Spurious ionic charge states in a tandem accelerator

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Abstract

Spurious projectiles appearing in a tandem accelerator when tuning highly ionized 35Cl were quantitatively analyzed in an attempt to understand their origin. Most of these ions can be explained in terms of multiple electron-capture and electron-loss reactions of the accelerating particle in the high-energy section of the accelerator. The fair agreement between the experimental yields of different spurious beams and the predictions of a theoretical model lends support to the hypothesis that such atomic collisions take place with molecules from the residual gas in the accelerator.

Introduction

Electrostatic tandem accelerators are designed to deliver monoenergetic beams of particles with a single charge state q after a magnetic rigidity selection by means of an analyzing magnet. However, many other spurious projectiles can be produced provided the correct magnetic rigidity, p/q, is reached (being p the linear momentum of the accelerated particle). Most of these spurious projectiles can be accounted for by atomic collisions that take place in the high-energy region of the accelerator (collisions with remaining gas molecules or slit scattering). In such a collision, the projectile may lose or capture several electrons and, depending on the exact place at which the collision occurs, the emerging ion may be accepted by the analyzing magnet. Therefore, in the absence of a velocity filter, these spurious ions may exit the accelerator thus contributing to a complicated pattern of background beams.

There are several sources of projectiles that might undergo the above mentioned atomic collisions. In the most usual case, spurious ions arise from collisions of the projectiles of interest, i.e. those corresponding to the tuned beam. An additional source of spurious ions, with masses and energies even farther apart from the selected projectile, arises from the injection of negative molecular ions and their break up into their component atoms at the stripper foil. Although these individual atomic components generally do not have the correct magnetic rigidity, some of them will also be able to get through the analyzing magnet as a consequence of a variation of their charge states. Finally, projectiles contributing to background beams may also originate in the injection of adjacent masses as a consequence of the acceptance of the injection magnet.

Background beams are usually much weaker than the desired beam and can therefore be ignored, as is typically the case in nuclear physics experiments. However, they can have a serious negative effect on measurements in which the beam of interest has an extremely low intensity thus requiring the use of particle detectors to record individual events, as in the case of many Accelerator Mass Spectrometry (AMS) experiments.

In particular, in this work we study a possible mechanism for the production of spurious beams via collisions of the projectiles with residual N2 molecules in the accelerator and we report new data on multiple electron stripping of highly ionized chlorine particles. The empirical values of the relative yields for the production of different background beams are compared with theoretical estimates based on an extension of the binary-encounter approximation [1], [2], [3] which takes into account the characteristics of the tandem accelerator.

Section snippets

Experimental considerations

The experiments have been carried out at the 20 UD tandem accelerator of the TANDAR Laboratory in Buenos Aires using beams of 35Cl accelerated to a 10 MV terminal voltage. The number of spurious ions is proportional to the pressure in the 17 m long high-energy accelerating tube, which is kept fairly constant along the accelerating path in the 10−8 Torr range by five ion pumps [4].

A detailed study of the charge-exchange processes requires the production of different ionic states emerging from

Analysis

The energy of an ion undergoing a charge exchange qiqf at a distance x from the stripper isEqf(x)=eVP+VT1+qixl+qf1−xl,where VP and VT are the pre-acceleration and terminal voltages, respectively, and l is the electrically active length of the high-energy side (l=12 m in our case). Isobaric ions with a final charge state qf and energy Eqf are accepted by the analyzing magnet if they fulfill the relation Eqf/q2f=Eo/q2o, where qo is the tuned charge state and Eo the corresponding energy. It

Theory

The binary encounter approximation (BEA), developed by Gryzinski and successfully used in describing similar data [1], [2], provides a simple model for the calculations of electron capture or loss cross sections in the cases of several atomic and ionic targets. In this scheme, the cross section for ionization of an electron is evaluated from an approximate expression for the two-body Coulomb scattering.

The cross section of a single ionization is the sum of the individual cross sections for the

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Cited by (0)

1

Present address: Centro Nacional de Aceleradores, Seville, Spain.

2

Present address: Biological Engineering Division, MIT, Cambridge, MA, USA.

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