Low Energy Neutrals Detection: MCP efficiency
Low Energy Neutrals Detection: MCP efficiency
Low Energy Neutrals Detection: MCP efficiency
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<strong>Low</strong> <strong>Energy</strong> <strong>Neutrals</strong> <strong>Detection</strong>:<br />
<strong>MCP</strong> <strong>efficiency</strong><br />
HEWG workshop - Porto Venere (La Spezia) 12-16 June 2011<br />
Rosanna Rispoli, Elisabetta De Angelis, Nello Vertolli,<br />
Luca Colasanti, Alessandro Mura, Jürgen Scheer and Peter Wurz
Outline<br />
The <strong>MCP</strong> detector: characteristic of ELENA<br />
stop element<br />
Description of <strong>efficiency</strong> measurements for <strong>Low</strong><br />
<strong>Energy</strong> Neutral Atoms at MEFISTO facility in<br />
Bern university<br />
Discussion of results<br />
HEWG workshop - Porto Venere<br />
(La Spezia, Italy) 12-16 June 2011
ELENA INSTRUMENT:<br />
ELENA is a Time-of-Flight (TOF) sensor. This kind of<br />
ToF system is able to directly measure lowenergetic<br />
particles identifying Tstart and Tstop with<br />
the following crucial ELENA elements:<br />
• The Start element: Shuttering System<br />
• The Stop element: <strong>MCP</strong> detector<br />
HEWG workshop - Porto Venere<br />
(La Spezia, Italy) 12-16 June 2011
The stop element: <strong>MCP</strong><br />
Chevron <strong>MCP</strong><br />
.<br />
The Microchannel Plate (<strong>MCP</strong>) consists of millions of<br />
very-thin, conductive glass channel (from 4 to 25<br />
micrometer) fused together and sliced into a thin plate.<br />
Each channel works as an independent secondaryelectron<br />
multiplier to form a two-dimensional<br />
secondary-electron multiplier. A particle that enters one<br />
of the channels hits the wall of the channel and the<br />
impact starts a cascade of electrons that propagates<br />
through the channel, which amplifies the original signal<br />
by several orders of magnitude depending on the<br />
electric field strength and the geometry of the micro- microchannel<br />
plate.<br />
For ELENA we have <strong>MCP</strong> in chevron configuration.<br />
In a chevron <strong>MCP</strong> the electrons that exit the first plate<br />
start the cascade in the next plate. The advantage of the<br />
chevron <strong>MCP</strong> over the straight channel <strong>MCP</strong> is<br />
significantly more gain at a given voltage. A single input<br />
event will generate a pulse of 10^7 or more electrons at<br />
the output.<br />
HEWG workshop - Porto Venere<br />
(La Spezia, Italy) 12-16 June 2011
Stop element:<br />
<strong>MCP</strong> <strong>efficiency</strong><br />
The detection <strong>efficiency</strong> of a <strong>MCP</strong> is an important<br />
performance parameter. In the ELENA application it’s<br />
crucial to understand the limit of this detector for very<br />
low impact energy.<br />
Several features can affect the detection <strong>efficiency</strong> :<br />
• the Open Area Ratio (OAR) the ratio of the open<br />
area to the total effective area of the <strong>MCP</strong> (Particles<br />
incident on the <strong>MCP</strong> between channels are not<br />
detected),<br />
• the Bias Angle: Channels are tilted to prevent<br />
incident particles from passing through the channels,<br />
• the coating that can efficiently enhance the<br />
secondary emission and consequently the detection<br />
<strong>efficiency</strong>.<br />
All these parameters can be accurately analysed to<br />
choice the better system.<br />
HEWG workshop - Porto Venere<br />
(La Spezia, Italy) 12-16 June 2011<br />
<strong>MCP</strong> EQM/FM<br />
Characteristics<br />
Dim 170x16<br />
Bias<br />
angle<br />
Pore<br />
size<br />
12°<br />
10μm<br />
C-C 12μm<br />
OAR 60%<br />
L/D 60:1<br />
Coating<br />
(tbc)<br />
CsI or<br />
MgO<br />
Dark cts ~1ct/s/cm
Existing data analysis for <strong>MCP</strong><br />
detector <strong>efficiency</strong> <strong>efficiency</strong><br />
In the past there have been many attempts to<br />
characterize and predict the detection efficiencies of<br />
<strong>MCP</strong> for electrons and ions. Less well studied is their<br />
detection efficiencies for low energy neutral species.<br />
This fact probably depends on the lack of facilities able<br />
to produce variable and well-controlled low energy<br />
beams of neutral atoms.<br />
Stephen and Peko has procured some data about this<br />
argument for O and H species (Stephen et al, 2000;<br />
Peko et al., 2000) in the range 30eV-1000eV.<br />
For the scientific goal<br />
of detecting sputtered<br />
neutral is crucial the<br />
study of the <strong>efficiency</strong><br />
of <strong>MCP</strong> in particular<br />
at lower energies and<br />
then starting from<br />
energies of about 10<br />
eV. Until today there<br />
are no works about it .<br />
HEWG workshop - Porto Venere<br />
(La Spezia, Italy) 12-16 June 2011
<strong>MCP</strong> Efficiency for LL-ENA<br />
ENA beam<br />
<strong>Low</strong> energy neutral beam (10-1000eV ) → Facility: MEFISTO, Bern University (Peter Wurz and<br />
Jürgen Scheer).<br />
Ion-beam neutralizer: positive ions enter the system from the left. The electrostatic analyser (ESA)<br />
separates any neutrals in the primary beam. Ions are neutralized by reflection at grazing incidence on<br />
a polished tungsten single crystal. After reflection, charged particles are removed by the charged-particle<br />
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deflection system on the right side resulting in a completely neutral particle beam.<br />
The current produced by the ions being neutralized on<br />
the tungsten surface of the neutralizer is measured with<br />
a sensitive pico-ammeter that is in the MEFISTO<br />
chamber to avoid background noise. This measurement<br />
is proportional to the rate of neutralization of the ion<br />
beam and therefore to the flux of neutrals in the neutral<br />
beam. Calibration curves for the surface neutralizer are<br />
therefore presented in units of flux per pico-Ampere
<strong>MCP</strong>s used for the tests<br />
PHYSICAL CHARACTERISTICS of<br />
<strong>MCP</strong>S<br />
SPECIFICATION<br />
Quality Diameter: 25mm Minimum<br />
Center-to-Center Spacing: 12µm Nominal<br />
Pore Size: 10µm Nominal<br />
Bias Angle: 12°± 1°<br />
Open Area Ratio: 55% Minimum<br />
Quality Level: <strong>Detection</strong><br />
ELECTRICAL CHARACTERISTICS of<br />
DETECTOR<br />
SPECIFICATION<br />
Electron Gain @ 2400 Volts: 1 x 107 Minimum<br />
Resistance: 83-600 Megohms<br />
Reference<br />
Dark Count: 5 (cts/sec/cm2)<br />
Maximum<br />
• First test:<br />
Efficiency measurements of<br />
a standard dimensions <strong>MCP</strong><br />
with low energy ENA beam<br />
(Hydrogen, Helium and<br />
Oxygen).<br />
• Second test:<br />
Efficiency measurements of a<br />
standard dimensions <strong>MCP</strong> with<br />
CsI Coating.
To reduce as much as possible the background<br />
level we use a setup that provides<br />
• some filter capacitors and resistance<br />
• one 78% transparency grid on front <strong>MCP</strong><br />
<strong>MCP</strong> Setup<br />
• Shielding box that contains the whole detector<br />
• Preamplifier box inside the vacuum chamber<br />
Capacitor<br />
<strong>MCP</strong><br />
HEWG workshop - Porto Venere<br />
(La Spezia, Italy) 12-16 June 2011<br />
The grid should have dual function:<br />
grid<br />
•Shielding for electron and secondary ions<br />
•Applying a small electric field between the<br />
input face of the channel plate and a grid<br />
above the face, electrons emitted by ions<br />
striking the interchannel web are returned to<br />
channels and then detected
<strong>MCP</strong><br />
First of all is necessary to establish a right threshold<br />
to distinguish the real signal from the noise that will have a<br />
pulse amplitude less<br />
than real signal induced by neutrals<br />
Acquisition Chain<br />
Oscilloscope<br />
Signal<br />
Preamplifier<br />
Pulse – Shaping<br />
Amplifier<br />
HEWG workshop - Porto Venere<br />
(La Spezia, Italy) 12-16 June 2011<br />
High Voltage Power<br />
Supply<br />
Multichannel Analyzer<br />
Frequency Counter<br />
PC
Measurement Details<br />
Then fundamental is the choice of <strong>MCP</strong><br />
operating high voltage.<br />
Observing variation of counts Vs <strong>MCP</strong><br />
voltage the best voltage will be that one<br />
where this curves reach a plateau where<br />
the count rate remains almost constant.<br />
It is this voltage that is chosen as the<br />
operating voltage.<br />
HEWG workshop - Porto Venere<br />
(La Spezia, Italy) 12-16 June 2011<br />
Although the same count rate is<br />
achievable at higher voltages, a lower<br />
voltage is chosen in order to preserve the<br />
<strong>MCP</strong>s functionality by avoiding to get<br />
fatigued too quickly<br />
This is a plot of gain vS extracted charge.<br />
There is an initial drop in gain as gasses<br />
are removed from the <strong>MCP</strong>, then there is a<br />
long period of stable gain.
Geometrical Area 5.47cm^2<br />
Open Area Ratio 60%<br />
Results<br />
Geometrical Area 5.47cm^2<br />
Open Area Ratio 60%<br />
Good agreement with Stephen<br />
and Peko’s results<br />
Efficiency improvement with a<br />
standard coating of CsI (in<br />
range of low energy about<br />
20%).
ANY IMPROVEMENT WITH GRID?<br />
There are some works in literature ( B.Deconihuout et al,1995) about this topic. By applying small<br />
electric field between the input face of the channel plate and a grid above the face electrons emitted<br />
by neutral striking the interchannel web are returned to channels and then detected. The <strong>efficiency</strong><br />
improvement is in the order of 15%.<br />
We didn’t observe a significant improvement in<br />
<strong>efficiency</strong> with the grid. Even if probably in this<br />
setup the distance of the grid was too big.<br />
Anyway we observed with the grid a reduction<br />
of dark count rate, that is very important to<br />
improve the signal to noise ratio.
Conclusions<br />
• Finally with this last results we cover the low energy range for<br />
<strong>MCP</strong> neutral <strong>efficiency</strong> that until now was missing.<br />
•We will want to study the behaviour of another standard coating<br />
(MgO) that should be more indicated for neutral detection and less<br />
hygroscopic than CsI.<br />
•We want further investigate the capability of the grid to improve<br />
the <strong>efficiency</strong> providing a setup with less distance between the<br />
front of the <strong>MCP</strong> and the grid and increasing its transparency from<br />
78% to 95%.<br />
HEWG workshop - Porto Venere<br />
(La Spezia, Italy) 12-16 June 2011