Experimental requirements for nuclear targets

Nuclear Instruments and Methods in Physics Research B56/.57 (1991) 933-937 North-Holland

933

Experimental requirements for nuclear targets

R. Pengo INF’N, Laboratari Nazionuii di Legnaro, Itab

A few examples of how the requirements for nuclear targets can be defined are given in the following. They refer to experiments where both y-rays and particles have to be detected and analyzed.

1. Introduction

Most nuclear physics experiments are carried out by using particle beams, accelerated by appropriate ma- clkes, which impinge on the nuclei of atoms of a generally very thin foil, known as a target. The chemical and physical characteristics of the targets depend on the experiment_ The preparation and quality of these foils are of prime importance for obtaining good data from the experiment. These relate to the choice of the correct isotopic enrichment and the selection of thickness in relation to the experiment, the choice of a chemical form which is compatible with the preparation techniques and the absence of contaminants, and the choice of a preparation method so that efficiency is compatible with availability and cost, given that the cost of en- riched isotopes is continuously increasing nowadays.

Consequently the preparation of a nuclear target for a specific experiment is very often the result of close collaboration between the experimenter and the person who is going to prepare the target, even though very often the experimenter thinks about the fact that a target is needed at the very last moment.

2. Example of target characteristics

As is very well known the probability of a nuclear reaction taking place is measured in terms of area (1 b = 10Mz4 cm’) and as a consequence it is more con- venient to measure the target thickness in mg/cm2. Therefore this will be the unit used hereafter. It is impossible to consider all the different types of reactions which can be produced. In order to give an example of the way a target is usually defined two examples will be given, i.e. a) when the product investi- gated is a y-ray, and b) when particles produced in a nuclear reaction have to be detected.

2. I. Experiments for the detecfion of gamma rays

Some of the characteristics of a nucleus can be studied from the y-rays of de-excitation of unstable levels. The measurement of the energy, angular distribu- tion and lifetime of such a y-ray provides information on the nuclei structure and on the forces which are responsible for the nuclear binding. The study of such characteristics belongs to the so-called nuclear spectroscopy techniques. The targets used in such types of

experiments are in general thicker than those used in experiments of type b). The characteristics of the target arise from the need for a reasonable number of nuclear reactions in a reasonable running time for the experiment, which has to be compatible with the availability of the accelerator. From the fact that the number of reactions is proportional, for the same amount of beam particles, to the number of nuclei in the target, the thicker the target is the larger the number of nuclei which take part in the reaction. On the other hand each ion passing through a material, at a certain velocity, is decelerated: that is of course the case for both the particles of the beam and the reaction products_ The attenuation of the y-ray can be neglected in most cases. The process is known as ion stopping power [l] and is essentially due to the interaction of an ion with the electrons in the material. The presence of this effect is such that any ion can be stopped by a sufficiently thick material.

Let us consider the reaction

114Cd(30Si, 4ny )‘%m

at an energy of 130 MeV of the incident particle [2]. As is well known, at a certain energy and because there exists a Q-value for a specific reactions, more than one reaction can take place, which have different efficien- cies.

0168-583X/91/$03.50 8 1991 - Elsevier Science Publishers B.V. (North-Holland) XII. ACCELERATOR TARGETS

934 R. Pengo / Experimental requirements for nuclear targets

In general the energy width of the beam in such an experiment has to be confined within 7 MeV. From ref. [l] the stopping power of Si in Cd at 4.3 MeV/amu is 5.2 MeV/(mg/cm’): as a consequence the thickness of the target has to be less than 1.3 mg/cm2. Suppose now that we want to detect the gammas by means of a ge~~urn detector at a typical counting rate N of lo4 counts/s. The following formula can be applied for practical purposes:

N = 3.7 X 106u(b)St(sr)I(pnA)d(mg/cm2)/.4(amu),

(1) where N is the typical counting rate of the detector (- 104), u is the total cross section of the reaction (- 0.5 b), Sz is the solid angle seen by the detector (- 3.1 x lo-‘), 1 is the current of the beam in particle nA, i.e. the electric current divided by the charge state (5-10 pnA), d is the thickness of the target to be found in mg/cm*, and A is the mass number of the target (A = 114). Substituting the typical values in brackets d has to be between 1 and 2 mg/cm*, i.e. compatible with the previous requirement. In order to define the target characteristics one has to consider whether the nucleus produced in the reaction is capable of emitting the y-ray before or after it has come to a complete stop. In other words, is the thickness of the target sufficient to stop the nucleus produced? The recoil energy of the

nucleus for the reaction under consideration is 21 MeV, and from ref. [I] the range of Sm in Cd is 2.5-3.0 mg/cm2, i.e. part of the nuclei are emitting their gammas in flight outside the target. As a consequence the energy E, seen by the detector, due to the Doppler shift, is:

E,=E,[1+/3(cos e)],

where E, is the y-ray energy, /3 = u/c, u is the velocity of the recoil nucleus, and B is the angle at the detector position with respect to the beam axis. In general it is sufficient to place a thick foil (20-50 mg/cm’) of a material of a high atomic number (in order to avoid a nuclear reaction) immediately after the target in order to bring the nuclei produced to rest. The materials used for such purposes are either gold or better “‘Pb even though more expensive. With the latter much cleaner y-ray spectra are obtained, its first excited level being sufficiently high. The stopper foil has to be placed immediately after the target, making sure there is no room left in between. The fact that the energy of a y-ray emitted in flight depends on the angle of detection causing the so-called Doppler broadening, can be exploited in some experiments for the measurements of lifetimes of the order of a few picoseconds.

At this point the major characteristics of the target have been defined, and the preparation method has to

ATOMIC NUMBER

19 1 20 1 21 1 22 123 124 125 I26 127 128 I29 130

K Ca SC Ti V Cr Mn Fe Co Ni cu Zn

7.0 0.5 0.6 0.3 0.5 - - 0.5 0.X 0.2 - 4.6

37 38 39 40 4, 42 43 “4 45 46 47 40

RbSt- Y Zr Nb MO 7-r: Ru Rh t=d Ag Ccl - 2.0 0.5 0.4 0.5 0.3 - 2.54 I.4 4.2 0.5 I..”

72 73 74 75 76 77 78 79 80

cs Sa L Hf l-a w Re OS It- Pt Au Hg - 2.P a.5 0.B IO.0 25.0 - iaa 0.8 0.s -

0.1 -

31 32 33 34 35 36

Ga Ge As Se Br Kr - __ - - _ _

49 50 51 52 53 54

tn Sn Sb l-e I Xe

5.0 4.4 - -

81 82 a3 a4 85 86

II Pb Bi PO At Rn 6.0 6.0 I.0 1.0 - -

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

L l-a Ce Pr Nd Pm Sm Eu Cd l-b Dy Ho Er Tm Yb Lu I.4 4.8 - 4.6 2.4 0.5 I.4 0.8 0.8 O..i 0.4 4.5 4.8 I.4 4.k

a9 90 91 92 93 92 95 96 97 98 99 100 101 102 103

A Ac l-h Pa U Np Pu AmCm Bk Cf Es Fm Md No Lr - 0.6 - 1.0 - 5.4 1.0 - - - - - - - -

Fig. 1. Ultimate thickness of elements which can be reduced to thin films by rolling. The data are from ref. [5] and they are a good indication, according to our experience, of the actual thickness obtainable in a laboratory. In some cases with special techniques the

ultimate thickness can be lowered.

R. Pengo / Experimental requirements for nuclear targets 935

be chosen. In general for nuclear spectroscopy experiments the thickness required is such that, if the material

is ductile, the pack rolling method can be selected. The material is placed between two sheets of polished stainless steel and rolled to the required thickness. This method allows either a gold or lead stopper to be positioned without leaving any space in between. A useful indication of the minimum thickness obtainable with this method can be found in fig. 1. If the current of the beam is of the order of 10 pnA, it is worthwhile coating the cadmium with a thin layer (- 0.1 mg/cm2) of gold in order to avoid the sublimation of the material during the experiment. Clear evidence of the need for such protective coating can be found in ref. [3], compar- ing the vapour pressures as a function of temperature.

2.2. Composite targets for g-facior me~urements

In order to study the g-factor of a certain level the transient fields created in ferromagnetic materials by a recoiling nucleus can be used. Consider for example the reaction [2]

where the projectile 56Ni is used to produce Coulomb excitation on ‘48Sm. If the nucleus is forced to recoil in

a ferromagnetic material, as Fe or Gd, a rotation 60 of the nucleus can be detected, which is given by

se - gB,rrf,ff 9

where g is the g-factor, B,_,r is the transient field (up to 10 MG) and feff is the transit time. It is therefore

necessary to prepare a target of 14%rn with a thickness typical of a y experiment (- 1 mg/cm’), whose excited nuclei can recoil into the Gd for a few picoseconds, which is the order of magnitude of the lifetime of the level, and such that they can be completely stopped in a foil with a cubic crystalline structure. The last requirement comes from the need to avoid any additional disturbing interaction such as a quadrupole interaction with an electric field gradient in noncubic elements. From the above-mentioned references the Gd thickness required is - 4-5 mg/cm’ and that of the copper is 20-50 mg/cm’. The three different films can be manu- factured, according to fig. 1, by means of the rolling procedure. The three different films are separately rolled and then “glued together” under the rolling mill. It is important to place a thin indium foil between the copper and the gadolinium, in order to have both better adhesion and better thermal conductivity. The gadolinium acquires its ferromagnetic characteristics when cooled to 77 K, i.e. at liquid nitrogen temperature, and the foil has to be maintained at this temperature during the whole experiment.

In many other cases the aim of the experiment is the detection of gammas requiring different targets from

those mentioned above. Using radiative capture reactions of the type 48Ca(p, Y)~~SC the main interest is to

populate a resonant level. The width of the resonance is of the order of 1 keV, which implies that the beam energy has to be very well defined and that its reduction due to the stopping power has to be minimal. A proton beam of 2 MeV has a stopping power of 0.1 MeV/(mg/cm’) in calcium, and as a consequence the target thickness has to be of the order of a few pg/cm2. The thickness is far removed from those of fig. 1, and the evaporation condensation (of calcium or 48CaF,) method in vacuum has to be used. In other cases the fact that a specific resonance is produced at a particular energy is exploited to investigate some properties of certain materials. A typical example is the reaction H(lSN, ay)12C which is very useful in order to measure the hydrogen content of metals simply varying the energy of the “*N beam. The resonance being very narrow, 1.8 keV, this is a very precise method for measuring the concentration of hydrogen in metals. The target for those experiments have to be first rolled and then warmed in a hydrogen atmosphere.

2.3. Reactions with the detection of particles produced

In order to study the mechanisms which rule a nuclear reaction, it is very often useful to detect the amount, the mass, the energy and the position of particles produced in a nuclear reaction. The methods used are different, such as the use of ionization chambers, parallel plate avalance counters (PPAC), silicon surface barrier (SSB) detectors and many others, which can be coupled to on-line magnetic or electrostatic separation devices. Let us consider a typical reaction [4] 58Ni(36S, olxnyp)94”6A where the aim of the experiment is to measure the amount of projectile and target nuclei which fuse to make a compound as a function of the incident energy. For this type of experiment a compromise has to be reached between the necessity of having a narrow beam energy spread, due to the stopping power of the projectiles in the target, and a reasonable counting rate in the detector. Using the formula (1) with the typical values of such a reaction, i.e. N = lo* counts/s, I= 5 pnA, (J = 10 b/sr, 52 = 5 x lo-’ sr, A = 58 the thickness d is - 0.6 mg/cm’.

A sulfur beam of 120 MeV has a stopping power of 10 MeV/(mg/cm’) in nickel so that its energy loss in the target would be 6 MeV, which is too high a value for the accuracy of the experiment. As a consequence the target has to be made as thin as 0.2 mg/cm*. The energy loss will then be acceptable and the counting rate accordingly reduced. From fig. 1 the thickness required is still within the range where the rolling method can be used. For the same reaction where the aim of the experiment is the detection of transferred particles, the thickness would have to be - 0.1 mg/cm2.

X11. ACCELERATOR TARGETS

936 R Pengo J Experimental requirements far nuclear targers

As can be seen in fig. 2 the efficiency of production of the evaporation condensation method that has to be used in this case is much iower, requiring a much higher amount of mategal to start with. Other methods such as sputtering in high vacuum, allow much higher efficiency to be attained, but they require apparatus only a~aiIable in a few laboratories for the preparation of nuclear targets.

It is important to mention the problem of contamination in a target. Particular care has to be taken when making a target, especially using the evaporation condensation method, in order not t,o evaporate part of the crucible together with the evaporand. This danger exists in particular when the crucible is heated exploit- ing the Joule effect, but it is practically inexistent if the material to be evaporated is heated by electrons accelerated with an electron gun, keeping the (copper) cruci- bie cooled at room ~rn~rat~~~ The latter method fias

0 i i %

0 2 4 6 8

RADIAL DISTANCE (an)

Fig. 2. Hot of the thickness of a film produce4 by evaporation condensation in vacuum, for different distances (from 8 to 20 cm) between the crucible and the substrate, starting with 100 mg af material. The curves are calculations FoBawing the formula in ref. [6] and have been very well reproduced in our laboratory for different materials. The curves also give the value of the deposit thickness as a function of the distance (from 0 to 10 cm) from the vertic& line above the crucible and

on the substrata

Fig_ 3. An exampk of contamination in a target. The contamination of oxygen and c&km is clearly seen in the bidi- mensiona? plot: time of flight (TOF) vs total kinetic energy (ET). The contamination was produced at the moment of the zirconium oxide reduction with calcium. Unfortunately it is not always possible to chscrirninate contamination from data as

in this case.

then to be preferred to the former> even though the first would normally have been chosen because d its much greater efficiency. Another possibfe source of contamination could arise from the choice of the reducing element (see fig. 3), of oxides for example: the m&&ant with the lowest vapour pressure at a high temperature has to be chosen, such as thorium, hafnium, zirconium in order to minimize the possibility of contamination. No contamination is usually introduced with the rolling procedure when using high quality stainless steel. Elec- trodeposition has to be in general avoided, where possible, because this method can give rise to possible ccn- tamination from the chemi& s&&on or from the electrode which usually has to be removed. Contamina- tion disturbs an experiments, in the sense that its effect is not usually seen at first sight. Only in rare cases such as in the one shown in fig. 3, the cont~nation of calcium and oxygen in a 2.ireonium target is clear Contamination was actually introduced at the moment of the reduction of zirconium oxide with calcium leaving part of the reductant (calcium) in the zirconium which had only been partially chemically reduced.

3. Conchlsion

Some indicative examples have been an8lysed as to how nuclear target characteristics are defmed, both for

R. Pengo / Experimental requirements for nuclear targets

gamma and particle detections. These are only two of References

937

the hundreds of experiments possible in nuclear physics. In general the preparation of nuclear targets should imply an active collaboration between the experimenter and the person preparing the target, in order to reach a suitable compromise between the possibility of carrying out an experiment and the making of a target. This is of course true for all experiments even though not mentioned here.

111

14 131

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J.F. Ziegler, Handbook of Stopping Power for Energetic

Ions in all Elements, ~01s. 5, 6 (Pergamon, 1980).

D. Bazzacco et al., 2. Phys. A238 (1987) 275.

Y. Shapira and D. Lichtman, in: Methods of Experimental

Physics, eds. G.L. Weisser and R.W. Carlson, vol. 14

(Academic Press, 1979) p. 350.

A.M. Stefanini et al., Nucl. Phys. A456 (1986) 509.

MRC Catalog, Munich (1981).

S. Schiller, U. Heisig and S. Panzer, in: Electron beam

technology (Wiley, 1982).

XII. ACCELERATOR TARGETS

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Experimental requirements for nuclear targets