Nuclear targets, recoil ion catchers and reaction chambers

Nuclear Instruments and Methods in Physics Research A 414 (1998) 239—260

Nuclear targets, recoil ion catchers and reaction chambers J.S. Dionisio!,*, Ch. Vieu!, C. Schu¨ck!, R. Collatz!, R. Meunier!, D. Ledu!, H. Folger", A. Lafoux#, J.M. Lagrange#, M. Pautrat#, B. Waast#, W.R. Phillips$, D. Blunt$, J.L. Durell$, B.J. Varley$, P.G. Dagnall$, S.J. Dorning$, M.A. Jones$, A.G. Smith$, J.C.S. Bacelar%, W. Urban&, T. Rzaca-Urban&, N. Amzal', Z. Me´liani', J. Vanhorenbeeck), A. Passoja* ! CSNSM, IN2P3-CNRS, 91405 Orsay Campus, France " GSI, 1 Planckstrass, 6100 Darmstadt, Germany # IPN, B.P. 1, 91406 Orsay Cedex, France $ Department of Physics and Astronomy, University of Manchester, Manchester M139PL, UK % KVI, University of Groningen, 9747 AA Groningen, The Netherlands & Institute of Experimental Physics, Warsaw University, Warsaw, Poland ' Institut of Physics, USTHB, ab Ezzouar, B.P. 32 El-Alia, Alger, Algeria ) ULB, Brussels, Belgium * University of Joensuu, Joensuu, Finland Received 2 March 1998

Abstract The main features of nuclear targets, recoil ion catchers and reaction chambers used in nuclear spectroscopic investigations involving in-beam multi-e-c spectrometers are discussed. The relative importance of the d-ray background due to the accelerated ion—target and the recoil-ion—target interaction is estimated. Its impact on the prompt low-energy electron measurements is stressed. Finally a few general features of the interplay between accelerated ion beams, targets and recoil ion catchers particularly relevant for these measurements are broadly discussed and illustrated with typical examples of in-beam e-c studies. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 29.25.Pj; 29.30.Dn Keywords: Nuclear targets; Recoil ions; Electron spectrometers

1. Introduction Some particular features of the nuclear targets, recoil ion catchers and reaction chambers coupled * Corresponding author. Fax: #33 1 691 55008; e-mail: [email protected]

to the in-beam e-c spectrometer operating in the experimental area of the MP tandem accelerator of IPN (Orsay) were described earlier (see Refs. [1—4] and references quoted therein). These studies included a detailed description of the main principles and applications of the recoil ion catcher method as well as the Doppler broadening of (prompt) IC

0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 4 8 3 - 5

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electron lines. However, they were restricted to the simplest e-c configurations involving very few Ge detectors and their AC shields (i.e. one or two detectors used in the earlier studies quoted in Refs. [1—3] and four Ge detectors reported in Ref. [4]). Furthermore, the increasing importance of e-c coincidence measurements required the use of more complex and larger detector arrangements surrounding the reaction chamber containing the target, the recoil ion catcher as well as their holders. The aim of the present work is to describe the main features of such devices coupled to any inbeam multi-e-c spectrometer suitable for the investigation of prompt and delayed low-energy transitions (i.e. with E (100 keV). In particular, % the nuclear targets, target holders and target chambers of such devices are briefly surveyed, respectively, in Sections 2—4 of this work. Similarly, the recoil ions, recoil catchers and their holders are broadly discussed in Section 5. An important feature of in-beam low-energy electron studies is the ratio of the two main components of the d-ray background due to the accelerated ion—target and recoil ion—target interaction. A rough estimate of that ratio (see Section 5.1) illustrates its importance while the main characteristics recoil catcher holders are condensed in Section 5.2. The functional relationship between target holder, recoil ion catcher, target chamber, accelerated ion beam line, electron spectrometer and their accessories is discussed (Section 6) in the context of different kinds of nuclear spectroscopic applications (Section 7). Several conclusions are drawn from this discussion (Section 8).

2. Targets and backings The main features and specifications of the targets used in e-c experiments, the optimum target thickness, the different types of target backings and protecting layers as well as the role of targets and target backing structure in such experiments are, respectively, described in Sections 2.1—2.4. 2.1. Target main features and specifications The main characteristics of the targets required by in-beam e-c experiments are the shape and the

smoothness of their boundaries (usually flat surfaces in the case of solid targets without holes and protuberances); their dimensions (i.e. size of their transversal cross-section and thickness of the mass layer between the target boundaries); the mass distribution (i.e. local and average densities); their position and orientation towards the accelerated ion beam as well as e-c spectrometer, the physicochemical stability of the target material (i.e. small oxidation, thermal diffusion, lack of amalgation with the target backing) during an experiment (or many experiments) under well-defined experimental conditions and the homogeneity of that material (in particular, their isotopic and chemical purity). The target specifications for nuclear spectroscopic measurements depend on the type of measurement and the characteristics of the radiations to be analysed (see Refs. [5—16], respectively, for earlier reviews or typical examples of targets and radioactive samples used in off-line and inbeam measurements). In particular, the electronand positron-measurements are much more sensitive to the target properties than the c-ray measurements. A similar relationship links the sensitivity of these measurements to the spatial distributions of the accelerated ion beams striking the targets. That sensitivity is very high for several types of electron measurements (such as those using the recoil ion catcher method) while for most c-ray measurements is less important (except when dealing with the Doppler broadening and hyperfine measurements). In such conditions, it is not surprising that most reviews concerning in-beam c-ray spectroscopy — including those dealing with multidetector arrays — omit general discussions (as well as practical considerations) concerning targets and accelerated ion beam properties and concentrate on the performances of the detector arrangements (see, for instance, Ref. [17]). Indeed these arrays are usually considered to be of primary importance for most in-beam c-ray measurements while the refined characteristics of the targets and ion beams are less important for such measurements than for in-beam electron measurements. This is true whenever they are available with a minimum purity and intensity (usually lower than is required for in-beam electron measurements).

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2.2. The optimum target thickness for e-c experiments The optimum thickness of a target used in an in-beam e-c experiment depends on several factors. The most important are the following: (i) the variation of the nuclear cross-section with the kinetic energy of the accelerated ions inducing that reaction; (ii) the accelerated ion slowing down inside the target before the occurrence of a nuclear reaction; (iii) the recoil ion scattering and slowing down inside the target before the e-c emission; (iv) the electron energy decrease and fluctuations occurring during the electron crossing of the effective target thickness (dependent on the relative target and electron spectrometer orientation); (v) the maximum electron line broadening accepted by the required accuracy of the planned measurements. The simultaneous consideration of all these factors in the planning of an e-c experiments is not easy and very much dependent on the particular type of experiment. In such conditions, it is easier to restrict that discussion to the general aspects common to a large number of nuclear experiments of that kind. In particular, the large nuclear cross-sections of light-ion-induced reactions allow the use of thinner targets in this kind of reaction than in heavy-ioninduced reactions. Indeed, the ratio between the maximum cross-sections in such cases can reach a factor three (or four) in medium mass nuclei (see, for instance, the examples quoted in Ref. [4]). Furthermore, the same argument can also be used for the study of complex low-energy electron spectra de-exciting low spin states of odd A and odd—odd heavy nuclei. The study of low-energy in-beam electron spectra de-exciting nuclear high spin states is outside the scope of light-ion-induced reactions and restricts the use of thin self-supported (or backed) targets to the investigation of prompt radiations emitted by very short lived nuclear states. Indeed, the radiations emitted by recoil ions in flight cannot be analysed and detected by the electron spectrometer unless they are stopped inside the target, its backing, or nearby (within a recoil ion catcher foil). Thus the study of prompt radiations de-exciting nuclear high spin states requires thick self-

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supported targets as well as thin backed ones. Furthermore, the high complexity of the nuclear structure of medium and heavy nuclei plus the occurrence of a wide variety of nuclear multiplets and isomeric states requires refined spectroscopic methods for their detailed investigation. The target thickness in all these experiments depends considerably on the energy range of the planned measurements. For electron measurements higher than 500 keV targets in the range of 0.7— 1.0 mg/cm2 are normally used. Between 500 and 100 keV the most favourable target thickness lies between 0.3 and 0.7 mg/cm2. Below 100 keV, the stronger scattering and absorption of electrons suggests the use of still thinner targets, i.e. in the thickness range 0.1—0.3 mg/cm2. These broad indications concerning the different range of target thickness suitable for in-beam electron measurements are coherent with previous investigations concerning the influence of the target thickness on the peak-to-background ratio of several measurements [2]. Indeed, this variation was especially investigated in different measurements performed with lanthanides covering the energy range below 500 keV (see Refs. [2—4]). A sharp decrease of peak-to-background ratio was observed in these measurements below 100 keV. This sharp decrease is mainly due to the stronger electron line broadening with the target thickness in the low-energy region (see Ref. [3]). However, a thinner target reduces the rate of production of nuclei in atomic collisions (assuming a constant ion beam intensity) as well as the residual recoil ion energy loss. Consequently, the most suitable target thickness is roughly a compromise between two opposite tendencies: (i) to use thinner targets in order to improve the electron line resolution (assuming a larger intrinsic resolution of the electron detector and a smaller contribution due to the Doppler broadening); (ii) to irradiate thicker targets (below the saturation thickness set by the corresponding cross-section) in order to increase the nuclear production rate. Nevertheless, it is important to be aware that the real situation is more complex than the equilibrium between these opposite requirements. Indeed, there are other factors — like the d-ray production rate and the electron line broadening which depend on the effective

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target thickness, i.e. the target and the electron spectrometer orientations relative to the accelerated ion beam axis. Furthermore, there is the following major difference between the d-ray emission and a nuclear reaction (induced by an accelerated ion beam bombarding a given target): (i) The same accelerated ion produces many drays during its interaction with the different atoms of the target material (or its backing). Furthermore, d-ray emission only ends when that ion is completely stopped — or leaves the target in flight and travels through the vacuum without further atomic collisions. (ii) Conversely, the same accelerated ion gives rise to a single-compound nucleus reaction during its interaction with the target atoms and no secondary particles identical to the accelerated primary ones are usually emitted during that nuclear reaction which are able to induce a chain reaction. The spallation reactions (p, xn#yp) partially contradict the latter statement because they can induce secondary nuclear reactions. However, they cannot give rise to a chain of nuclear spallation reactions (like a sequential electron emission in the d-ray production) because the kinetic energy of secondary protons is much lower than the kinetic energy of the primary protons. More generally, the low probability of nuclear chain reactions induced by accelerated ions makes a major difference with neutron-induced reactions. Another important difference between d-ray emission and nuclear reaction production by accelerated ions is the following: the d-ray emission arises not only in the interaction between the incoming ions and the target atoms but also in the collisions between the recoil ions and the target atoms which slow them down to rest (or to thermal motion). This means that the accelerated ions bombarding a target and the recoil ions issued from their interactions with the target both contribute to the d-ray production (see Section 5.1). 2.3. Target backings and protecting layers The mechanical strength of very thin metallic layers (surface density, m/s4200 lg/cm2) prepared by vacuum evaporation or ion collection in an EM

isotope separator on a suitable thin backing (m/s430—40 lg/cm2) is usually insufficient to allow the removal of that layer from the backing without its destruction. In other cases, the fast chemical reactivity of the metal (i.e. the fast oxidation of most rare earth and actinides) prevents the handling of thin metallic foils uncovered with protecting layers (except under vacuum). In all these cases thin backed targets are practically unavoidable either to increase the mechanical strength or to prevent the fast oxidation of thin metallic layers. Moreover, there are several reasons to advise the use of thin and light backings whenever the recoil ion stopping — or slowing down — is not essential in the planned e-c measurements. The following are most important: (i) The target backing thickness increases the d-ray background especially when it is made of heavy elements; (ii) the backing material bombarded by fast accelerated ions above the Coulomb barrier induces spurious nuclear reactions and increases the e-c background. Unfortunately, the simultaneous fulfillment of both conditions is somewhat contradictory because thin backings are less resistant and more porous than thick ones while light backings have a much lower Coulomb barrier than heavy backings. Consequently, a compromise must be found to fulfil these contradictory requirements. Thin carbon foils (m/s&30—40 lg/cm2) are usually preferred as standard backings of nuclear targets whenever high mechanical strength and few spurious reactions are required simultaneously. Indeed, Be and Al backings give a much stronger nuclear background than C. Moreover, the fact that carbon is a good electrical and thermal conductor is also favourable for this type of application — especially for highly concentrated layers of very rare isotopes prepared by direct collection on an EM isotope separator. It is also important to remember that electron backscattering is particularly disturbing for low-energy electron measurements (i.e. E 420 keV) which favours carbon compared % with Al (or Cu). A good illustration of the crucial role of electron backscattering in low-energy electron spectra is shown by a hafnium-178 m very thin layer collected in a thin Cu foil placed on the ion collector of an EM isotope separator [18,19].

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Indeed, the lack of continuous b-spectra (as well as d-rays) in this standard electron source puts clearly in evidence the backscattered tail of the main IC lines of this radioisotope down to 5—8 keV. Another example of the same kind is given by the low-energy internal conversion and L Auger spectra of highquality standard sources of the active deposit of thoron and radon as well as by high specific activity samples of other radionuclides prepared by direct ion collection techniques in EM isotope separators (see the earlier review of the Auger effect [20] and more detailed descriptions of L Auger spectra obtained with radioactive samples [21—23]). Thin aluminium layers backed on glass lamels and selfsupporting aluminium foils were mostly used in such cases (as well as a few others involving lowenergy electrons [24—27]). Very thin backings of aluminized plastic foils were also successfully used in many investigations concerning the spectral shape of b$ continuous spectra [28]. However, most of these plastic backings are unsuitable for in-beam experiments on account of their spurious nuclear background and their poor performance under the impact of accelerated ions. Thus the improvement of carbon backings in a wide range of foil thickness is a crucial step for the progress of low-energy in-beam electron measurements both for prompt and delayed transitions. 2.4. The role of the target and backing structure The target and backing structure (i.e. their atomic composition, effective density and thickness) play a very important role in the Doppler shift attenuation method as well as in most spectroscopic methods involving recoil ions. In particular, the recoil ion stopping time versus the target thickness as well as the evolution of the mean recoil ion penetration through different backings and the corresponding evolution of the recoil ion velocity distributions in single unbacked and backed targets were evaluated earlier for 30 MeV terbium-147 (see Ref. [3] and the results obtained in that work with the code of Biersack and Ziegler [29]). According to those results the mean recoil ion penetration of such ions in different backings is much larger for light elements (Be, Al, C) than heavy elements (Ta, Au, Pt). Indeed, one has 5—7 lm, in the first case

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and 2.5—3.0 lm in the second case (see Fig. 4 of Ref. [3]). The slowing down times of the same ions in those backings were derived from those numerical results (i.e. t"2.67 ps for Be and t"0.72 ps for Pt). Similar results were obtained in Ref. [3] with DSAM computer code [30] applied to terbium-147 recoil ions produced by 140 MeV 31P accelerated ions bombarding isotopically enriched samples of 120Sn (350 lg/cm2) backed on carbon (m/s" 1.31 mg/cm2) and gold (m/s"5.16 mg/cm2). The penetration difference of these recoil ions is not very large for a very short slowing down time (t(0.2 ps) but the carbon backing is more favourable for low-energy electron measurements than the gold backing. Indeed, the gold backing gives a much stronger d-ray background, particularly disturbing in the low-energy region (E ( % 100 keV). For higher electron energies the influence of that background is smaller but the nuclear background due to the accelerated ion—target backing interaction is more disturbing in that energy domain. Similar calculations with both computer codes were performed for the penetration of radium-219 recoil ions produced in lead-208 targets (m/s" 350 lg/cm2) supported by carbon (m/s"230 lg/ cm2) and gold (m/s"606 lg/cm2) backings (see Fig.5 of Ref. [3]). The d-ray background reduction is very important for heavy ion as well as light-ion-induced reactions whenever low-energy electron measurements are planned or required. In the former type of nuclear reactions the d-ray background is stronger than that for light-ion-induced reactions. However, the c-ray multiplicity is large in that case and its evaluation is a powerful method of discriminating between atomic (i.e. d-ray emission) and nuclear reactions (c-rays and IC electrons) leading to this multiple event. Another way to improve the quality of the electron spectra is to adjust the target (or target plus backing) thickness to the range of the recoil ions and orient the rear face of the target (or target plus backing outer face stopping the recoil ions) towards the electron spectrometer. In such a way the effective electron source thickness will be minimized and the main disturbing effect will be electron backscattering in the target (or target plus backing).

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This possibility is particularly favourable in the forward collinear geometry because the target can be easily oriented perpendicularly — or slightly tilted — to the accelerated ion beam axis. The same procedure is also currently used in the electron measurements made at 90" of the accelerated ion beam axis. The evolution of the recoil ion reduced velocity (b "v /c) distributions corresponding to the same 3 3 recoil ions (i.e. terbium-147 and radium-219 produced in nuclear reactions induced by 31P and 14C accelerated beams) during their slowing down in carbon and gold backings was evaluated with the same computer code [30] for the collinear as well as the perpendicular orientation (see Fig.7 and Fig. 8 of Ref. [3]). The main features of these reduced ion velocity distributions at a given instant of the recoil ion slowing down process in those backing foils can be summarized in the following items: (i) the projection of the recoil ion speed distributions perpendicular to the initial ion beam direction is roughly symmetrical (relatively to the initial ion beam input value of the speed used in those calculations); (ii) the width of the recoil ion speed distributions decreased with the recoil ion slowing down process while the input recoil ion velocity peak was smoothed out during the same process; (iii) the projections of the recoil ion speed distributions collinear with the initial ion beam direction were sharply asymmetrical on the low energy side of the recoil ion input speed. These general features of single backed targets illustrate the complexity of the recoil ion slowing down process and stimulate more refined developments towards the study of nuclear isomerism with e-c measurements. Indeed, the situation is still more complex if one wants to take advantage of a multiple target over a single one for in-beam e-c measurements concerning nuclear high spin spectroscopy [31]. However, the same advantage is not so clear for low-energy electron measurements (see the discussion developed in Ref. [3]). For that reason multiple targets are not considered in the present context. Instead, it is important to consider the relative thickness of the target and its backing. In particular, the backing thickness is the smallest possible (i.e. 20% or less than the target thickness) whenever the role of the backing is only a mechan-

ical support or a protecting layer. However, if the backing role is a recoil ion stopper then the backing thickness can be much larger than the target thickness (one order of magnitude or more). This fact leads to a natural classification of targets and backings (see Section 6).

3. Target holders The main role of the target holders, the importance of Compton scattering and ion beam instabilities on their dimensions and orientation as well as their general characteristics are, respectively, discussed in Sections 3.1—3.3. 3.1. Main role of target holders A target holder is essentially a mechanical support with a secondary role in many nuclear spectroscopic experiments. Indeed, the role of such devices is not critical in most c-ray experiments unless scattering events are important like in escape suppression shields or the target position must be very accurately known (as in the plunger method discussed in Refs. [32,33]. However, their influence for low and medium energy electron spectroscopy experiments is more important on account of the strong electron background due to Compton scattering of c-rays emitted by the target on their holders. 3.2. Compton scattering and ion beam instabilities The attenuation of the Compton effect requires target holders with large dimensions and small thicknesses in order to decrease the solid angle of the target holder subtended by the target centre (assumed in the middle of the accelerated ion beam impact with the target). Furthermore, the Compton scattering also depends on the target holder material and the target holder orientation relative to the electron spectrometer entrance aperture. Moreover, this background can be attenuated by proper shields of the electron detector against electron emission due to spurious sources (like photo-electron emission). These different types of spurious effects were investigated earlier [34,35] with electron source

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holders having several sizes and shapes suitable for axial magnetic lenses of the triangular’s type operated off-beam in the classical step-by-step differential mode. According to these investigations a thin electron source holder (made of Al and with a radius of 5 mm) oriented perpendicularly to the magnetic lens axis gives a weaker Compton background than smaller holders made with heavier materials oriented in the same way. It is important to stress that the target orientation is a useful variable parameter for in-beam measurements (which is usually neglected in off-line measurements). Indeed, by changing the target orientation it is possible to gradually vary the effective target thickness as well as the rate of production of radioactive nuclei. Furthermore, the recoil ion stopping inside a target can also be changed in the same way. However, this change of orientation has a deep effect on the amount of Compton scattering. Indeed, a target oriented perpendicularly to the magnetic lens axis is a suitable configuration for off-beam measurements but unsuitable for low-energy electron in-beam measurements (assuming the magnetic lens oriented perpendicularly to the ion beam bombarding flat targets lying in the ion beam axis). The perpendicular orientation of the target towards the accelerated ion beam axis is good for in-beam electron collinear measurements but inconvenient for in-beam perpendicular measurements because the electron source thickness around the magnetic lens is nearly maximum in the latter case. Consequently, the usual target orientation for in-beam electron measurements is tilted both to the accelerated ion-beam and the magnetic lens axis. However, this orientation gives rise to a larger Compton scattering than the perpendicular one used for off-beam measurements. Furthermore, e-c and e—e coincidence measurements require a large angular aperture of the electron spectrometer. For that reason, the distance between the target and the magnetic lens aperture should be reduced in order to increase the transmission of the spectrometer. Of course, this improvement also increases the amount of Compton scattering (unless thinner and wider target holders are used) as well as the Doppler broadening (see Ref. [4] for a general formulation). There is another physical argument that favours the use of large target holders for in-beam electron

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measurements. This argument is founded on the frequent occurrence of small ion beam instabilities shifting the beam position on the target from the normal focused position by a few mm. In such cases, the target holder effective dimensions are extremely important. If they are rather small the ion beam occasionally hits the target frame and produces many spurious radiations completely spoiling the electron (and c-ray) spectra. However, if the effective target holder opening is larger than the ion-beam shifted distance (from the centrum of the target frame) there will be no serious perturbation on the c-ray spectra and very little (if any) on the electron spectrum measured by a magnetic energy selector. 3.3. Target holder main characteristics The main characteristics (shape, dimensions, orientation, chemical composition and physical status) of the target holders used for in-beam electron measurements — as well as their main advantages and drawbacks — have been condensed in Table 1. The physical status of these devices can be particularly important whenever low temperatures, high accelerating or retarding electric fields, weak (or strong) magnetic fields are involved in special measurements.

4. Target chambers The main role and characteristics of target chambers coupled to in-beam e-c spectrometers are, respectively, reviewed in Section 4.1 and Section 4.2. 4.1. The target chambers main role The target chambers of nuclear spectrometers operating in-beam with atomic particle accelerators are intermediate devices between those instruments and the accelerator beam line (see Ref. [8] for a general survey on target chambers of particle accelerators). Their role has essentially a triple aim: (i) to make easier the performance of in-beam experiments, (ii) to allow a fast insertion and removal of nuclear targets kept under vacuum inside that chamber, (iii) to install radiation

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Table 1 Target holder characteristics (shape, dimensions, orientation, chemical composition and physical status) 1. Flat metallic disk with an inner circular hole z Main advantage: Simplest target shape suitable for spectroscopic measurements. 2. Target orientation fixed or changeable under vacuum orientable at 90° collinear electron to the ion beam axis for orientable at 30—60° perpendicular measurements z Main advantages: — small Doppler broadening — large target effective thickness for orientation O 90°. 3. Effective target diameter: larger than accelerated ion beam halo z Main advantage: Avoid beam-target holder interactions z Main drawback: A large thin self-supporting target is normally more fragile than a small identical target with the same thickness. z Other restrictions: (i) dimensions smaller than available space in the e-c spectrometer source region are needed, (ii) thermal insulation of cryogenic targets is indispensable, (iii) electrical insulation of HV targets is necessary. 4. Target holder thickness: the thinnest possible with good mechanical strength z Main advantage: Attenuate the Compton background due to X and c ray interaction with the target holder frame (by reducing the solid angle subtended by this frame from the irradiated zone of the target). Indeed a weak spurious ion beam hitting a thick target holder disturbs the spectra emitted by a strong ion beam striking a very thin target. 5. Target holder chemical structure: light non-magnetic metals z Main advantage: Reduce the photoelectric emission and to avoid electrical charge accumulation on the target as well as spurious magnetic fields. 6. Target holder physical status (a) kept at room temperature or cooled to low temperatures, (b) kept at earth electrical potential or at positive (or negative) high voltage, (c) kept at earth magnetic field or the stray field of a magnetic electron spectrometer. z Main advantages: (a) Increase the target density whenever required (especially for volatile products with small nuclear cross-sections) by solidifying liquid and gas target materials. (b) Prevent pile up effects due to a large number of low-energy d-electrons. (c) Reduce the number of electrons backscattered in the target reaching the electron detector by using the magnetic mirror effect. 7. Target holder coupled (or uncoupled) to recoil ion catcher holder z Main advantages: Using the same carrier for the target holder and recoil ion catcher holder. Allowing their insertion and extraction through the same vacuum lock.

G

H

G

detectors (or beam diagnostic devices) near by the targets. Furthermore, the insertion of a target chamber coupling the beam line to the nuclear spectrometer allows a fast insulation between the vacuum chamber of that spectrometer and accelerator beam line which is essential for the protection of their most delicate components (i.e. accelerator tube and cooled radiation detectors unprotected by windows). Finally, the target chamber has usually several thin windows allowing the passage of soft (X, c) radiations emitted by the irradiated target before their detection by one or more individual detectors. There is a wide variety of target chambers fulfilling the previous general requirements and

HG

few others specific for different experimental purposes. 4.2. Main characteristics Table 2 recalls the main characteristics of target chambers coupled to e-c-spectrometers. In particular, their dependence on the electron optical properties and the geometrical configuration of each type of electron spectrometer is shown. However, for simplicity, nothing is mentioned concerning the insertion and operation of recoil ion catchers — which are required in many recoil ion experiments. Consequently, the design of the target chambers must take into account the configuration of

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Table 2 Target chamber characteristics (shape, dimensions, orientation and composition) 1. Tubular chambers are the most compact target chambers suitable for in-beam multi-e-c experiments z Main advantages: Simple mechanical coupling between the beam line and the electron spectrometer entrance aperture (when the electron source lies outside the electron spectrometer chamber). This is the case of toroidal spectrometers, sector magnets and magnetic lenses. Target holder carriers can easily be slid through tubular chambers. z Main drawbacks: The available space inside a tubular chamber is rather small especially for double solenoidal spectrometers. 2. Transversal cross-section of the target chamber fulfilling a double condition: (i) larger than maximum cross-section of the accelerated ion beam, (ii) compatible with the available space in the source region of the electron spectrometer. z Main advantage: Reduce amount of spurious radiation. z Main limitations: The fulfilment of these conditions is difficult for double solenoidal spectrometers and eB pair spectrometers with large transmission. 3. Thin target chamber walls compatible with required mechanical strength and vacuum tightness z Main advantage: Attenuation of spurious radiations and avoidance of mechanical (or vacuum) failures. 4. Miniaturized target holder and recoil ion catcher carrier movable under vacuum z Main possibilities: The accuracy of the mechanism positioning the target and (or) the recoil ion catcher depends on the type of measurement (RCM, RSM). The target holder can also be used for transporting a scintillation detector to visualize the beam impact. 5. Target chamber made of an Al tube with squared cross-section z Main advantage: Simple arrangement for an electron spectrometer with a small entrance aperture of the electron beam. 6. Thin perspex window fitting the target chamber wall allowing the visualization of the ion beam impact on the target position. This transparent window is covered by a thin graphite cover for in-beam measurements of electrons and photons (including soft X- and c-rays) z Main advantage: Beam visualisation as well as detection of soft X-, c-radiation especially in delayed measurements where strong prompt radiations can be greatly attenuated.

these devices (see Section 5) and their important role in such e-c in-beam experiments.

of the recoil ion catcher holders suitable for inbeam e-c experiments are evaluated and discussed.

5. Recoil ions, recoil catchers and their holders

5.1. d-ray emission from accelerated ions versus recoil ions

The interaction between fast ions and target atoms gives rise to recoil ions including — or not — compound nuclei emitted in flight. Whenever the EM transitions emitted by these nuclei should be investigated at rest (or nearly) recoil ion catchers (or stoppers) held by catcher holders must be placed near the target holders supporting the thin targets where the recoil ions are produced. A detailed description of the recoil ion catcher method coupled to in-beam e-c spectrometers was given earlier [1]. A few typical applications of this method to the study of nuclear isomerism in several medium heavy nuclei were also described elsewhere [1,36—38]. In Sections 5.1 and 5.2 the d-ray emission ratio from accelerated ions and recoil ions target interaction as well as the main characteristics

5.1.1. Ratios of non-relativistic total cross-sections A simple way of making a rough estimate of the relative importance of the d-ray emission due to the interaction between the accelerated and recoil ions with target atoms is to apply the non-relativistic total differential cross-section of d-ray emission (derived by Huus et al. [39]) to both interactions. Indeed, assuming that (Z A E ) are the character1 1 1 istics of the accelerated ion hitting the target (Z ,A ) 5 5 lying at rest in the LAB system and (Z "Z # # 1 Z , A "A #A , E ) are the corresponding para5 # 1 5 # meters of the recoil ions generated by this nuclear collision one obtains the following relations:

A B

E 4 e2m c2 0 dE , dp "10~18 Z2 1 Z4 15 1 A 5 E9 d d 1

(1)

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A B

E 4 e2m c2 0 dE , dp "10~18 Z2 # Z4 5 E9 # A #5 d d #

(2)

where E is the kinetic energy of the d-electron d emitted in both interactions (i.e. accelerated ion and recoil ion—target interactions) and dE the differend tial of the corresponding energy spectrum. The most striking feature of these non-relativistic cross-sections (1,2) is their very fast variation with the kinetic energy of the d-electrons in the lowenergy domain (i.e. when E ;m c2). Indeed, this d 0 variation is much faster than the variation of the internal conversion coefficients (ICC) with the energy of the corresponding c-ray which is roughly as (kr)~L~1 where r is the radial distance between the nucleus (behaving as 2L multipole oscillator emitting the c-ray) and the electron of its atomic cloud interacting with that c-ray (see Ref. [40]). In the particular case of a heavy atom (Z"81), the variation of the ICC is roughly k~2 for ¸"1 and k~3 for ¸"2 which is much smaller than the k~9 variation of the non-relativistic cross-section predicted by Huus et al. [39] for the d-ray emission. Furthermore, the experimental shape of the d-ray continuous spectra is not so sharp as that of the theoretical prediction. However, in spite of that quantitative disagreement the simplicity of the relations (1,2) is particularly attractive and suggests the comparison of similar features of the d-ray spectra emitted by the accelerated ion and the recoil ion—target interactions assuming the same kinetic energy for both d-electrons and the validity of the semi-classical model restricted to monopole excitations (i.e. 0P0 transitions, see Refs. [41,42]). This leads to the relation,

A BA B BA BA B

dp Z2 E 4 A 4 # R " 15" 1 1 5 dp E Z2 A #5 # 1 #

A

+

2 A #A 4 E 4 Z 1 1 5 1 , Z #Z A E 1 5 1 #

(3)

where R "dp /dp is the ratio of the total differ5 15 #5 ential cross-sections of d-ray production of the accelerated and recoil ions crossing the same target (given by the relations (1) and (2), respectively) and the d-electrons emitted in both interactions (i.e. particle—target and recoil ion—target) have the same

kinetic energy. The last relation Eq. (3) can be simplified by introducing the relationship between E E A and A . Indeed E /E "A /A &(A #A )/ 1 # 1 5 1 # # 1 1 5 A leads to 1 2 Z A 8 1 R+ 1# 5 . (4) 5 Z #Z A 1 5 1 If a light-ion hits a medium or heavy atom (Z ;Z 1 5 and A ;A ), Eq. (4) becomes 1 5 Z 2 A 8 5 (5) R+ 1 5 Z A 5 1 while for a inverse heavy-ion-induced reaction Z
A

BA

B

A BA B

5.1.2. Application to unbacked targets The ratios of the (non-relativistic) total crosssections for d-ray production due to accelerated and recoil ion—target interactions evaluated from the relation (4) for several medium and heavy targets [(Z ;A )"(50;119);(60;144);(70;173);(80;201) and 5 5 (90;233)] bombarded with a few light and medium ions are shown in Fig. 1 a—d. This figure shows a very wide variation of that ratio for different bombarding ions (i.e. 1H; 2H; 4He; 6Li; 9Be; 11B; 1 1 2 3 4 5 12C; 14N; 16O; 19F; 31P and 32S). 6 7 8 9 15 16 According to these estimates the d-ray emission due to recoil ions produced in light-ion-induced reactions, within a very thin target, is negligible compared with the corresponding emission from accelerated ions. Furthermore, the d-ray production due to the recoil ions from heavy-ions-induced reactions in the same target is more important than from the light-ion-induced reactions but still negligible compared with the accelerated ions. However, for inverse heavy-ion-induced reactions the ratio between these total cross-sections is larger than in the previous cases. Nevertheless, all these non-relativistic estimates assume the same kinetic energy of the d-electron emitted in both interactions (i.e. accelerated ion—target and recoil ion—target) and the compound nucleus is produced in the first target layer — or a very thin target. Thus, it is important to consider other cases besides this very special one. The simplest case to consider is a thin unbacked target bombarded by the same ions. If E and 15

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E are the residual kinetic energies of the acceler#5 ated particle and the recoil ion after crossing the target (with corresponding energy losses *E and 1 *E , described as variable fractions, f and f , of # 15 #5 the maximum energies, E and E , such as 1 # *E "f E and *E "f E ), then their ratio is 1 15 1 # #5 # given by the relation: E E (1!*E /2E ) f E 15" 1 1 1 , 15 1, (6) E E (1!*E /2E ) f E #5 # # # #5 # where f and f are the mean relative residual 15 #5 energies of accelerated and recoil ions after crossing the target. Finally, introducing relation (6) into the relation (3) with E and E instead of E and 15 #5 1 E then the following formula arises: # 2 Z A 8 f 4 1 15 R + 1# 5 5. Z #Z f A 1 5 #5 1 2 A #A 81!2(*E /E ) Z 1 1 5 1 1 + (7) Z #Z A 1!2(*E /E ) 1 5 1 # # showing the influence of the ratios f /f (or 15 #5 *E /E /*E /E ) over the ratio R between the 1 1 # # 5. cross-sections of d-ray production due to the accelerated and recoil ions with mean residual energies E and E . 15 #5 If the target is very thin and the energy loss of the accelerated and recoil ions crossing the target is negligible then both correcting terms in the last fraction can be negelcted and relation (7) is reduced to relation (3). If the target is thick and the energy loss of the accelerated and recoil ions slowed down within the target cannot be neglected, then *E increases and # can reach E /2. In that limiting case the recoil ion is # stopped in the rear face of the target where its contribution to the d-ray production vanishes. Otherwise for smaller energy losses of the compound nuclei (such as *E "E /4) the ratio R is # # 5. roughly twice the value given by relation (3). Consequently, the influence of the f and f correcting 15 #5 terms in relation (7) is not critical for thin targets (compared with the recoil ion range in the target material) and relations (4) and (5) are acceptable rough estimates. It is important to stress that relations (3) and (7) are essentially valid for a single target layer with

A A

BA BA

BA B B

249

a definite ratio f /f or the corresponding relative 15 #5 energy losses of the accelerated and recoil ions inside the target. Thus the global contribution to the same slice of the d-ray spectrum can be roughly estimated by integrating relation (3) between the maximum and minimum values of E and E , i.e. 1 # (E ,E ) and (E ,E ). The corresponding mean 1 1. # #. value, R , is given by the relation, 5 2 A #A 4 [E !E ] Z 1 1 5 # #. R" 5 Z #Z A [E !E ] 1 5 1 1 1. :E1 (E )4 dE 15 ] E1. 15 (8) :E# (E )4 dE #5 E#. #5 which leads to the final result:

A

BA

B

A

BA

B

2 A #A 8 Z 1 1 5 R" 5 Z #Z A 1 5 1 [1!E /E ] [1!(E /E )5] #. # 1. 1 . ] (9) [1!E /E ] [1!(E /E )5] 1. 1 #. # In most cases, the energy loss of the accelerated ions inside the target is smaller than the recoil ion energy loss, which makes 15E /E 'E /E . 1. 1 #. # Then the corrective terms of relation (7) introduced in relations (8,9) rarely exceed a factor four and are much smaller than the variation of the main terms in those relations (3)—(5). This large difference justifies the previous detailed discussion of those terms. 5.1.3. Extension to backed targets All the previous relations (3)—(9) are restricted to collisions between an accelerated ion and a selfsupporting target. If the residual energies of the accelerated and recoil ions leaving the target (and entering the backing characterized by their atomic and mass numbers Z and A ) are f E and f E the " " 15 1 #5 # following ratio for the kinetic energies of the accelerated ion and the recoil ion in the same layer of the target backing arises (instead of relation (6)) E f f E 13"& 1" 15 1. (10) E f f E #3" #" #5 # Consequently, within the validity of these assumptions the accelerated ion and recoil ion slowing down inside the target backing can be accounted for either replacing the ratio f /f by 15 #5

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Fig. 1. Ratios of (non-relativistic) total cross-sections, R , for d-ray production due to accelerated and recoil ions—target interaction. 5 These ratios are evaluated from the relation (4) (see text) for several medium and heavy targets [Z ;A )"(50; 119); (60; 144); t t (70; 173); (80; 201) and (90; 233)] bombarded with a few light and medium ions. For simplicity neither the ratio R "R (f /f )4 (relation 5. 5 15 #5 (7)) nor the average ratio, R (relation (9)), are represented in the figure; (a) R for 1H; 2H; (b) R for 2H; 4He; 6Li; (c) R for 6Li; 9Be; 11B; 5 5 1 1 t 1 2 3 5 3 4 5 12C; 14N; 16O; 19F; (d) R for 16O; 19F; 31P; 32S. All these non-relativisitic estimates assume the same kinetic energy of the d-electron 6 7 8 9 5 8 9 15 16 emitted in both interactions (i.e. accelerated ion—target and recoil ion—target).

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251

Fig. 1. Continued.

f f /f f in the formulae (3)—(7) or including the 1" 15 #" #5 corresponding corrective terms in the relations (8) and (9). However, it is essential to keep in mind that

all the previous relations were derived from the semi-classical model applied to monopole excitations of atomic states with l"0 (i.e. s atomic states)

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ionized by accelerated ions (as discussed in Refs. [41,42]). Furthermore, these relations are valid only for accelerated ion—target atom and recoil ion—target atom interactions fulfilling the energy and momentum conservation principles. Unfortunately, the latter conditions are particularly restrictive because the maximum energy of the d-rays emitted in both interactions is very different for the accelerated and recoil ion interactions. E "4(m /m )E cos2u d1. % 1 1 "2.18]10~3(E /A )cos2u, (11) 1 1 E "4(m /m )E cos2u d#. % # # "2.18]10~3(E /A )cos2u, (12) # # where u(u490") is the observation angle of the d-electrons (see Refs. [41—43]). The corresponding ratio of the maximum d-ray energies of the accelerated ion and recoil ion—target interactions is quite simple

A B A

B

E A 2 A 2 d1." # + 1# 5 , (13) E A A d#. 1 1 taking into account the relation between the maximum kinetic energies of the accelerated ion, E , 1 and recoil ion, E , i.e. E /E "A /A . # 1 # # 1 The maximum kinetic energy of the d-rays due to the accelerated ion—target interactions according to relation (13) is always larger than the maximum kinetic energy of the d-rays arising in the recoilion—target interaction. Instead, the minimum kinetic energy of the d-rays arising in both interactions is practically the same. However, the previous relations neglect the scattering (and the slowing down) of the d-rays inside the target as well as the possibility of recoil ions produced in the backing due to spurious reactions induced by the accelerated ions. Indeed, both effects considerably change the initial spectral distribution of the d-rays and make the experimental test of the theoretical predictions, more difficult. Then, the ideal target thickness for minimizing the d-ray background overlapped with discrete electron lines is difficult to estimate accurately for collisions involving heavy ions or heavy targets. Further theoretical calculations are required for that purpose using binary [43—45] or multiple [46] ion—electron interactions.

5.2. Recoil ion catcher holders The recoil distance method is particularly useful for the study of nuclear isomerism in the range of a few ps to one hundred nanoseconds. Several varieties of this method were developed for different kinds of delayed nuclear measurements (see Refs. [32,33]). All these varieties have a common feature. They avoid — or reduce as much as possible — the interactions of the accelerated ions and any material emitting prompt radiations overlapped with the delayed transitions to be investigated. In particular, two main varieties of the recoil distance method were used for the study of delayed IC electrons. They are (i) the recoil ion catcher method and (ii) the recoil ion shadow method. The main characteristics of recoil ion catcher holders coupled to in-beam e-c spectrometers are condensed in Table 3. The recoil ion catcher method — like the recoil distance method — requires a variable distance between the target and the recoil catcher. Consequently, either the target position is fixed and the recoil catcher movable or the target is movable and recoil catcher is fixed. However, in both cases at least one of them (target or catcher foil) should be inserted on the axis of the electron spectrometer. If this arrangement is excluded (in order to increase the space available for placing c-ray detectors around the electron source) then both target and recoil ion catcher holder must be inserted on the target chamber far away from that axis on a carrier through a vacuum lock placed upstream (or downstream) the target chamber. Indeed, this condition is particularly important for multi-e-c spectrometers excluding bulky devices around the target placed at (or near) the electron spectrometer axis. Most experiments performed with the recoil ion catcher method used thin aluminium self-supporting foils (thickness 2—4 lm) as recoil ion catchers because carbon foils with the same areal density are not easily available especially with a small central hole allowing the accelerated ion beam to cross that foil without any noticeable interaction. However, one can expect that carbon foils available with the same areal density and mechanical strength will replace progressively in the future the Al recoil ion

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253

Table 3 Characteristics of recoil ion catcher holder (shape, dimensions, orientation and composition) 1. Flat metallic disk with a inner circular opening oriented at 45° to the beam axis for electron measurements z Main purpose: Reduction in the effective thickness of the recoil ion catcher foil seen by the electron spectrometer entrance aperture. 2. Effective recoil ion catcher diameter larger than the cross-section of most probable recoil ion angular spread at the chosen target recoil ion catcher distance. Diameter of inner circular opening larger than accelerated ion beam halo in that position z Main purpose: Increasing the efficiency of recoil ion collection. 3. Recoil ion catcher target distance chosen according to the half-life of nuclear isomeric state to be investigated z Main purpose: To discriminate roughly between prompt and isomeric transitions according to their lifetimes. 4. Relative position of target, recoil ion catcher holder and electron spectrometer axis: Target upstream to that axis and Investigation of low!energy transitions z P recoil ion catcher put on the axis de!exciting an isomeric state. Target placed on the axis and Investigation of transitions feeding z P recoil ion catcher downstream that axis that isomeric state. z Main purpose: To improve the discrimination between these transitions through simultaneous e-c measurements as well as coincidence measurements. 5. Recoil ion catcher thickness larger than recoil ion range z Main advantage: To increase the efficiency of this recoil method. 6. Recoil ion catcher made of light metals z Main advantage: To reduce the amount of d-rays due to recoil ion-catcher interaction. z Main drawback: Induced reactions with accelerated ions scattered by the target.

G G

H G

H G

collectors especially in experiments involving heavy ions with a strong d-ray background.

6. Functional relationship The functional relationship between a target holder, a recoil ion catcher, a target chamber coupled to an accelerated ion beam line and an electron spectrometer is described in Table 4. In particular, this table illustrates the high complexity of that relationship especially if cryogenic targets and (or) cooled charged particle detectors are requested by such experiments. Moreover, the general requirement for a compact design and the serious limitations set by the lack of miniaturized vacuum components increases considerably the difficulty to build a multi-purpose target chamber suitable for a wide variety of prompt and delayed e-c measurements with the recoil ion catcher and recoil ion shadow method operated with an electron-spectrometer coupled to an array of several X, c-ray detectors. A few devices of that kind for special in-beam measurements with light or heavy ions are reported in the next section.

Finally, it is important to stress that all atomic processes arising in the interaction between a fast accelerated (light or heavy) ion and a single homogeneous (self-supporting or backed) target can be classified according to the target (or target plus backing) structure. That means that the usual selfsupporting target classification according to the relative energy loss of an accelerated ion crossing the target and the half-width of a nuclear resonance leads to a more general one for backed targets: (i) a thin unbacked target; (ii) a thin backed target; (iii) a thick unbacked target and (iv) a thick backed target. The first two cases (i, ii) illustrate the major role of the accelerated ions interaction with the target atoms for the production of d-rays and confirm the previous discussion (see Section 2) concerning the main differences between the d-ray emission and the compound nucleus formation induced by light ions as well as heavy ions. Usually, the recoil energy of compound nuclei produced with light bombarding ions is small. Consequently, the recoil ion range of medium heavy nuclei produced by fusion reactions with light ions is also small and a thin backing may be enough to stop the recoil ions. The third case (iii) is similar to the first one (i) when the target

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Table 4 Functional relationship between target holder, recoil ion catcher, target chamber, accelerated ion beam line, electron spectrometer and their accessories 1. Target holder z fixed or movable under vacuum, z insertion and extraction from target chamber. 2. Holder of recoil ion catcher z fixed or movable under vacuum, z insertion and extraction from target chamber. 3. Target chamber z Mechanical coupling to: z Accelerated ion-beam line. z Nuclear spectrometers (under vacuum for electron and recoil spectrometers, in air for X- and c-ray spectroscopy). z Target holder z target carrier z vacuum lock for target holder. z Transparent window for target external visualisation as well as observing beam impact on scintillator film or disk. Movable opaque cover of that window transparent to soft X- and c-rays. z Holder of beam profiler. z Holder of recoil ion catcher: z carrier of recoil ion catcher; z vacuum lock for holder of recoil ion catcher; z holder of compact high density radiation shields for delayed transition studies filtering the delayed transitions emitted by the catcher foil from the strong transitions emitted by the target. z Other important devices and accessories (a) cryogenic targets, (b) cryostats for charged particle detectors around the target, (c) vapour traps made with cooled pipes, (d) cryogenic vacuum valves for (a, b, c).

thickness is smaller than the recoil ion range. However, for thick targets the d-ray contribution increases which makes them unfavourable for lowenergy electron measurements. The fourth case (iv) is formally identical to the second case (ii). However, the recoil ion energy in heavy-ion-induced reactions occurring inside medium (or heavy) targets is larger than in light-ioninduced reactions on the same nuclei. Thus in case (iv) thicker backings are usually required to stop the recoil ions.

7. Applications to in-beam e-c spectroscopy There is a wide range of different kinds of e-c spectroscopic studies using the target chambers and accessories broadly described in this work (Sections 2—6). Many of these applications were

dealt with in earlier investigations concerning the influence of nuclear target characteristics and different spectroscopic arrangements on low-energy electron measurements (see Ref. [1] and references quoted therein). In particular, the influence of the electron spectrometer orientation towards the accelerated ion beam axis was extensively discussed in Ref. [1] from the point of view of the Doppler broadening of the electron lines and the d-ray background reduction. Furthermore, a few special requirements on the most suitable accelerated ion beam needed by these measurements were surveyed earlier [4] and recently described [47]. This section has a triple purpose: (i) to compare a few characteristic features of two different spectroscopic methods used for the study of delayed nuclear transitions in the subnanosecond and several nano-second time range (i.e. the recoil ion shadow and the recoil ion catcher methods developed

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in Section 7.1); (ii) to illustrate the main applications of collinear and non-collinear e-c configurations for the study of prompt nuclear transitions and (iii) to prospect the evolution of these spectroscopic devices towards multi e-c detector arrays. 7.1. Recoil ion shadow versus recoil ion catcher methods The main experimental conditions suitable for the recoil ion shadow and the recoil ion catcher methods applied to the study of short lived nuclear isomeric states are condensed in Table 5. The comparison between the main requirements and possibilities of both methods shows a few important differences as well as complementary aspects for the study of low energy strongly converted transitions. Indeed, the recoil ion shadow method is better suited for the study of subnanosecond delayed transitions than the recoil ion catcher method. Conversely, the latter method is more convenient for the investigation of delayed transitions in the nanosecond range than the former method. A common requirement of both methods is the need for thin and homogeneous targets. Indeed, a rather small energy spread of the recoil ion is indispensable in the first method and a well-defined angular spread of the recoil ions is needed in the second method. Furthermore, the low-energy electron emission by the recoil ions in flight is rather sensitive to their electric charge state which depends on the recoil ion charge stripping inside the target as well as its electronic rearrangement afterwards. This feature is particularly important for the study of heavy nuclei on account of their complex electronic cloud and different modes of rearrangement. The recoil ion catcher method does not have the same strict requirements but the minimization of the d-ray emission during the recoil ion slowing down inside the catcher foil is a very important condition for the success of this method. Consequently, the target features play an important role in both methods while the catcher foil has a crucial role in the recoil ion catcher method. It is not suprising in such conditions hat the optimization of the experimental conditions in both cases is rather difficult and accelerated ion beam time consuming. This remark is particularly valid for the recoil ion

255

shadow method on account of the difficult adjustment of the most suitable accelerated ion beam, target and target chamber characteristics needed for such experiments. Naturally, this drawback has restricted considerably the use of this method for short delayed e-c measurements. The same reason plus the wider range of time delayed measurements performed with the recoil ion catcher method explains why this method has been used more than the recoil shadow method in many medium-heavy nuclei (i.e. in particular, 152~153Ho [48,49], 187~189Au [50,51], 190~194Pb [52,53]). 7.2. Transition from non-collinear to collinear electron spectrometer configurations The large Doppler broadening of the electron lines de-exciting fast rotating nuclei in flight produced with heavy-ion-induced reactions requires the use of collinear electron spectrometer configurations instead of non-collinear configurations. This requirement implies that not only should the reaction chamber coupled to the in-beam e-c spectrometer be changed, but also the replacement of the cylindrical central absorber around the electron-source detector axis, by a similar absorber with a Faraday cup collecting the accelerated ion—beam or a tubular absorber (without a Faraday cup) according to the characteristics of the Si(Li) detection and the ion-beam direction. The forward collinear geometry of an axial symmetric — or toroidal — magnetic spectrometer operating with circular Si(Li) detectors requires usually a central absorber headed by a Faraday cup (see Refs. [2] and [54] for the corresponding cases). A typical application of this method is the study of nuclear high spin isomers of terbium-149 populated by heavy-ion-induced reactions [55]. However, the target-Si(Li) detector distance, d, is rather small in a typical mini-orange electron spectrometer (d+ 10—15 cm) compared with the corresponding distance of a SK magnetic lens or a standard toroidal spectrometer (d"50—60 cm). Then it is more convenient to use annular Si(Li) detectors and stop the accelerated ion—beam in a Faraday cup placed far away downstream the electron spectrometer. The backward collinear geometry of the same axial symmetric or toroidal spectrometer requires

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Table 5 Recoil shadow versus recoil catcher method Experimental conditions

Ion beam

G

continuous pulsed

G

recoil ion in flight

e-c emission

at rest

Target position

Recoil ion

G

fixed movable

G

slowing down stopping within

Electron spectrometer type

G

Electron, ion B!B0(static field) focusing and B"B(t)(swept field) deflecting field E"E (static field) 0

G

Shielding of target and surrounding materials radiation emitted by recoil ions catcher foil

Time filtering of delayed transitions by fast c!ray detectors Time filtering of delayed transitions by particle (a,ff ) detectors

H

placed around the target

Recoil shadow

Recoil catcher

I&100 nA;/45 mm;*h44 mrad not currently used

I&100 nA;/42 mm;*h44 mrad Same I,/,*h and ¹ &2—4 ns; 1@2 *¹"400—800 ns

Target(m/s"0.1—0.4 mg/cm2) placed upstream the e-spectrometer axis (*x(3 cm) Unsuitable for this method

Unfavourable for this method

Discrimination between prompt and short delayed transitions emitted by recoil ions in flight Nuclear ¹ (ns) measurements 1@2

Discrimination between prompt transitions emitted in flight and long delayed transition emitted at rest Not used yet in this method

By inelastic scattering in the target The target (unsuitable for this method)

And the catcher foil The catcher foil placed on the espectrometer axis (1—10 lm C, Al)

High transmission spectrometer (toroidal or solenoidal)

Field free electron source region (triangular magnetic lenses)

Electrons focused in a narrow (broad) *p within a toroidal (solenoidal) % magnetic spectrometer Not used in this method Electrons pre-accelerated (or retarded) in flight (before their detection) by an electrostatic lens

Idem

Narrow collimator for e-c emitted by recoil ions in flight Not used in this method

Narrow collimator for e,c emitted by recoil ions stopped in the catcher foil Reduction of catcher foil activity by allowing the accelerated ions to pass through a (/"3—5 mm) hole

Suitable for short flight distances (*x(3 cm) off-axis of solenoidal spectrometers Main application: EM transitions following a-decay

Suitable for long flight distances (8'*x'3 cm) off-axis triangular magnetic lenses Main application: decay of fission isomers

compulsory annular Si(Li) detectors allowing a free passage of the accelerated ion beam throughout all the spectrometer axis. Thus in such cases, the cylindrical central absorber (shielding the Si(Li) detector

Recoil catcher foil placed on the e-spectrometer axis downstream the target m/s"0.2—0.1 mg/cm2; 8 cm '*x'3 cm

Electrons swept in a wide *p % Recoil ions with different electric charges

from the irradiated target) is necessarily replaced by a tubulal central absorber (fulfilling the same aim). Three typical arrangements of a SK magnetic lens mounted in the backward collinear geometry

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257

Fig. 2. (a) Schematic view of an array of eight EUROGAM1 (Ge#ACS) plus three LEPS detectors and six BaF crystals coupled to 2 the ML. Si(Li) spectrometer (oriented perpendicularly to the accelerated ion beam axis). The Ge#ACS are mounted on two rotating platforms around a vertical axis passing through the target position. Six BaF #PM are mounted in the vertical plane colinear with the 2 ion beam axis. Only one vertical PM is represented in this figure slightly above the horizontal plane containing the beam line axis. All the (Ge#ACS) detectors can be moved radially into (or out from) the target by sliding radially their holders in these rotating platforms. (b) Schematic view of the same array of c-ray detectors showing the reaction chamber, the ML (placed on the top of the figure) and three BaF detectors seen from above the tubular target chamber centered on the beam line. The rotational and translational movements of 2 these detectors are schematically represented.

coupled to several c-ray and Si(Li) detectors as well as a similar configuration in the perpendicular orientation are illustrated in Fig. 2 of Ref. [47]. This configuration is the most suitable one for reducing the d-ray background (as shown in earlier

radium-219 measurements quoted in Ref. [3]). However, the rather strict requirements in the accelerated ion beams needed for such measurements (i.e. a nearly parallel ion beam with a small halo throughout all the electron spectrometer and

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detector cryostat) reduces its applicability to the best accelerated ion beams available with the Tandem accelerator (12~14C,16~18O,19F,31P). Furthermore, the more complex structure of the Si(Li) cryostat cooling the annular Si(Li) detector (compared with the corresponding device of a circular detector) contributes also to reduce the practical use of this method. Nevertheless, its advantages are strong enough to make sure that further developments are welcome to generalize and improve its application. 7.3. Evolution towards multi-e-c spectroscopy The transition from single to multi-e-c spectroscopy has a considerable impact on the design of the target chamber as well as the holders of the electron spectrometer and c-ray detectors. Indeed, the interplay between these different devices reduces the choice of the different possibilities accounting for the geometrical configuration of the electron spectrometer, different types of X and c-ray detectors as well as their AC shields or photomultipliers (in the case of BaF crystals). 2 Fig. 2 is a typical illustration of the tubular target chamber coupled to the SK magnetic lens (oriented perpendicularly to the accelerated ion beam) combined up to eight Ge#AC shields. This set-up has been used recently in a systematic search for octupole correlations in promethium isotopes (1505A5145). Similar studies are planned for europium-153 [56] to extend earlier studies of lighter Eu isotopes. All these e-c prompt studies were performed in the perpendicular orientation with light accelerated ions because the Doppler broadening was negligible or rather small. The extension of the same arrangements to the collinear orientation has not been attempted yet but will be tried in the near future.

8. Conclusions A wide choice of target characteristics is often an important factor for the success of an in-beam experiment. Indeed, it is very rare that one can easily and suddenly make an isotopic enriched target. For in-beam e-c spectroscopy the target im-

portance is critical in the low-energy domain. The same is true for the e-c measurements with recoil ion catchers. The characteristics of the target holder, the recoil ion catcher holder and the target chamber are less important than the target — or recoil catcher — itself but their role is far from being negligible especially if the ion beam has small unstabilities. The interaction between accelerated ion beams and targets gives rise to the emission of d-rays as well as recoil ions travelling through the target (and eventually stopped within their boundaries). Furthermore, the interaction between the target atoms and moving recoil ions emit d-rays overlapped with prompt internal conversion electrons. A non-releativistic estimate of the relative crosssections of d-ray production by these different procedures shows the primary importance of that interaction. Acknowledgements This work is dedicated to the memory of Salomon Rosenblum who contributed to the development of very thin and strong radioactive sources indispensable for the discovery of the a-ray fine structure — an important landmark in the earlier build-up of Nuclear Spectroscopy. Later Robert Wallen developed still further these important physicochemical techniques for the expansion of that scientific branch while Rene´ Bernas contributed to the development of nuclear isotope separation methods and techniques at Orsay indispensable for many investigations dealing with off-beam — as well in-beam-nuclear and electron spectroscopy. Finally, we are very grateful to the contribution of the staff of CSNSM, IPN (Tandem) and GSI Target Laboratory to the preparation of targets and complementary devices required by this work. References [1] J.S. Dionisio, Ch. Vieu, J.M. Lagrange, M. Pautrat, J. Vanhorenbeeck, A. Passoja, Nucl. Instr. and Meth. A 282 (1989) 10.

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