- Email: [email protected]
Studies on a cluster target
Nuclear Instruments and Methods in Physics Research
A295 (1990) 44-52
North-Holland
Studies on a cluster target W. Bickel, M. Buschmann, H. Dombrowski, G. Gaul, D. Grzonka, G. Hölker, R. Santo and M. Wähning Institut für Kernphysik der Universität Münster, D-4400 Münster, FRG Received 13 February 1990 and in revised form 19 April 1990 Cluster beams of various gases have been produced by a versatile test setup, where the configuration of nozzles, skimmers and collimators has been studied and optimized. The parameters determining the cluster yield were investigated using elastic electron scattering . Functional dependences of the various quantities have been extracted . A detailed evaluation of the pumping capacities shows that, compared to existing cluster targets, much lower pumping capacities are required for gases of a mass higher than that of hydrogen . 1. Introductión Nuclear and elementary-particle reactions are predominantly performed with solid targets of various thicknesses adjusted to energy resolution and other experimental boundary conditions. Although requiring m.),-e technical effort, gas targets - in particular gas jet and cluster targets - have several advantages in comparison with solid targets, because they are chemically pure and do not alter during irradiation, they can tolerate large currents without deterioration and they CL- be varied down to thicknesses much smaller than achievable with solid targets. On the other hand, only a very limited number of materials is at disposal and the maximum target thicknesses are severely limited. With the advent of a new generation of storage rings La--n stochastic and electron cooling, there is a strong err°,:.r°st in suitable and variable internal targets. The advantage of storage-ring beams in conjunction with internal targets is the high luminosity which allows the measurement of very low cross sections or rare events . Internal target operation with a cooled beam requires, of course, low target densities, so that the energy loss of the circulating beam can at least partially be compensated . This completely excludes even the thinnest target foils available, which are in the range of some !Lg/cm'` . Different other solutions have been proposed [1] like dust targets, fibre targets or frozen-pellet targets. In the field of gaseous targets one has to distinguish between atomic-beam, gasjet and cluster targets . A simple source of gas used for atomic beams is a vessel at temperature To and pressure po with a small hole allowing the gas to flow into a region of pressure p < po. Such an effusive source yields a broad Maxwellian velocity distribution for the gas particles, where the
preparation of a gas beam is simply achieved by a collimator. The disadvantage of such a gas source is the very low target density of typical 109-10'1 atoms/cm2. To increase the density of the gas beam, nozzles have been used instead of a simple hole. With a nozzle of convergent-divergent structure, the principle of a Laval nozzle, one can obt,-.in a supersonic gas beam, which yields target densities in the range of 1015 atoms/cm2 close to the nozzle. The expansion of the gas through a nozzle further has the consequence of cooling the gas . Looking at the gas flow at the beginning, the gas atoms or molecules have a broad distribution of their relative velocities, but during the expansion through the nozzle the velocities assimilate and the velocity distribution shrinks, which can be described as a reduction of the temperature . In the course of further expansion the vapour pressure may be reached and this saturated gas starts clustering. If one follows the expanding gas in the nozzle one passes the different phases of saturation, supersaturation and nucleation where the clustering sets in. In this condensation process cluster sizes of 105-10 6 molecules can be achieved [1] . '_)
Fvrwrimontai r -. ""--- ..r çphin
2.1 . Cluster target
Fig . 1 shows the cluster target setup [2,3] . The target gas enters the nozzle after passing a cooling system consisting of a two-stage cold head (Leybold RG210). The temperature can be checked by a silicon diode temperature sensor (DT-450 from Lake Shore Cryotronics ire .), which is placed in the gas supply close to the
016&9002/90/$03.50 0) 1990 - Elsevier Science Publishers B.V . (North-Holland)
W. Bickel et al. / Studies on a cluster target
gas
scattering chamber 10cm cluster beam dump Fig. 1 . Setup of the cluster target.
nozzle. As nozzle we use a trumpet-shaped one with a throat diameter of 160 Wm machined at CERN . To prepare a fine cluster beam without loading the vacuum system of the scattering chamber with too
45
much gas, we apply different pressure stages . In the skimmer stage, most of the gas is peeled off from the clusters which are preferentially produced in the center of the beam . Then the beam passes the collimator, where an additional fraction of the gas is held back, and reaches the target position with the desired dimensions. These dimensions are controlled by varying collimator distance and opening. Then the cluster beam passes the scattering chamber, where the beam is subjected to
diagnostics . As a beam dump a 10001/s turbomolecular pump without a collimator to reduce the backflow is used. In this setup all critical components i .e . nozzle, skimmer and collimator are mounted on one unit so that they are always exactly aligned. To avoid a heating up of the cluster beam by the skimmer and to reduce the loss of cooling capacity, the skimmer support is surrounded by a tank with liquid nitrogen. Only a few percent of the gas expanding through the nozzle forms clusters and therefore nearly the complete gas volume has to be pumped off, which requires an enormous pumping capacity for the cluster target setup . Fig. 2 shows the pumping scheme of our setup and in table 1 the various pumps are summarized . The skimmer stage, where more than 9096 of the gas is peeled off, is equipped with two roots pumps with a pumping capacity of 500 m3 /h each, with two rotatingvane pumps with 200 m3 /h capacity as backing pumps . For the collimator stage we use a 10001/s turbomolecular pump, at which the pressure in the skimmer stage forms the low-vacuum side . The same concept, a 1000 1/s turbomolecular pump and the skimmer stage at the low-vacuum side, is realized at the beam dump. The scattering chamber is pumped by a 360 1/s turbomolecular pump with a 52 m3 /h rotating-vane backing pump . Furthermore, the region w the electron gun must be evacuated separately with a 360 1/s turbomolecular pump to get a sufficient low pressure. The electron gun and the scattering chamber are separated by a collimator of 2 mm diameter, which allows a pressure lower
Table 1 Pumping system for the cluster target Pumping stage
Pumps
Backing pump
Skimmer
roots pump : 2 x 500 m-'/'h (Leybold Ruvac WSU 500)
rota y, 3 2 x 200 m /h
Collimator
turbomolecular pump : 10001/s (Leybold Turbovac 1000)
skimmer stage
Scattering chamber
turbomolecular pump : 3601 ; s (Leybold Turbovac 360)
rotary, 52 m3/h
Beam dump
turbomolecular pump : 10001/s (Leybold Turbovac 1000)
skimmer stage
Typical pressure (G -=1000 Pa 1/s C02)
n v.vc n aa
7 x 10 3 Pa 1 X10-3 Pa
W. Bickel et al. / Studies on a cluster target
Fig. 2. Pumping scheme for the cluster target .
than 1 x 10-4 Pa at the electron gun, if the pressure in the scattering chamber is less than 1 x 10-1 Pa. 2.2. Beam diagnostics
The beam diagnostics is performed by elastic electron scattering. The advantage of this method compared with other methods is the simple availability of electrons and the hish sensitivity . which is only limited by the electron detector. r y r This system consists of an electron gun, a set of deflection plates to scan the target region with the electron beam, a detector for the elastic scattered electrons and a Faraday cup to stop the primary electron beam. As electron gun we use a compact module for standard TVs, operating with an accelerating voltage of 18 kV . This module, a Valpo-type A 61-120 W, contains an indirect heated Ba/Sr-oxide cathode and five internal electrodes for the extraction, the acceleration and
the focussing of the electrons . Contrary to the normal working conditions we had to put the whole module on negative high voltage . At typical operating conditions we get electron currents of 1-10 [LA for an electron beam diameter smaller than 1 mm at the target zone. With this system we can analyze the structure and the density distribution of the cluster beam. But also it can be used as a monitor system by fixing the electron beam at a position in the center of the target and control the density during an experiment. For applications of the systera in accelerator experiments the monitor system has to be operated automatically. For this purpose the diagnostic system is controlled by a personal computer with a specially designed NIM module. It contains digital-analog converters to set the required deflection voltage, and voltage-tofrequency converters which allow the simple measurement of pressure or other slowly varying parameters without using ADCs. The communication with the com-
W. Bickel et al. / Studies on a cluster target
47
Fig. 3. Setup of the analysis and monitor system with elastic electron scattering .
puter is made by a PC I/O-card with 48 I/O-channels and six counters . Fig. 3 shows a sketch of the setup for the diagnostics.
a) C0 2
To select the elastic-scattered electrons and suppress the background counts we use an electron spectrometer . This spectrometer is a double-focussing 90* sector
b) N2
Fig. 4. Density distribution of a carbon dioxide and a nitrogen gas beam at a flow rate of 500 Pa 1/s and a temperature of 290 K. The profiles perpendicular to the beam axis cover a range of 30 mm and the distance from the nozzle varies between 2 and 22 mm. (These measurements were performed with another setup, similar to fig. 1 without the cooling system, but with the possibility of measuring profiles close to the nozzle .)
W. Bickel et al. / Studies on a cluster target
48
magnet with a nominal trajectory radius of 80 mm [3] . As detector we use a plastic scintillator with photomultiplier readout or a channeltron . Comparing these two detectors, the channeltron has the advantage of a very low background level and all electrons, nearly independent of their energy, yield a pulse so that also low-energy electrons, which are produced in the target zone, can be selected with the spectrometer and detected with the channeltron . On the other hand, the channeltron 10-4 Pa) to work satisneeds a very low pressure (p < factorily. Therefore a further turbomolecular pump is needed for the channeltron chamber. The photomultiplier also needs a low pressure to avoid a breakthrough on the base, but there a pressure of p < 10-2 Pa is safficient. The absolute target densities are obtained by normal :ing the yield Ndet to the Rutherford cross section. The t,?-get layer PF, which results from an integration of the tarret density F_ v along the electron beam is given by VF -
1 Ndet
*10 3.2
N
15
a
2.8
0 2 .4 ~,
2. 1 .6
" b
1 .2 0.8 0 .4 0.
12
16
20
24
28
32 36 position / mm
1
c N;,, (da/dQ) 0 dû'
where E = detector efficiency, Ndet = number of detected electrons, Ni,, = number of electrons (from cup current), d2 = solid angle, (do/d2) 0 = Rutherford cross section for the elastic scattering of electrons with the energy E on a nucleus with atomic number Z at the scattering angle t$. The deviations from pure Rutherford scattering at 90' are less than 5ß'o. The efficiency of our detector system was determined by a comparison with a solid target of known thickness. For these measurements we used thin carbon foils of pdx -- 10 Etg/cm2. An extra check can be made by a double-slit unit which defines the interaction length seen by the spectrometer . Then, for a fixed pressure in the scattering chamber without a cluster beam the number of scattered electrons can be calculated. For the plastic scintillator with photomultiplier the detector efficiency is 80%, while for the channeltron only a value of - 10% is obtained. 3. Experimental investigations shnurc turn Aençity! Aiçtri}tntionç of the region -c from ? to 22 mm behind the nozzle for a C02 and a N2 gas beam at a temperature of 290 K. Both pictures are very similar and show a broad gas distribution. For nitrogen - room temperature no clustering is expected, but for carbon dioxide, due to the high condensation temperature of 195 K, clustering is possible with the nozzle used. However, the density distributions show no indication of clustering. In both profiles a steep decrease of the density in beam direction can be seen. The above measurements were made without any skimmer, b'
Fig
d,
Fig . 5. Profile of a gas beam 8 mm behind the nozzle and a cluster beam 8 mm behind the skimmer . In both cases C02 was used. In (a) the cluster beam is covered by the gas distribution but in (b) the gas is peeled off so that the cluster beam .led up by a factor of .becomes visible . (The axis in (b) is sc --100.) hence no separation of clusters from the gas was made. Recalling the fact that only a few percent of the gas undergoes clustering it is not surprising that no effects of clustering are detected in the measured density distributions . A completely changed situation arises if a skimmer is used whereby the nonclustered gas molecules are peeled off from the cluster beam and the vacuum in the scattering chamber is strongly improved so that the clusters can survive and become visible in the density profile . In fig. 5 we show two profiles of a gas beam and a cluster beam. In fig. 5a we measured a profile 8 mm below the nozzle (no skimmer mounted), and in fig. 5b a skimmer is used and the profile was again measured 8 mm below the skimmer. At first sight the, drastic difference in the widths of the two profiles can be seen. While the gas beam shows a broad distribution with a smooth decrease of the density, in the case of the cluster beam we observe a rapid change between the cluster beam and the background of the homogeneous gas density in the
W. Bickel et al. / Studies on a cluster target scattering chamber . Due to the fact that the scattering of two gas particles with equal masses leads to large scattering angles, the gas jet always yields a broad density distribution . When, on the other hand, a cluster of up to 106 atoms strikes a gas molecule, the average change in direction of the cluster is negligible. Therefore the clustering leads to a beam of nearly constant divergence, where the beam dimensions can be calculated in good approximation by geometrical considerations . 3.1 . Conditions for clustering
When preparing a cluster target, the correct adjustment of the various parameters of the experimental setup, like nozzle and skimmer form, kind of gas, input pressure and temperature is the basic requirement . Measurements of Gspann [11 and other authors have shown that the form of the nozzle is very crucial in order to get a well-defined target volume of high density. The form of the skimmer is also important for a satisfactory operation . The function of the skimmer is to peel off the gas from the clusters without disturbing the gas flow. To fulfill these requirements the deflection angle 8 of the skimmer should be as small as possible. An upper limit for 0 is given by the value where the attached shock wave is detached, which leads to a disturbance of the cluster beam. For a typical gas jet with a Mach number of - 10 we get a ®max of - 40 ° . On the other hand, for an effective pumping of the region below the skimmer the opening angle should be as large as possible. A large internal angle of the skimmer cone also helps to minimize the problem of perturbation of the skimmer flow by particles reflected from the inner skimmer surface . For the skimmers in operation we chose a deflection angle of 30 ° and were able to get a mechanically stable skimmer with an opening angle of 20 ° . For the collimator, angles of 35 ° and 25° were chosen . The distance between nozzle and skimmer was adjusted to a value of 20 mm, and for the distance between skimmer and collimator we have chosen a value of 40 rrim. The orifices of the skimmer and the collimator with diameters of 1 .2 mm and 3.6 mm have been chosen in such a way that at the target region, 260 mm below the nozzle, target densities in the range of 1013-1014 atoms/em2 can be achieved. Because of this geometry the target zone is coniiiieu to a uiametei of - 12 mm. Furthermore, the pressure in the different pumping stages, skimmer, collimator and scattering chamber, has to be sufficiently low, so that the clusters can reach the target zone without being destroyed by collisions with the residual gas. To test the possible vacuum conditions in the skimmer and collimator region for a stable cluster target, we performed measurements, where a CO2 cluster beam was prepared and a density profile perpendic-
49
ular to the beam direction was measured by elastic electron scattering. Then we increased the pressure in the skimmer or the collimator region by opening a needle valve and allowing the streaming in of carbon dioxide to simulate a reduced pumping capacity. For the skimmer, a sudden disappearance of the cluster beam at a pressure below 14 Pa in the skimmer chamber was observed (which is in agreement with the maximum pressure value of - 10 Pa given by Macri et al. [11). Similar results were obtained for the collimator stage. 3.2. Thermodynamic effects In fig. 6 we show the central target density as a function of the nozzle temperature for different gases. This measurements were performed at constant flow rate using a mass-flow controller (Brooks Instrument BV type 4250). The temperature dependence of the target density can be divided into two parts. At high temperatures no clustering is possible and we obtain a constant yield because the flow rate was kept fixed. At >, 100 -v 80 60 40 20 0
L__
I
14E
1
I
160
I
i
180
140
I
-- 1
160
1
I
200 temperature / K
i0 1
1
I
1
1Z- "1.
200 180 temperature / K
Fig. 6. The dependence of the central target density on the gas temperature for N2 at a gas flow of 780 Pa 1 //s . The solid ilne is a fit to the data for low temperatures up to -150 K with a power law (y = T-4.7) . For the temperature range from 150 to 180 K the data can be described by an exponential function (y = exp(0 .8T ), broken line).
W. Bickel et al. / Studies on a cluster target
50
low temperatures we get a drastic increase of the target density, which can only be explained by clustering effects. The solid lines in the figure represent fits to the data with a T-X power-law dependence for the lowtemperature part, which is expected from the scaling relations of Macri [11 and Hagena [5] based on an isentropic expansion of the gas. A rough description of the data is possible with such a power law, but in the region of the onset of clustering strong deviations are seen . In this temperature range the cluster yield depends exponentially on the temperature (fig. 6; broken line). In the very first stage of clustering, the mean size of the clusters is supposed to be very low so that the target density is not primarily determined by the isentrope but by the stability condition of a single cluster. The vapour pressure of a droplet is given by Pd ) - Mm 2a pRT r Pinf where pd, pinf =vapour pressure at the surface of a droplet and of the liquid, respectively, r = radius of the droplet, a = surface tension. Hence the probability of nucleation, J, will show an exponential temperature dependence, J - exp(-1/kT), which can explain the observed deviations from the power law. Furthermore, a minimum radius is needed to build a stable cluster so that the power law is only valid if this critical radius is exceeded .
Fig. 7. Clustering temperatures Tc, for different input pressures. The solid lines are fits to the data with a power-law dependence, which are within the upper and lower limits for the slope shown by the broken lines.
U
U
12
12
N 24
2
U
12
20
O
12
b
8 4 4 -12-8 -4 0 8 12 axis / mm distance to beam
0
-8 -4 0 4 8 12 distance to beam axis / mm
Fig. 8. Profiles of cluster beams from N2( PO = 4 bar, To =120 K) and C02( PO ~ 3 bar, To = 250 K). In this plots the background level is subtracted . The solid lines are fits to the data with the assumption of a rectangular radial density distribution .
210
220
230
240
250
260
distance from the nozzle / mm
200
680
160
840
920[-
.10 12
210
-8
4 8 12 --4 0 distance from the beam axis / mm Fig. 9. Target density as a function of the distance from the nozzle in the range from 215 to 530 mm. Several profiles at various distances (230, 242, 254 and 530 mm) are shown. The line which connects the profiles results if the broadening only is due to geometrical divergence. On the right side of the figure the measured density decrease for a smaller range of nozzle distance (215-254 mm) is shown.
-4 0 4,/ 8 12 distance from the beam axis / mm
12 8 -4 4/ 0 distance from the beam axis / mm
0.
0.2
po = 4 bar
To = 290 K'
0.4
C02
W. Bickel et al. / Studies on a cluster target
52
In fig . 7 the clustering temperatures defined by the onset of clustering are displayed for different input pressures. The solid lines are fits to the data with a power-law dependence, which are within the lower limit pTK~cl-"? = resulting from an isentropic expansion ( const.) and the upper limit which is described by pT (1 .5K-1)J(1-K) = const. [6] . Although a unique description of all gases is not possible in the model of corresponding jets [6], the measurements can be used to determine the clustering conditions for other gases.
gen, which allow an easier investigation and analysis of the cluster formation and performance . Since scaling laws are found to apply rather well to the various parameters, the modifications of the setup for hydrogen, which is of major interest for nuclear-reaction studies, is straightforward. The only change concerns the pumping capacity which has to be increased appreciably. A hydrogen cluster target to be used at the future cooler synchrotron COSY at Jülich is presently under construction at the Institut fur Kernphysik in Munster .
3.3 . Stability of the cluster beam
In fig . 8 we show the background-corrected measurements of a N2 and a C02 cluster beam. The solid lines were fitted to the data with the assumption of a rectangular radial density distribution of the clusters which demonstrates the excellent stability of the cluster beam (the slight deviations can be attributed to the finite dimension of the electron beam which is in the range of 1 mm). Fig. 9 shows cluster beam profiles for various distances from the nozzle (230, 242, 254 and 530 mm) . The line connecting the different profiles would be obtained, if the broadening is only due to geometric divergence. As an inset in the figure, the measured density decrease in beam direction is shown for the range between 215 and 254 mm. Again, the straight line, calculated by assuming that the density decrease results only from the cluster beam divergence, describes the experimental data very well. So an enlargement of the flight path for the clusters, which is necessary for most cluster-target experiments, should not influence the quality of the cluster beam. 4. Application to hydrogein All the experiments discussed above have been performed with gases of a mass higher than that of hydro-
Acknowledgements We are greatly indebted to Leybold for kindly providing various turbomolecular pumps for test purposes. We would like to thank Mr. N. Mezin at CERN for manufacturing the excellent nozzles used in the cluster beam studies . We also thank Mr. H. Baumeister for continuous assistance and advice in the various stages of construction and assembling. This work was supported in part by the Bundesministeriurn fur Forschung and Technologie, the Gesellschaft fur Schwerionenforschung GSI at Darmstadt and the Forschungszentrum KFA at Jillich .
References [1] Workshop Internal targets for COSY, Jül-Spez-409 (1987) . [2] H. Dombrowski. Diplomarbeit, Institut für Kernphysik, Universität Münster (1989). [3] M. Wähning, Diplomarbeit, Institut für Kernphysik, Universität Münster (1988) . [4] G. Hólker, Diplomarbeit, Institut für Kernphysik, Universität Münster (1988). [5] O.F. Hagena, Z. Phys. D4 (1987) 291. [6] O.F. Hagena and W. Obert, J. Chem. Phys. 56 (1972) 1793.
A295 (1990) 44-52
North-Holland
Studies on a cluster target W. Bickel, M. Buschmann, H. Dombrowski, G. Gaul, D. Grzonka, G. Hölker, R. Santo and M. Wähning Institut für Kernphysik der Universität Münster, D-4400 Münster, FRG Received 13 February 1990 and in revised form 19 April 1990 Cluster beams of various gases have been produced by a versatile test setup, where the configuration of nozzles, skimmers and collimators has been studied and optimized. The parameters determining the cluster yield were investigated using elastic electron scattering . Functional dependences of the various quantities have been extracted . A detailed evaluation of the pumping capacities shows that, compared to existing cluster targets, much lower pumping capacities are required for gases of a mass higher than that of hydrogen . 1. Introductión Nuclear and elementary-particle reactions are predominantly performed with solid targets of various thicknesses adjusted to energy resolution and other experimental boundary conditions. Although requiring m.),-e technical effort, gas targets - in particular gas jet and cluster targets - have several advantages in comparison with solid targets, because they are chemically pure and do not alter during irradiation, they can tolerate large currents without deterioration and they CL- be varied down to thicknesses much smaller than achievable with solid targets. On the other hand, only a very limited number of materials is at disposal and the maximum target thicknesses are severely limited. With the advent of a new generation of storage rings La--n stochastic and electron cooling, there is a strong err°,:.r°st in suitable and variable internal targets. The advantage of storage-ring beams in conjunction with internal targets is the high luminosity which allows the measurement of very low cross sections or rare events . Internal target operation with a cooled beam requires, of course, low target densities, so that the energy loss of the circulating beam can at least partially be compensated . This completely excludes even the thinnest target foils available, which are in the range of some !Lg/cm'` . Different other solutions have been proposed [1] like dust targets, fibre targets or frozen-pellet targets. In the field of gaseous targets one has to distinguish between atomic-beam, gasjet and cluster targets . A simple source of gas used for atomic beams is a vessel at temperature To and pressure po with a small hole allowing the gas to flow into a region of pressure p < po. Such an effusive source yields a broad Maxwellian velocity distribution for the gas particles, where the
preparation of a gas beam is simply achieved by a collimator. The disadvantage of such a gas source is the very low target density of typical 109-10'1 atoms/cm2. To increase the density of the gas beam, nozzles have been used instead of a simple hole. With a nozzle of convergent-divergent structure, the principle of a Laval nozzle, one can obt,-.in a supersonic gas beam, which yields target densities in the range of 1015 atoms/cm2 close to the nozzle. The expansion of the gas through a nozzle further has the consequence of cooling the gas . Looking at the gas flow at the beginning, the gas atoms or molecules have a broad distribution of their relative velocities, but during the expansion through the nozzle the velocities assimilate and the velocity distribution shrinks, which can be described as a reduction of the temperature . In the course of further expansion the vapour pressure may be reached and this saturated gas starts clustering. If one follows the expanding gas in the nozzle one passes the different phases of saturation, supersaturation and nucleation where the clustering sets in. In this condensation process cluster sizes of 105-10 6 molecules can be achieved [1] . '_)
Fvrwrimontai r -. ""--- ..r çphin
2.1 . Cluster target
Fig . 1 shows the cluster target setup [2,3] . The target gas enters the nozzle after passing a cooling system consisting of a two-stage cold head (Leybold RG210). The temperature can be checked by a silicon diode temperature sensor (DT-450 from Lake Shore Cryotronics ire .), which is placed in the gas supply close to the
016&9002/90/$03.50 0) 1990 - Elsevier Science Publishers B.V . (North-Holland)
W. Bickel et al. / Studies on a cluster target
gas
scattering chamber 10cm cluster beam dump Fig. 1 . Setup of the cluster target.
nozzle. As nozzle we use a trumpet-shaped one with a throat diameter of 160 Wm machined at CERN . To prepare a fine cluster beam without loading the vacuum system of the scattering chamber with too
45
much gas, we apply different pressure stages . In the skimmer stage, most of the gas is peeled off from the clusters which are preferentially produced in the center of the beam . Then the beam passes the collimator, where an additional fraction of the gas is held back, and reaches the target position with the desired dimensions. These dimensions are controlled by varying collimator distance and opening. Then the cluster beam passes the scattering chamber, where the beam is subjected to
diagnostics . As a beam dump a 10001/s turbomolecular pump without a collimator to reduce the backflow is used. In this setup all critical components i .e . nozzle, skimmer and collimator are mounted on one unit so that they are always exactly aligned. To avoid a heating up of the cluster beam by the skimmer and to reduce the loss of cooling capacity, the skimmer support is surrounded by a tank with liquid nitrogen. Only a few percent of the gas expanding through the nozzle forms clusters and therefore nearly the complete gas volume has to be pumped off, which requires an enormous pumping capacity for the cluster target setup . Fig. 2 shows the pumping scheme of our setup and in table 1 the various pumps are summarized . The skimmer stage, where more than 9096 of the gas is peeled off, is equipped with two roots pumps with a pumping capacity of 500 m3 /h each, with two rotatingvane pumps with 200 m3 /h capacity as backing pumps . For the collimator stage we use a 10001/s turbomolecular pump, at which the pressure in the skimmer stage forms the low-vacuum side . The same concept, a 1000 1/s turbomolecular pump and the skimmer stage at the low-vacuum side, is realized at the beam dump. The scattering chamber is pumped by a 360 1/s turbomolecular pump with a 52 m3 /h rotating-vane backing pump . Furthermore, the region w the electron gun must be evacuated separately with a 360 1/s turbomolecular pump to get a sufficient low pressure. The electron gun and the scattering chamber are separated by a collimator of 2 mm diameter, which allows a pressure lower
Table 1 Pumping system for the cluster target Pumping stage
Pumps
Backing pump
Skimmer
roots pump : 2 x 500 m-'/'h (Leybold Ruvac WSU 500)
rota y, 3 2 x 200 m /h
Collimator
turbomolecular pump : 10001/s (Leybold Turbovac 1000)
skimmer stage
Scattering chamber
turbomolecular pump : 3601 ; s (Leybold Turbovac 360)
rotary, 52 m3/h
Beam dump
turbomolecular pump : 10001/s (Leybold Turbovac 1000)
skimmer stage
Typical pressure (G -=1000 Pa 1/s C02)
n v.vc n aa
7 x 10 3 Pa 1 X10-3 Pa
W. Bickel et al. / Studies on a cluster target
Fig. 2. Pumping scheme for the cluster target .
than 1 x 10-4 Pa at the electron gun, if the pressure in the scattering chamber is less than 1 x 10-1 Pa. 2.2. Beam diagnostics
The beam diagnostics is performed by elastic electron scattering. The advantage of this method compared with other methods is the simple availability of electrons and the hish sensitivity . which is only limited by the electron detector. r y r This system consists of an electron gun, a set of deflection plates to scan the target region with the electron beam, a detector for the elastic scattered electrons and a Faraday cup to stop the primary electron beam. As electron gun we use a compact module for standard TVs, operating with an accelerating voltage of 18 kV . This module, a Valpo-type A 61-120 W, contains an indirect heated Ba/Sr-oxide cathode and five internal electrodes for the extraction, the acceleration and
the focussing of the electrons . Contrary to the normal working conditions we had to put the whole module on negative high voltage . At typical operating conditions we get electron currents of 1-10 [LA for an electron beam diameter smaller than 1 mm at the target zone. With this system we can analyze the structure and the density distribution of the cluster beam. But also it can be used as a monitor system by fixing the electron beam at a position in the center of the target and control the density during an experiment. For applications of the systera in accelerator experiments the monitor system has to be operated automatically. For this purpose the diagnostic system is controlled by a personal computer with a specially designed NIM module. It contains digital-analog converters to set the required deflection voltage, and voltage-tofrequency converters which allow the simple measurement of pressure or other slowly varying parameters without using ADCs. The communication with the com-
W. Bickel et al. / Studies on a cluster target
47
Fig. 3. Setup of the analysis and monitor system with elastic electron scattering .
puter is made by a PC I/O-card with 48 I/O-channels and six counters . Fig. 3 shows a sketch of the setup for the diagnostics.
a) C0 2
To select the elastic-scattered electrons and suppress the background counts we use an electron spectrometer . This spectrometer is a double-focussing 90* sector
b) N2
Fig. 4. Density distribution of a carbon dioxide and a nitrogen gas beam at a flow rate of 500 Pa 1/s and a temperature of 290 K. The profiles perpendicular to the beam axis cover a range of 30 mm and the distance from the nozzle varies between 2 and 22 mm. (These measurements were performed with another setup, similar to fig. 1 without the cooling system, but with the possibility of measuring profiles close to the nozzle .)
W. Bickel et al. / Studies on a cluster target
48
magnet with a nominal trajectory radius of 80 mm [3] . As detector we use a plastic scintillator with photomultiplier readout or a channeltron . Comparing these two detectors, the channeltron has the advantage of a very low background level and all electrons, nearly independent of their energy, yield a pulse so that also low-energy electrons, which are produced in the target zone, can be selected with the spectrometer and detected with the channeltron . On the other hand, the channeltron 10-4 Pa) to work satisneeds a very low pressure (p < factorily. Therefore a further turbomolecular pump is needed for the channeltron chamber. The photomultiplier also needs a low pressure to avoid a breakthrough on the base, but there a pressure of p < 10-2 Pa is safficient. The absolute target densities are obtained by normal :ing the yield Ndet to the Rutherford cross section. The t,?-get layer PF, which results from an integration of the tarret density F_ v along the electron beam is given by VF -
1 Ndet
*10 3.2
N
15
a
2.8
0 2 .4 ~,
2. 1 .6
" b
1 .2 0.8 0 .4 0.
12
16
20
24
28
32 36 position / mm
1
c N;,, (da/dQ) 0 dû'
where E = detector efficiency, Ndet = number of detected electrons, Ni,, = number of electrons (from cup current), d2 = solid angle, (do/d2) 0 = Rutherford cross section for the elastic scattering of electrons with the energy E on a nucleus with atomic number Z at the scattering angle t$. The deviations from pure Rutherford scattering at 90' are less than 5ß'o. The efficiency of our detector system was determined by a comparison with a solid target of known thickness. For these measurements we used thin carbon foils of pdx -- 10 Etg/cm2. An extra check can be made by a double-slit unit which defines the interaction length seen by the spectrometer . Then, for a fixed pressure in the scattering chamber without a cluster beam the number of scattered electrons can be calculated. For the plastic scintillator with photomultiplier the detector efficiency is 80%, while for the channeltron only a value of - 10% is obtained. 3. Experimental investigations shnurc turn Aençity! Aiçtri}tntionç of the region -c from ? to 22 mm behind the nozzle for a C02 and a N2 gas beam at a temperature of 290 K. Both pictures are very similar and show a broad gas distribution. For nitrogen - room temperature no clustering is expected, but for carbon dioxide, due to the high condensation temperature of 195 K, clustering is possible with the nozzle used. However, the density distributions show no indication of clustering. In both profiles a steep decrease of the density in beam direction can be seen. The above measurements were made without any skimmer, b'
Fig
d,
Fig . 5. Profile of a gas beam 8 mm behind the nozzle and a cluster beam 8 mm behind the skimmer . In both cases C02 was used. In (a) the cluster beam is covered by the gas distribution but in (b) the gas is peeled off so that the cluster beam .led up by a factor of .becomes visible . (The axis in (b) is sc --100.) hence no separation of clusters from the gas was made. Recalling the fact that only a few percent of the gas undergoes clustering it is not surprising that no effects of clustering are detected in the measured density distributions . A completely changed situation arises if a skimmer is used whereby the nonclustered gas molecules are peeled off from the cluster beam and the vacuum in the scattering chamber is strongly improved so that the clusters can survive and become visible in the density profile . In fig. 5 we show two profiles of a gas beam and a cluster beam. In fig. 5a we measured a profile 8 mm below the nozzle (no skimmer mounted), and in fig. 5b a skimmer is used and the profile was again measured 8 mm below the skimmer. At first sight the, drastic difference in the widths of the two profiles can be seen. While the gas beam shows a broad distribution with a smooth decrease of the density, in the case of the cluster beam we observe a rapid change between the cluster beam and the background of the homogeneous gas density in the
W. Bickel et al. / Studies on a cluster target scattering chamber . Due to the fact that the scattering of two gas particles with equal masses leads to large scattering angles, the gas jet always yields a broad density distribution . When, on the other hand, a cluster of up to 106 atoms strikes a gas molecule, the average change in direction of the cluster is negligible. Therefore the clustering leads to a beam of nearly constant divergence, where the beam dimensions can be calculated in good approximation by geometrical considerations . 3.1 . Conditions for clustering
When preparing a cluster target, the correct adjustment of the various parameters of the experimental setup, like nozzle and skimmer form, kind of gas, input pressure and temperature is the basic requirement . Measurements of Gspann [11 and other authors have shown that the form of the nozzle is very crucial in order to get a well-defined target volume of high density. The form of the skimmer is also important for a satisfactory operation . The function of the skimmer is to peel off the gas from the clusters without disturbing the gas flow. To fulfill these requirements the deflection angle 8 of the skimmer should be as small as possible. An upper limit for 0 is given by the value where the attached shock wave is detached, which leads to a disturbance of the cluster beam. For a typical gas jet with a Mach number of - 10 we get a ®max of - 40 ° . On the other hand, for an effective pumping of the region below the skimmer the opening angle should be as large as possible. A large internal angle of the skimmer cone also helps to minimize the problem of perturbation of the skimmer flow by particles reflected from the inner skimmer surface . For the skimmers in operation we chose a deflection angle of 30 ° and were able to get a mechanically stable skimmer with an opening angle of 20 ° . For the collimator, angles of 35 ° and 25° were chosen . The distance between nozzle and skimmer was adjusted to a value of 20 mm, and for the distance between skimmer and collimator we have chosen a value of 40 rrim. The orifices of the skimmer and the collimator with diameters of 1 .2 mm and 3.6 mm have been chosen in such a way that at the target region, 260 mm below the nozzle, target densities in the range of 1013-1014 atoms/em2 can be achieved. Because of this geometry the target zone is coniiiieu to a uiametei of - 12 mm. Furthermore, the pressure in the different pumping stages, skimmer, collimator and scattering chamber, has to be sufficiently low, so that the clusters can reach the target zone without being destroyed by collisions with the residual gas. To test the possible vacuum conditions in the skimmer and collimator region for a stable cluster target, we performed measurements, where a CO2 cluster beam was prepared and a density profile perpendic-
49
ular to the beam direction was measured by elastic electron scattering. Then we increased the pressure in the skimmer or the collimator region by opening a needle valve and allowing the streaming in of carbon dioxide to simulate a reduced pumping capacity. For the skimmer, a sudden disappearance of the cluster beam at a pressure below 14 Pa in the skimmer chamber was observed (which is in agreement with the maximum pressure value of - 10 Pa given by Macri et al. [11). Similar results were obtained for the collimator stage. 3.2. Thermodynamic effects In fig. 6 we show the central target density as a function of the nozzle temperature for different gases. This measurements were performed at constant flow rate using a mass-flow controller (Brooks Instrument BV type 4250). The temperature dependence of the target density can be divided into two parts. At high temperatures no clustering is possible and we obtain a constant yield because the flow rate was kept fixed. At >, 100 -v 80 60 40 20 0
L__
I
14E
1
I
160
I
i
180
140
I
-- 1
160
1
I
200 temperature / K
i0 1
1
I
1
1Z- "1.
200 180 temperature / K
Fig. 6. The dependence of the central target density on the gas temperature for N2 at a gas flow of 780 Pa 1 //s . The solid ilne is a fit to the data for low temperatures up to -150 K with a power law (y = T-4.7) . For the temperature range from 150 to 180 K the data can be described by an exponential function (y = exp(0 .8T ), broken line).
W. Bickel et al. / Studies on a cluster target
50
low temperatures we get a drastic increase of the target density, which can only be explained by clustering effects. The solid lines in the figure represent fits to the data with a T-X power-law dependence for the lowtemperature part, which is expected from the scaling relations of Macri [11 and Hagena [5] based on an isentropic expansion of the gas. A rough description of the data is possible with such a power law, but in the region of the onset of clustering strong deviations are seen . In this temperature range the cluster yield depends exponentially on the temperature (fig. 6; broken line). In the very first stage of clustering, the mean size of the clusters is supposed to be very low so that the target density is not primarily determined by the isentrope but by the stability condition of a single cluster. The vapour pressure of a droplet is given by Pd ) - Mm 2a pRT r Pinf where pd, pinf =vapour pressure at the surface of a droplet and of the liquid, respectively, r = radius of the droplet, a = surface tension. Hence the probability of nucleation, J, will show an exponential temperature dependence, J - exp(-1/kT), which can explain the observed deviations from the power law. Furthermore, a minimum radius is needed to build a stable cluster so that the power law is only valid if this critical radius is exceeded .
Fig. 7. Clustering temperatures Tc, for different input pressures. The solid lines are fits to the data with a power-law dependence, which are within the upper and lower limits for the slope shown by the broken lines.
U
U
12
12
N 24
2
U
12
20
O
12
b
8 4 4 -12-8 -4 0 8 12 axis / mm distance to beam
0
-8 -4 0 4 8 12 distance to beam axis / mm
Fig. 8. Profiles of cluster beams from N2( PO = 4 bar, To =120 K) and C02( PO ~ 3 bar, To = 250 K). In this plots the background level is subtracted . The solid lines are fits to the data with the assumption of a rectangular radial density distribution .
210
220
230
240
250
260
distance from the nozzle / mm
200
680
160
840
920[-
.10 12
210
-8
4 8 12 --4 0 distance from the beam axis / mm Fig. 9. Target density as a function of the distance from the nozzle in the range from 215 to 530 mm. Several profiles at various distances (230, 242, 254 and 530 mm) are shown. The line which connects the profiles results if the broadening only is due to geometrical divergence. On the right side of the figure the measured density decrease for a smaller range of nozzle distance (215-254 mm) is shown.
-4 0 4,/ 8 12 distance from the beam axis / mm
12 8 -4 4/ 0 distance from the beam axis / mm
0.
0.2
po = 4 bar
To = 290 K'
0.4
C02
W. Bickel et al. / Studies on a cluster target
52
In fig . 7 the clustering temperatures defined by the onset of clustering are displayed for different input pressures. The solid lines are fits to the data with a power-law dependence, which are within the lower limit pTK~cl-"? = resulting from an isentropic expansion ( const.) and the upper limit which is described by pT (1 .5K-1)J(1-K) = const. [6] . Although a unique description of all gases is not possible in the model of corresponding jets [6], the measurements can be used to determine the clustering conditions for other gases.
gen, which allow an easier investigation and analysis of the cluster formation and performance . Since scaling laws are found to apply rather well to the various parameters, the modifications of the setup for hydrogen, which is of major interest for nuclear-reaction studies, is straightforward. The only change concerns the pumping capacity which has to be increased appreciably. A hydrogen cluster target to be used at the future cooler synchrotron COSY at Jülich is presently under construction at the Institut fur Kernphysik in Munster .
3.3 . Stability of the cluster beam
In fig . 8 we show the background-corrected measurements of a N2 and a C02 cluster beam. The solid lines were fitted to the data with the assumption of a rectangular radial density distribution of the clusters which demonstrates the excellent stability of the cluster beam (the slight deviations can be attributed to the finite dimension of the electron beam which is in the range of 1 mm). Fig. 9 shows cluster beam profiles for various distances from the nozzle (230, 242, 254 and 530 mm) . The line connecting the different profiles would be obtained, if the broadening is only due to geometric divergence. As an inset in the figure, the measured density decrease in beam direction is shown for the range between 215 and 254 mm. Again, the straight line, calculated by assuming that the density decrease results only from the cluster beam divergence, describes the experimental data very well. So an enlargement of the flight path for the clusters, which is necessary for most cluster-target experiments, should not influence the quality of the cluster beam. 4. Application to hydrogein All the experiments discussed above have been performed with gases of a mass higher than that of hydro-
Acknowledgements We are greatly indebted to Leybold for kindly providing various turbomolecular pumps for test purposes. We would like to thank Mr. N. Mezin at CERN for manufacturing the excellent nozzles used in the cluster beam studies . We also thank Mr. H. Baumeister for continuous assistance and advice in the various stages of construction and assembling. This work was supported in part by the Bundesministeriurn fur Forschung and Technologie, the Gesellschaft fur Schwerionenforschung GSI at Darmstadt and the Forschungszentrum KFA at Jillich .
References [1] Workshop Internal targets for COSY, Jül-Spez-409 (1987) . [2] H. Dombrowski. Diplomarbeit, Institut für Kernphysik, Universität Münster (1989). [3] M. Wähning, Diplomarbeit, Institut für Kernphysik, Universität Münster (1988) . [4] G. Hólker, Diplomarbeit, Institut für Kernphysik, Universität Münster (1988). [5] O.F. Hagena, Z. Phys. D4 (1987) 291. [6] O.F. Hagena and W. Obert, J. Chem. Phys. 56 (1972) 1793.