Low Energy Neutrals Detection: MCP efficiency

Low Energy Neutrals Detection: 

MCP efficiency 

HEWG workshop - Porto Venere (La Spezia) 12-16 June 2011 

Rosanna Rispoli, Elisabetta De Angelis, Nello Vertolli, 

Luca Colasanti, Alessandro Mura, Jürgen Scheer and Peter Wurz

Outline 

The MCP detector: characteristic of ELENA 

stop element 

Description of efficiency measurements for Low 

Energy Neutral Atoms at MEFISTO facility in 

Bern university 

Discussion of results 

HEWG workshop - Porto Venere 

(La Spezia, Italy) 12-16 June 2011

ELENA INSTRUMENT: 

ELENA is a Time-of-Flight (TOF) sensor. This kind of 

ToF system is able to directly measure lowenergetic 

particles identifying Tstart and Tstop with 

the following crucial ELENA elements: 

• The Start element: Shuttering System 

• The Stop element: MCP detector 



The stop element: MCP 

Chevron MCP 

. 

The Microchannel Plate (MCP) consists of millions of 

very-thin, conductive glass channel (from 4 to 25 

micrometer) fused together and sliced into a thin plate. 

Each channel works as an independent secondaryelectron 

multiplier to form a two-dimensional 

secondary-electron multiplier. A particle that enters one 

of the channels hits the wall of the channel and the 

impact starts a cascade of electrons that propagates 

through the channel, which amplifies the original signal 

by several orders of magnitude depending on the 

electric field strength and the geometry of the micro- microchannel 

plate. 

For ELENA we have MCP in chevron configuration. 

In a chevron MCP the electrons that exit the first plate 

start the cascade in the next plate. The advantage of the 

chevron MCP over the straight channel MCP is 

significantly more gain at a given voltage. A single input 

event will generate a pulse of 10^7 or more electrons at 

the output. 



Stop element: 

MCP efficiency 

The detection efficiency of a MCP is an important 

performance parameter. In the ELENA application it’s 

crucial to understand the limit of this detector for very 

low impact energy. 

Several features can affect the detection efficiency : 

• the Open Area Ratio (OAR) the ratio of the open 

area to the total effective area of the MCP (Particles 

incident on the MCP between channels are not 

detected), 

• the Bias Angle: Channels are tilted to prevent 

incident particles from passing through the channels, 

• the coating that can efficiently enhance the 

secondary emission and consequently the detection 

efficiency. 

All these parameters can be accurately analysed to 

choice the better system. 


(La Spezia, Italy) 12-16 June 2011 

MCP EQM/FM 

Characteristics 

Dim 170x16 

Bias 

angle 

Pore 

size 

12° 

10μm 

C-C 12μm 

OAR 60% 

L/D 60:1 

Coating 

(tbc) 

CsI or 

MgO 

Dark cts ~1ct/s/cm

Existing data analysis for MCP 

detector efficiency efficiency 

In the past there have been many attempts to 

characterize and predict the detection efficiencies of 

MCP for electrons and ions. Less well studied is their 

detection efficiencies for low energy neutral species. 

This fact probably depends on the lack of facilities able 

to produce variable and well-controlled low energy 

beams of neutral atoms. 

Stephen and Peko has procured some data about this 

argument for O and H species (Stephen et al, 2000; 

Peko et al., 2000) in the range 30eV-1000eV. 

For the scientific goal 

of detecting sputtered 

neutral is crucial the 

study of the efficiency 

of MCP in particular 

at lower energies and 

then starting from 

energies of about 10 

eV. Until today there 

are no works about it . 



MCP Efficiency for LL-ENA 

ENA beam 

Low energy neutral beam (10-1000eV ) → Facility: MEFISTO, Bern University (Peter Wurz and 

Jürgen Scheer). 

Ion-beam neutralizer: positive ions enter the system from the left. The electrostatic analyser (ESA) 

separates any neutrals in the primary beam. Ions are neutralized by reflection at grazing incidence on 

a polished tungsten single crystal. After reflection, charged particles are removed by the charged-particle 

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deflection system on the right side resulting in a completely neutral particle beam. 

The current produced by the ions being neutralized on 

the tungsten surface of the neutralizer is measured with 

a sensitive pico-ammeter that is in the MEFISTO 

chamber to avoid background noise. This measurement 

is proportional to the rate of neutralization of the ion 

beam and therefore to the flux of neutrals in the neutral 

beam. Calibration curves for the surface neutralizer are 

therefore presented in units of flux per pico-Ampere

MCPs used for the tests 

PHYSICAL CHARACTERISTICS of 

MCPS 

SPECIFICATION 

Quality Diameter: 25mm Minimum 

Center-to-Center Spacing: 12µm Nominal 

Pore Size: 10µm Nominal 

Bias Angle: 12°± 1° 

Open Area Ratio: 55% Minimum 

Quality Level: Detection 

ELECTRICAL CHARACTERISTICS of 

DETECTOR 

SPECIFICATION 

Electron Gain @ 2400 Volts: 1 x 107 Minimum 

Resistance: 83-600 Megohms 

Reference 

Dark Count: 5 (cts/sec/cm2) 

Maximum 

• First test: 

Efficiency measurements of 

a standard dimensions MCP 

with low energy ENA beam 

(Hydrogen, Helium and 

Oxygen). 

• Second test: 

Efficiency measurements of a 

standard dimensions MCP with 

CsI Coating.

To reduce as much as possible the background 

level we use a setup that provides 

• some filter capacitors and resistance 

• one 78% transparency grid on front MCP 

MCP Setup 

• Shielding box that contains the whole detector 

• Preamplifier box inside the vacuum chamber 

Capacitor 

MCP 



The grid should have dual function: 

grid 

•Shielding for electron and secondary ions 

•Applying a small electric field between the 

input face of the channel plate and a grid 

above the face, electrons emitted by ions 

striking the interchannel web are returned to 

channels and then detected

MCP 

First of all is necessary to establish a right threshold 

to distinguish the real signal from the noise that will have a 

pulse amplitude less 

than real signal induced by neutrals 

Acquisition Chain 

Oscilloscope 

Signal 

Preamplifier 

Pulse – Shaping 

Amplifier 



High Voltage Power 

Supply 

Multichannel Analyzer 

Frequency Counter 

PC

Measurement Details 

Then fundamental is the choice of MCP 

operating high voltage. 

Observing variation of counts Vs MCP 

voltage the best voltage will be that one 

where this curves reach a plateau where 

the count rate remains almost constant. 

It is this voltage that is chosen as the 

operating voltage. 



Although the same count rate is 

achievable at higher voltages, a lower 

voltage is chosen in order to preserve the 

MCPs functionality by avoiding to get 

fatigued too quickly 

This is a plot of gain vS extracted charge. 

There is an initial drop in gain as gasses 

are removed from the MCP, then there is a 

long period of stable gain.

Geometrical Area 5.47cm^2 

Open Area Ratio 60% 

Results 

Geometrical Area 5.47cm^2 

Open Area Ratio 60% 

Good agreement with Stephen 

and Peko’s results 

Efficiency improvement with a 

standard coating of CsI (in 

range of low energy about 

20%).

ANY IMPROVEMENT WITH GRID? 

There are some works in literature ( B.Deconihuout et al,1995) about this topic. By applying small 

electric field between the input face of the channel plate and a grid above the face electrons emitted 

by neutral striking the interchannel web are returned to channels and then detected. The efficiency 

improvement is in the order of 15%. 

We didn’t observe a significant improvement in 

efficiency with the grid. Even if probably in this 

setup the distance of the grid was too big. 

Anyway we observed with the grid a reduction 

of dark count rate, that is very important to 

improve the signal to noise ratio.

Conclusions 

• Finally with this last results we cover the low energy range for 

MCP neutral efficiency that until now was missing. 

•We will want to study the behaviour of another standard coating 

(MgO) that should be more indicated for neutral detection and less 

hygroscopic than CsI. 

•We want further investigate the capability of the grid to improve 

the efficiency providing a setup with less distance between the 

front of the MCP and the grid and increasing its transparency from 

78% to 95%.

Low Energy Neutrals Detection: MCP efficiency

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