6.2.2 Neutral Source

As the CASYMS chamber only provides an ion beam, the needed neutral particles had to be produced in the setup itself. A polished tungsten(110) surface of 25x30mm was chosen to neutralize the ion beam from the CASYMS source [33]. The beam hits the neutralization surface (NS) at grazing incidence (11 \ensuremath{}). This neutralizes almost 100% of the ions (see also Section 3.2.1). The neutralization of the positive ions is also associated with dissociation of molecules making it possible to use CO\( ^{+}_{2}\) primary ions to produce neutral oxygen atoms and N\( ^{+}_{2}\) ions to produce nitrogen atoms. Molecular beams have a far higher intensity than corresponding atomic ion beams at CASYMS. The drawback of using molecules is that at the low energies used a considerable fraction of molecules might have survived and hit the conversion surface in the actual instrument as molecules instead of atoms. But these surviving molecules would have been easily be identifiable by the time-of-flight (CODIF). The measurements showed no substantial molecule fraction even at the lowest energies investigated. Thus the dissociation was assumed to be very effective even at lower energies (< 200eV per oxygen atom) contrary to results reported in [37,38]. The high dissociation yield might be due to the different material used (W instead of Pd) and also due to a possible overlayer on the surface most probably water as the NS was not heated or cleaned as described in [33]. Figure 6.2 depicts a schematic view of the neutralizer including the beam path.

Figure 6.2: NICE beam neutralizer (with part of the shielding removed)

A front shield limits the primary ion beam width from 100x100mm\( ^{2}\) (CASYMS beam width) to about 30x10mm\( ^{2}\). This beam then hits the tungsten neutralization surface. The reflected particles pass through a grid with at least 98% transmission into the charged particle deflection unit. Charged particles (positive or negative) get deflected towards the capacitor plates while all neutrals propagate undisturbed to the exit aperture. This slit delimited the angular divergence of the beam such that only the conversion surface mounted in the extraction lens would be hit by the neutrals and as little as possible of the surrounding structure. This was verified optically. The whole neutralizer was electrostatically shielded such that from the outside only surfaces at ground potential were visible. To minimize background the exit of the neutralizer was linked by an aluminum foil tunnel to the hole in the shielding around the extraction lens. This was necessary because the conversion surface had to be kept on a high negative voltage (minus 19kV) and would have attracted accidental positive ions from the background gas with a very high efficiency. Figure 6.3 depicts the view from the CASYMS ion source into the neutralizer.

Figure 6.3: Neutralizer as seen from the CASYMS ion source. The dotted line depicts the three tungsten neutralization surfaces. The black rectangle mirrored in the tungsten surfaces is the entry aperture of the charged particles deflection section. This entry aperture was later enlarged and covered by a high transmission grid in order to use the whole neutralization surface available. The misalignment of the three tungsten surfaces is clearly visible.

The primary ion beam flux impinging on the neutralizer is estimated in Equation 6.1. \( I_{B\! S} \) denotes the primary molecular ion flux measured using the beam scanner in CASYMS. This scanner has a sensitive area \( A_{B\! S} \) of 10mm\( ^{2}\) and is realized with a channeltron. The neutralization surface has a active area \( A_{N\! S} \) of 30x25mm\( ^{2}\) and is tilted to an angle \( \beta \) of 11 \ensuremath{}. The total primary ion flux \( I_{P} \) is given by

I_{P}=I_{B\! S}\frac{A_{N\! S}\: \sin (\beta )}{A_{B\! S}}
\end{displaymath} (6.1)

The neutral particle intensity after the charged particles deflection plates is estimated in Equation 6.2. \( F_{n} \) denotes the fraction of neutral particles either molecules or atom pairs) after reflection (\( \approx \)0.95 for tungsten), \( \sigma \) the reflection efficiency into an solid angle of 22.5 \ensuremath{}x22.5 \ensuremath{} (see Chapter 4.2.2). The reflection efficiency is estimated to be 0.2 for tungsten at an incidence angle of 11 \ensuremath{}. \( \sigma _{rel} \) denotes the fraction of the into the 22.5 \ensuremath{}x22.5 \ensuremath{} cone reflected particles that are within a solid angle of 9 \ensuremath{}x1.3 \ensuremath{} (this is the effective beam divergence after the exit aperture). The latter angle was selected such that the neutral beam covers the conversion surface (CS) completely even when the CS is moved around using the x-translation table. \( \sigma _{rel} \) was estimated using angular scattering images from tungsten single crystals and has a value of 0.16. An additional factor \( k_{miss} \) accounts for the possible misalignment of the into three tungsten crystal pieces divided neutralizer surface. This loss was estimated to be of the order of 0.5. Additional loss \( k_{f\! ov} \) is due to the fact that only particles scattered from the middle of the conversion surface would make it through the exit aperture when scattered inside the 9 \ensuremath{}x1.3 \ensuremath{} solid angle. Particles impinging near the edge of the neutralization surface are more likely to be absorbed by the exit aperture. Although more accurate calculations should be done to estimate the loss due to this effect, a value of 0.5 was used for \( k_{f\! ov} \). The neutral flux \( I_{N} \) at the exit of the neutralizer is then given by

$\displaystyle I_{N}$ $\textstyle =$ $\displaystyle I_{P}\, 2\, F_{n}\, \sigma \, \sigma _{rel}\, k_{miss}\, k_{f\! ov}$ (6.2)
  $\textstyle \approx$ $\displaystyle 0.2\, I_{B\! S}$ (6.3)

The factor of 2 accounts for the dissociation of molecules yielding two usable particles (either two nitrogen atoms when using a N\( ^{+}_{2}\) primary beam or an oxygen and a carbon monoxide molecule when using a CO\( ^{+}_{2}\) primary beam [39]). With typical count rates \( I_{B\! S} \) at the beam scanner of 30kHz this yields a neutral beam intensity \( I_{N} \) of about 6000 neutral particles per second.

March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland