M. Massi, G. Pratesi and E. Vittone, The ionoluminescence apparatus at the LABEC external microbeam facility, Nucl.Instr.&Meth.B 266 (2008), Issue 10.

4) M. Massi, L. Giuntini, M.E. Fedi, C. Arilli, N. Grassi, P.A. Manḍ, A. Migliori and E. Focardi, Use of micro-PIXE analysis for the identification of contaminants in the metal deposition on a CMS pitch adapter, Nucl.Instr.&Meth.B 219-220 (2004), 722.