The Chandigarh variable energy cyclotron

The characteristics and performance of the variable energy cyclotron at Chandigarh have been described. The machine operates for protons at 1 to 5 MeV, for deuterons at 4 MeV, for alphas from 1 to 2 MeV and 7 to 8 MeV, and He³⁺⁺ upto 11 MeV. The resolved beams of different particles from 30 nA to 1 μA have been obtained at the target with a resolution of about 30 keV. The magnetic field and the beam profile in the chamber are discussed. Various gamma-ray and charged particle spectra are given to indicate the performance of the accelerator.     

Surface erosion of TiC coatings under 4He+ and D+ ion irradiation

The surface damage and erosion of chemically vapor-deposited TiC coatings irradiated by 20, 40 and 60 keV D⁺ and ⁴He⁺ were studied for as-deposited and polished surfaces. Scanning electron micrographs of irradiated TiC reveal surface damage and erosion due to blistering and surface exfoliation. The erosion yield increases with increasing ion energy for both D⁺ and He⁺ irradiations, and it is generally larger for D⁺ than for He⁺ irradiations at a given energy. The erosion yield is larger for polished than for as-deposited surfaces for both D⁺ and He⁺ irradiations. The relationship between the blister diameter and the skin thickness agrees well with a relationship derived from a gas pressure model but disagrees with a relationship derived from a model of integrated lateral stresses as a prime mechanism for blister formation.     

Some effects at ion beam modification of polymethyl methacrylate

Measurements of the sensitivity of polymethyl methacrylate to ion irradiation are presented. The results of these studies show for ion energies (40 keV—1.8 MeV) and for the heavier ions (He⁺, Ar⁺) a higher sensitivity of the resist than in H⁺, electron or X-ray exposures. The measured sensitivity shows a dependence on electronic stopping power. In order to understand this relation an electron excitation spike mechanism is suggested. This mechanism is in accordance with investigations of sputtering rate of PMMA during ion bombardment. The effects of low energy ion induced nuclear reactions in the polymer layer and in masks and wafer are also discussed.     

Processes of Charge State Change at Collisions of He2+ and ArZ+ Ions with Fullerenes

Abstract

Using coincidence technique the relative cross-sections of charge state change and fragmentation at one and two electron capture processes have been measured for the first time at collisions of He²⁺ and Ar^z+ projectiles with fullerenes in the energy range (3-35)z keV.     

Effects of surface track potential on secondary electron emission and surface stopping power

In ion-surface scattering a positive surface track potential is induced on the surface behind the projectile due to ionizing collisions. The surface track potential is expected to affect secondary electron emission as well as the energy loss process of the projectile ions. We measure secondary electron yield induced by 0.5 MeV/u H⁺, He²⁺, Li²⁺ and B³⁺ ions during grazing angle scattering at a KCl(0 0 1) surface. The position-dependent secondary electron production rate was derived from the observed secondary electron yield. The secondary electron production rate is normalized by the mean square charge of the reflected ions. The normalized rate decreases with Z₁ suggesting that the surface track potential recapture the secondary electrons. We also measure the energy losses of 0.5 MeV/u H⁺, He²⁺, Li²⁺, B³⁺ and C⁴⁺ ions during grazing angle scattering at a KCl(0 0 1) surface. The observed result suggests that the surface stopping power is reduced by the surface track potential.     

Back-scattering study of heavy-ion distribution in semiconductors

The three-dimensional distribution of ⁸⁴Kr ions implanted into a Si substrate with 180 keV energy to a dose of 2 × 10¹⁶ ions/cm² has been measured by a back-scattering technique with 1.5 MeV ⁴He⁺. The distribution was observed as a distorted curve due to the influence of the ⁸⁴Kr ions on the specific energy loss of the ⁴He⁺. By considering the correction of the distortion, it is shown that the experimental results are in good agreement with theoretical predictions, and it is concluded that in quantitative discussions on the profile of impurity atoms in heavily doped material the influence of the impurity atoms on the specific energy loss cannot be neglected.     

Carbon coatings for fusion applications

The use of low atomic number material coatings in a fusion reactor plasma chamber may ease concern over maintaining plasma purity. A deposition process involving the discharge activated deposition of carbon coatings from methane at about 1 Pa pressure has been investigated. A coating of thickness 10 μm on copper has survived 1000 cycles of pulsed thermal heating at 37 MW m^-2 with only minor flaking. The surface damage of similar coatings on stainless steel surfaces has been investigated for irradiation with 20, 40, 60 and 120 keV D⁺ and He⁺ ions at ambient temperature. Scanning electron microscopy of irradiated surfaces revealed no significant surface damage for either D⁺ or He⁺ irradiation with energies of 40 and 60 keV for doses of 4 × 10²² and 8.1 × 10²² ions m^-2 respectively. For 120 keV D⁺ and He⁺ irradiations for a dose of 2.2 × 10²³ ions m^-2, surface damage in the form of ridges was observed. A comparison of the results for carbon coatings with those obtained for ATJ graphite reveals that this type of graphite shows surface damage for all irradiations performed, while the carbon coating appears to be more resistant to damage for many of these irradiations. These results reflect favorably on the possible use of these coatings on various components in the plasma chamber of a fusion reactor.     

The detection sensitivity of heavy impurities in Si using 280 keV He2+ and C2+ back-scattering

Background limitations, especially those involving pulse pile-up, on the sensitivity to detection of heavy elements near the surface of Si have been investigated with back-scattered 280 keV He²⁺ and C²⁺. Using gated electronics, pulse pile-up was reduced by as much as a factor of 1/60, which permitted a detection sensitivity, i.e. equal signal and background, of approximately 2×10¹¹ Tl atoms/cm² for He²⁺ back-scattering, while C²⁺ back-scattering data showed a detection sensitivity of about 2×10¹⁰ Tl atoms/cm². Back-scattering from Si implanted with 20 keV Tl⁺ to doses of 1×10¹³ and 1×10¹² ions/cm² was used for experimental confirmation of the He²⁺ and C²⁺ back-scattering sensitivities.     

Applications of large barium fluoride scintillators to in-beam gamma-ray spectroscopy

Properties of large-volume BaF₂ scintillators have been studied and first experiences of applications to in-beam γ-ray spectroscopy are reported.Energy resolutions of 9.1% at 662 keV and 6.3% at 1332 keV γ-ray energies were obtained. A time resolution of 375 ps FWHM was measured using a ⁶⁰Co source. Gain variations of the phototubes from changes in count rate could be limited to a 2% level.Lifetimes have been measured for the first 2⁺ state of ¹⁵²Sm using a ¹⁵²Eu source and the 5₁⁻ state of ⁴⁰Ca at 4.49 MeV populated in the (α, α′) reaction.     

Proton or helium ion beam written channel waveguides in Nd:YAG ceramics

We report on the fabrication of channel waveguides in Nd:YAG ceramics, using either focused proton beam writing (PBW) or He beam writing (HeBW) techniques. Energies of ions used in the writing process were at 1 MeV and 2 MeV, respectively, with different writing fluence. High quality channel waveguides were produced in both H⁺ and He⁺ implanted regions. Characteristics of the waveguides were explored, and refractive index distribution of the waveguide was reconstructed.     

Diffusion of Pb in (100) Si under electron beam annealing following dual ion implantations of Pb/Ne, Pb/O and Pb/N

(100) Si was dual-implanted with the ions Pb⁺/²²Ne⁺ (7 and 30 keV), Pb⁺/¹⁶O⁺ (7 and 26 keV) and Pb⁺/¹⁴N⁺ (7 and 24 keV) to peak concentrations of typically 10 at.%. The implanted samples were then electron beam annealed at 900 °C for 30 s with a temperature gradient of 5 °C s⁻¹ under high vacuum conditions. Channelled RBS measurements performed with 1.5 MeV ⁴He⁺ ions showed that annealing of the Pb/Ne and Pb/O samples resulted in an almost complete recrystallisation of the amorphous layers caused by the ion implantations and a total loss of the implanted Pb. For the Pb/Ne samples the Ne diffused out to leave a rough surface sprinkled with deep craters; for the Pb/O samples some SiO₂ formed below the surface. In contrast, for the Pb/N samples most of the amorphous layer survives annealing and almost all the Pb is retained. A striking feature is that annealing causes the Pb to diffuse away from the surface to be trapped in a deep diffusion sink provided by the implanted N. XRD analyses exhibited Pb (111) and Pb (220) reflections suggesting that Pb nanoclusters have grown in the understoichiometric silicon nitride layer. These structures offer an interesting opportunity for controlled carbon nanotube (CNT) growth on silicon nitride.     

Performance of a BGO calorimeter with photodiode readout and with photomultiplier readout at energies up to 10 GeV

Bismuth germanate (BGO) calorimeter arrays, consisting of up to 12 elements of 30 × 30 × 200 mm³ have been tested at the CERN PS with pions and electrons of up to 10 GeV/c momentum, and at SIN with pions, electrons and protons up to 450 MeV/c. Both photomultiplier (PM) and photodiode (PD) readouts were used. Accurate calibration in the 100 MeV energy range was achieved with stopping protons, stopping pions and minimum ionizing pions. With 212 MeV electrons and PM readout, a time resolution of the BGO signal of 640 ps fwhm has been measured. The energy resolution for electrons above 1 GeV (PD readout) was found to be roughly constant at σ/E ～ 1%. This is consistent with a negligible intrinsic resolution for BGO at these energies, after taking into account shower leakage and PD noise. For electrons of 92 and 200 MeV, we obtained (PM readout) energy resolutions close to the theoretical limit given by photon statistics and shower leakage. The electron/hadron separation was better than 1:500 over the energy range of 0.5 to 10 GeV, and improved to better than 1:1000 after a simple pattern cut. The energy deposition of the e.m. showers, both laterally and longitudinally (rear leakage), was found to be in agreement at the 0.1% level with Monte Carlo calculations using the SLAC-EGS program.     

He+ ION BOMBARDMENT OF C60 FULLERENE: AN FT-IR AND RAMAN STUDY

ABSTRACT

C₆₀ fullerene films have been bombarded with He⁺ ions at 30?keV at room temperature in vacuum. The structural changes undergone by C₆₀ have been followed by both FT-IR and Raman spectroscopy. Raman spectroscopy was the most useful tool for this scope. It has been clearly discovered that at low radiation dose C₆₀ forms oligomers but at higher radiation doses it is converted into an amorphous carbonaceous matter. The implications of these results on the possible survival of C₆₀ fullerene in the interstellar space have been discussed briefly in connection with the previous results on the effects of various types of electromagnetic radiation over C₆₀.     

Neutron detection efficiency of a thick lithium glass detector

The neutron detection efficiency of a 9.55 mm thick NE-912 lithium glass scintillator has been determined for the energy range from 25 keV to 2 MeV. It was measured relative to a thin, 0.835 mm thick NE-908 glass, which was determined by Monte Carlo calculations. The measured efficiency curve shows the marked effect of the ¹⁶O resonance at 442 keV, and a strong increase with energy for E_n > 1.2 MeV, due to (n,n′γ) reactions.     

Low capacity drift diode

Based on the drift chamber principle a low capacity diode has been built with one large area p⁺n junction at the front, and p⁺ and n⁺ patterns at the rear. The capacitance of the device is only 1.5 pF at an area of 1 cm² and a thickness of 280 μm. The energy resolution for Am α-particles is 13.5 keV, the optical sensitivity is homogeneous over the whole area, and the charge collection times lie between 10 and 300 ns.     

In Situ Mitigation of Subsurface and Peripheral Focused Ion Beam Damage via Simultaneous Pulsed Laser Heating

Focused helium and neon ion (He⁺/Ne⁺) beam processing has recently been used to push resolution limits of direct‐write nanoscale synthesis. The ubiquitous insertion of focused He⁺/Ne⁺ beams as the next‐generation nanofabrication tool‐of‐choice is currently limited by deleterious subsurface and peripheral damage induced by the energetic ions in the underlying substrate. The in situ mitigation of subsurface damage induced by He⁺/Ne⁺ ion exposures in silicon via a synchronized infrared pulsed laser‐assisted process is demonstrated. The pulsed laser assist provides highly localized in situ photothermal energy which reduces the implantation and defect concentration by greater than 90%. The laser‐assisted exposure process is also shown to reduce peripheral defects in He⁺ patterned graphene, which makes this process an attractive candidate for direct‐write patterning of 2D materials. These results offer a necessary solution for the applicability of high‐resolution direct‐write nanoscale material processing via focused ion beams.     

High-sensitivity and high-resolution low-energy ion scattering

Low-Energy Ion Scattering (LEIS or ISS) is used to selectively analyze the atomic composition of the outer atomic layer of surfaces. In addition, the spectrum gives (non-destructively) the in-depth distribution. Using a double toroidal energy analyzer with parallel energy detection and time-of-flight filtering a high sensitivity and mass resolution of LEIS is achieved. This is demonstrated for a highly dispersed catalyst of Pt/Au on γ-alumina. The improved depth resolution is illustrated for self-assembled monolayers of alkanethiols (12-20 carbon atoms) on gold. Even for these low Z carbon atoms a clear shift of 8 eV/carbon atom is observed (using 1.5 keV ⁴He⁺ ion scattering). This opens many new possibilities for studies of ultra-thin diffusion barriers, high-k dielectrics and biosensors.     

Radiation damage study of MeV ions-implanted Nd:YVO4 crystal

Damage formation mechanism of Nd:YVO₄ implanted with MeV ions is investigated. MeV Si⁺ ions were implanted into Nd:YVO₄ crystal, and the lattice damage was measured using Rutherford backscattering spectroscopy/channeling (RBS/C) method. The damage creation kinetic indicates a significant contribution from electronic energy loss to the surface damage. A detailed analysis allows us to deduce the different contributions from electronic and nuclear stopping powers to the lattice damage production. An obvious difference in extent of damage from 1 MeV and 3 MeV Si⁺ implantations also implies that there exists a threshold value of the electronic energy deposition for damage formation. The exact value of threshold is obtained by comparison with the experimental data obtained from 3 MeV O⁺, F⁺ and Si⁺ implantation results, which turns out to be (1.7 ± 0.1) keV/nm.     

Growth kinetics of the θ phase in AlCu thin film bilayers

The growth kinetics of the θ (Al₂Cu) phase in AlCu thin film bilayers were studied by MeV He⁺ backscattering and X-ray diffraction. In the temperature range 160–200°C the growth is diffusion limited with an activation energy of 1.02 eV and a pre-exponential factor of 7 × 10^?3cm²s^?1. Nucleation of the γ₂ phase occurs at 200°C with long annealing times. There is no evidence of the presence of other phases in the investigated temperature range.     

Surface modification of biodegradable plastics by ion beams

Biodegradable plastics can be used as conventional plastics, while on disposal they decompose to water and carbon dioxide by microorganisms existing in natural environment. Products using biodegradable plastics have recently been developed in many companies pursuing ecology. In this study, surface modification of biodegradable plastics was carried out by inert ion beams for improvement of photo deterioration under an ultraviolet ray. The hardness of biodegradable plastics tended generally to decrease with irradiation of an ultraviolet ray. In this method, the hardness of ion-bombarded biodegradable plastics was kept at an initial value under an ultraviolet ray, because the modified layer by ion bombardment intercepted an ultraviolet ray. The hardness of He⁺ ion-bombarded biodegradable plastics showed larger value than that of Ar⁺ ion bombardment. He⁺ ion bombardment at ion energy of 10 keV produced the suitable property with both of high transmittance of a visible ray and high interception of an ultraviolet ray in a surface layer of biodegradable plastics.