Design and ®rst results of the nuclear microprobe in Cracow

S. Lebed a,b, M. Cholewa a,c, Z. Cioch a, B. Cle� d, P. Golonka a, D.N. Jamieson c,G.J.F. Legge c, S. èazarski a, A. Potempa a, C. Sarnecki a, Z. Stachura a,*

a Niewodnicza�nski Institute of Nuclear Physics, ul. Radzikowskiego 152, PL-31-342, Cracow, Polandb Institute of Applied Physics, Sumy, Ukraine

c School of Physics, MARC, University of Melbourne, Parkville 3052, Australiad Institute of Nuclear Physics, University of M�unster, Germany

Abstract

A new nuclear microprobe has been constructed at the Van de Graa� accelerator (HVEC, KN-3000) in the Institute

of Nuclear Physics in Cracow. The focusing system is based on two doublets of magnetic quadrupole lenses forming a

``Divided Russian Quadruplet'' geometry. Numerical calculations of the ion optics are published elsewhere [V. Brazh-

nik, S. Lebed, W. Kwiatek, Z. Stachura, M. Cholewa, D. Jamieson, G. Legge, Nucl. Instr. and Meth. B 130 (1997) 104].

In the present work details of the design are shown and the ®rst experimental results are compared with the theoretical

expectations. The advantage of the described construction is a short total length of the microprobe of 230 cm. A so

short microprobe may be very convenient in some application to biophysics where a vertical position of the beam line is

preferable. Also, because of the short length of the system, spherical and chromatic aberrations only slightly in¯uence

the ®nal resolution of the microprobe. Ó 1999 Elsevier Science B.V. All rights reserved.

Keywords: Microprobe; Divided Russian Quadruplet; Facilities; Scanning system

1. Introduction

The nuclear microprobe (MP) in Cracow isbased on a HVEC KN-3000 Van de Graa� accel-erator with a proton beam energy 6 3 MeV andan energy stability of DE/E � 10ÿ3. The focusingelements of the MP (two doublets of magneticquadrupole lenses) were manufactured in theMicro Analytical Research Centre (MARC).

To ®nd the optimum performance of the setup,a series of ion optics calculations was performed.The results are presented in Ref. [1]. Carefulanalysis of the optimising calculations revealed apossibility to construct a compact microprobe withits total length below 2.5 m, allowing a submicronresolution. The expected current for a beam spotsize of 2 lm is su�cient for diagnostics by PIXE,RBS and other analytical methods (see Table 1).The spherical and chromatic aberration coe�-cients are about 10 times larger than those calcu-lated for a conventional MP composed of a closelyspaced quadruplet. However, due to the shortlength of the focusing system, the aberrations only

Nuclear Instruments and Methods in Physics Research B 158 (1999) 44±47

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* Corresponding author. Tel.: +48-12-6-37-02-22; fax: +48-

12-6-37-18-81; e-mail: stachura@alf.ifj.edu.pl

0168-583X/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 3 8 7 - 0

slightly in¯uence the ®nal resolution of the system.The microprobe was manufactured in the me-chanical workshop of the Institute of NuclearPhysics in Cracow.

2. Details of the construction

The total length of the microprobe focusingsystem is 230 cm, the object distance is 118 cm andthe demagni®cation factor 17.7 for working dis-tance of 15 cm. The charged particle beam of theaccelerator is delivered to the object slit of themicroprobe with the help of a dipole magneticsteerer, a double focusing bending magnet and adoublet of magnetic quadrupole lenses. The mag-netic ®eld of the bending magnet is stabilized usinga power supply equipped with a Hall probe. Thestability of the magnetic ®eld is better than 10ÿ5.The momentum of charged particles beam is sta-bilized by a feedback loop to the corona dischargecurrent. The error signal for the feedback loop istaken from the slit edges located at a focus afterthe bending magnet or alternatively from slitsplaced before the object slit of the MP.

The high resolution of the MP requires an ap-propriate damping of mechanical vibrations. Themain source of vibration are mechanical pumps inthe experimental hall. To reduce vibrations, themicroprobe is mounted on a heavy concrete blockplaced in a sand bath. To this block a heavy steeltable is fastened. The elements of the MP line arefastened to the table through hard rubber attenu-ators. This construction damps vibrations by afactor of 10 (comparing to the neighbouring beamlines), as measured by the microphonic e�ect. The

vacuum tubes of the MP are connected to thepumps and to the accelerator vacuum system bytwo neoprene sleeves in series at each connection.The dumping of vibrations by these sleeves is alsobetter than 10.

In Fig. 1 a schematic view of the microprobe isshown. The legs of the table (1) are fastened tothe anti-vibration concrete block (not shown in the®gure). Following a gate valve (2) separating theMP from the accelerator vacuum system slit edgesare mounted (3) for the energy stabilization feed-back loop. A water-cooled circular aperture (4)

Fig. 1. Schematic view of the microprobe. For details see the

text.

Table 1

Expected resolution and beam current on the target

Object aperture (lm) 20.0 10.0 5.0

Angular aperture (lm) 200.0 50.0 25.0

Beam size on the target, (lm2) for 0.1% parasitic

aberrations

1.3 ´ 1.2 0.6 ´ 0.6 0.32 ´ 0.30

Beam size on the target, (lm2) for 1% parasitic

aberrations

6.0 ´ 1.3 0.8 ´ 0.6 0.35 ´ 0.30

Beam current at the target, It (pA) assuming the

normalized brightness of the RF source equal to

10 A/m2/rad2/eV

P 100 P 1 P 0.25

S. Lebed et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 44±47 45

lets only the central part of the beam pass to theobject slit (5). Following the experience of theMARC group, we use as the object slit a com-mercial diaphragm made for a scanning electronmicroscope. Several such diaphragms are mountedon a linear feed-through rod equipped with mic-rometer screws. In the same way an angular col-limator (6) is constructed. Two doublets ofquadrupole lenses (7) are mounted on separatesupports after the angular collimator. A two-di-mensional scanning system (10) is mounted be-tween the last doublet of the quadrupole lensesand the target chamber (11). In several positionsalong the ion beam path boxes are located en-abling a beam diagnosis. Each box is equippedwith a Faraday cup and a quartz or bismuth ger-manate (BGO) plate. In particular, such a box islocated at the expected position of the beam cross-over (9) between the doublets of the quadrupolelenses. At the end of the MP line a target chamber(11) is mounted, equipped with a rotatable x±y±zmicro-manipulator (14) that is used as a targetholder. The target chamber is surrounded by sev-eral detectors (13). The target position is observedby a CCD camera equipped with a microscope lens(12). The position of each element of the focusing

system (the object slit and the angular collimator,the doublets of quadrupole lenses and the targetchamber) as well as their angles of inclination withrespect to the beam axis are adjustable by highprecision screws.

3. Scanning system

A magnetic scanning system of the MP for eachscanned direction is composed of coils wound on apair of ferrite rings with a hole in the central partbetween them (see Fig. 2). The ion beam tube ismounted within this hole (B� 11 mm). With acurrent of 5 A this system de¯ects the 2.5 MeVprotons by 0.5 mm at the target position. Detailsof the scanning system design are described in [2].

4. Ion beam trajectory

There are two modes of focusing the beam atthe target position for a ``Divided Russian Qua-druplet'' geometry. At smaller excitations ofquadrupole lenses the object slit is imaged at thetarget with a demagni®cation factor of D�ÿ1.5

Fig. 2. The scanning coils.

46 S. Lebed et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 44±47

only. At larger excitation values an image withD� 17.7 is formed. The solution with the larger Dvalue is characterized by a beam cross-over at thebeam axis between the quadrupole doublets. In theposition of the expected cross-over a diagnosticbox is mounted (see Fig. 1), allowing to quickly®nd the right position of the quadrupole lenses andthe correct mode of focusing.

5. Expected resolution of the microprobe

Expected values of the resolution and of thebeam current at the target are presented in Table 1.In the calculations the chromatic and sphericalaberrations were taken into account as well as twovalues of parasitic, sextupole components in themagnetic ®eld of the quadrupole lenses of 1% or

0.1% of the quadrupole ®eld were included. Thebeam current at the target position was estimatedassuming that the normalized brightness of the RFsource could be 10 A/m2/rad2/eV.

The proton beam current (Ep� 1.5 MeV) ac-tually measured at the target position measuredafter the 25 lm object aperture was It� 0.11 nA,and thus similar to the expected value shown inTable 1.

References

[1] V. Brazhnik, S. Lebed, W. Kwiatek, Z. Stachura, M.

Cholewa, D. Jamieson, G. Legge, Nucl. Instr. and Meth. B

130 (1997) 104.

[2] V. Khomenko, S. Lebed, S. Mordik, Nucl. Instr. and Meth.

B 130 (1997) 86.

S. Lebed et al. / Nucl. Instr. and Meth. in Phys. Res. B 158 (1999) 44±47 47