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Study of radiative capture reactions with radioactive ion beams
Nuclear Instruments and Methods in Physics Research A 418 (1998) 355—364
Study of radiative capture reactions with radioactive ion beams K.E. Rehm *, C.L. Jiang , M. Paul, D. Blumenthal , L.A. Daniel, C.N. Davids , P. Decrock , S.M. Fischer , D. Henderson , C. Lister , J. Nickles, J. Nolen , R.C. Pardo , J.P. Schiffer , D. Seweryniak , R.E. Segel Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA Hebrew University, Jerusalem, 91904 Israel University of Wisconsin, Madison, WI 53706, USA Northwestern University, Evanston, IL 60208, USA Received 9 January 1998; received in revised form 8 June 1998
Abstract A technique for separating and identifying reaction products from radiative capture reactions induced by radioactive ion beams has been developed. The F(p,c)Ne and O(p,c)F reactions have been measured in inverse kinematics by identifying the F and Ne reaction products with respect to mass and charge at forward angles in the Fragment Mass Analyzer. A total detection efficiency of 30% and a suppression factor for the incident beam relative to the (p,c) reaction products of better than 10:1 was achieved. Details of the detection technique, the beam monitoring procedure as well as experience in the use of polypropylene targets in experiments with radioactive beams are discussed. 1998 Published by Elsevier Science B.V. All rights reserved. PACS: 25.40.Lw; 25.60.-t; 29.30.-h Keywords: Radiative capture reactions; Radioactive ion beams; FMA
1. Introduction Radiative capture reactions play an important role in the synthesis of elements in stars [1]. In the CNO cycle, which is a sequence of (p,c), (p,a) reactions and b> decays, [C(p,c) N(b>)C(p,c)N(p,c) O(b>)N(p,a)C] providing the energy in the burning of second generation stars with masses
* Corresponding author. Tel.: #1 630 252 4073; #1 630 252 6210; e-mail: [email protected].
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heavier than the mass of the sun, the three (p,c) reactions contribute more than 60% to the total energy production. At higher temperatures (p,c) and (a,c) reactions e.g. Ne(p,c)Na or O(a, c)Ne, provide a path for the production of heavier elements via the so-called rapid-proton (rp) process [2] which occurs in nova and supernova explosions and in X-ray bursts [3]. Finally, in the r-process, a series of (n,c) reactions followed by b\ decays, the heavier elements are formed [1]. Experimental studies of radiative capture reactions induced by charged particles have been
0168-9002/98/$19.00 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 8 8 5 - 7
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studied extensively in the past, mainly using gamma detectors. The low efficiency of this technique combined with the small cross sections, especially at low bombarding energies, requires the use of high-intensity proton or a-beams. As shown in various network calculations [3] the majority of the radiative capture reactions of interest to astrophysics involves nuclei with half-lifes that are too short for the production of suitable targets which could be used in proton or a-induced reactions. Some of these unstable nuclei, however, are now available as beams at radioactive beam facilities [4] which opens up the possibility of using Coulomb breakup [5] or reactions with inverse kinematics [p(X,c)] on hydrogen targets [6] for radiative capture studies. Because at the current generation of radioactive beam facilities these secondary beams are typically 3—6 orders of magnitude weaker than stable ion beams, new detection techniques with higher efficiencies have to be developed. An additional difficulty in experiments with radioactive beams is the presence of isobaric beam impurities which sometimes can be much more intense than the ion beam of interest [7] and can lead to considerable background. In a recent experiment [8] studying the p(Ne,Na)c reaction at Louvain-la-Neuve, a specific property of the residual Na nuclei was utilized to increase the detection efficiency. Some of the excited states in Ne populated in the b> decay of Na are unstable with respect to a emission into O. By detecting these a particles a detection efficiency of &1% was achieved. Other techniques which have been investigated employ a solenoidal field for separating the positrons from the decay of the beam particles and the (p,c) reaction products, respectively [9]. In this contribution we describe the use of a recoil mass separator for the detection of the reaction products produced in a radiative capture reaction studied in inverse kinematics. Such recoil mass separators [10] which consist of a combination of electric and magnetic fields have been used in many experiments for measurements of processes with small cross sections, ranging from proton decay studies to the search for superheavy nuclei. Similar separators consisting of a Wien filter and a magnetic dipole have been used for radiative capture
studies involving stable C beams [11,12]. The experiment described below studied the p(F, Ne)c reaction at energies around 650 keV/u. It is the first study where a recoil separator has been used in radiative capture experiments involving low-intensity radioactive beams [13]. The beam production technique has been described previously in Ref. [14].
2. Description of the technique Because of the small momentum of the outgoing c-ray, the products of inverse-kinematics capture reactions are emitted within a narrow forward cone which makes separation from the incident beam very difficult. For a 13.4 MeV F beam the opening cone for the Ne ions produced via the p(F,Ne)c reaction is &0.59°, while the halfangle for small-angle scattering in a 100 lg/cm CH target is &0.21°. Since the momentum p of the Ne reaction products is identical to that of the incident F beam, a magnetic field which separates the particles according to p/q cannot be used for separating the particles of interest. For an incident energy of 13.4 MeV the energy and velocity of the Ne reaction products after the target (E"11.2$0.2 MeV, v/c"0.0355$0.0003), however, are sufficiently different from the corresponding values of the F particles (E"12.0 MeV, v/c"0.0378) that an electric dipole or a Wien filter [11] can be used to spatially separate the radiative capture products from the primary beam. In our experiment an electric dipole has been used. Because of the low probability for the Ne> charge state at these energies the technique of using fully stripped ions cannot be applied for particle separation. The reaction products were analyzed by the Fragment Mass Analyzer (FMA) [15] installed at the superconducting heavy ion accelerator ATLAS. The FMA consists of two magnetic quadrupole doublets, two electric dipoles and one magnetic dipole as shown in Fig. 1. With an angular acceptance of $2.3° and an energy acceptance of $20%, it completely covers the energy and angle range expected for the Ne recoil products from the F(p,c) experiment.
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Table 1 Energies, kinematic opening angles, small angle scattering angles and energy widths used as input parameters in the GIOS [16] calculations Fig. 1. Schematic layout of the ion-optical elements of the FMA used for a measurement of the inverse F(p,c)Ne reaction. (TGT: target station, Q: quadrupole, ED: electric dipole, MD: magnetic dipole, DET: focal plane detector. The overall length is 8.2 m.)
Particle
E (MeV)
H (°)
H (°)
H (°)
*E (keV)
O F
13.40 11.38
0.0 0.68
0.18 0.21
0.18 0.71
27 160
2.1. Test measurements of the p(O,F)c reaction Before the actual experiment with F beams, test measurements with stable O beams were performed. The separation of the reaction products from the incident beam particles was first studied with the ion-optics code GIOS [16]. This code was modified to include energy and small-angle straggling as well as the effects of a finite beam spot size. The transport calculations were performed for an O beam of E "13.4 MeV bombarding a 100 lg/cm CH target. The input parameters for the GIOS calculations are summarized in Table 1. The centroids of the trajectories for O> and F> ions are separated after the first electric dipole by 26 mm (Fig. 2). Installing a collimator at this location will stop the dominant fraction of the primary beam before it reaches the magnetic dipole. The other charge states (also shown schematically in Fig. 2) are deflected to larger (8>) or smaller (6>) angles and are stopped either at the collimator or at the anode plate of the first electric dipole. At the focal plane of the FMA the particles are dispersed according to m/q and are detected by an xy-position sensitive parallel grid avalanche counter (PGAC) [15] followed by a large-volume ionization chamber (IC) equipped with a 5;5 cm Si detector. This setup provides x—y position signals for m/q determination, two *E signals (from the PGAC and the IC, respectively) and an E sig0 nal for Z-identification. Fig. 3a shows a plot of energy loss versus residual energy, *E—E , mea0 sured with the ionization chamber for an incident O beam at E "16 MeV which corresponds to an energy slightly above the 1/2> resonance at E "8.971 MeV in F. The solid lines are simulaV tions for the expected *E—E curves for O and 0
Fig. 2. Envelopes for the trajectories of O> > > and F> ions in the first part of the FMA calculated with the ion-optics code GIOS [16] using the input parameters summarized in Table 1. The horizontal dimensions in this figure are strongly compressed.
F, respectively, taking the dimensions of the IC and the energy loss in the two entrance foils into account. The F reaction products from the p(O,F)c reaction are well separated from the O background. These O events are caused by the scattering of the incident O beam at various locations in the FMA (e.g., the anode plate of the first electric dipole). The suppression factor for the incident beam relative to the F reaction products measured in these experiments was better than 10:1. Changing the incident energy to E "13.4 MeV results in a *E—E spectrum which is shown in 0 Fig. 3b. The yield for the F events is considerably reduced but is still clearly visible. The main contribution to the yield at this energy is attributed to a small fluorine contamination in the targets which were produced by stretching thin polyethylene foils
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Fig. 3. a) *E—E spectrum measured at the focal plane of the 0 FMA for the O(p,c)F reaction at E "16 MeV. This en ergy is close to the 1/2> resonance at E "8.791 MeV in F. V The solid lines are the result of detector simulations of *E versus E for different ions. (b) *E—E spectrum measured at the 0 0 focal plane of the FMA for the O(p,c)F reaction at E "13.4 MeV.
over a heated teflon piston which contains fluorine material. Fluorine impurities have also been observed in measurements of other (p,c) reactions [17]. The recoil F ions from the fluorine impurities in the angular range h"0—2.3° (produced by backward-scattered O particles) have an integrated cross section of &3 mb which is considerably larger than typical (p,c) cross sections which are of the order of tens of lb. Thus, even small amounts of impurities can influence low cross section measurements. This is a general problem for radiative capture measurements in inverse kinematics when the reaction product is stable and might be present as an impurity in the target. Ions of F can also be produced via the O(d,n)F reaction on the small (0.015%)
deuterium content in the CH target. With typical cross sections of &10 mb/sr [18] for this reaction one calculates background yields corresponding to a cross section of &2 lb. This is about a factor of 10 smaller than the background caused by the F recoil ions caused by the fluorine impurity in the target. Another source of F background that was discovered in these experiments originates from impurities in the incident beam. An electrical discharge in the ion source of the tandem accelerator which is set nominally for injection of O\ particles can cause F\ ions to be transmitted for a short time through the injection magnet. Through charge-exchange processes with the residual gases in the high-energy section of the tandem, F particles with the same magnetic rigidity, i.e. an energy of ;E(O) can be produced. Since this is the exact energy of the (p,c) reaction products, the F particles are transmitted with high efficiency to the focal plane of the FMA and identified as F in the ionization chamber. This effect was visible through an increased count rate at the appropriate energy in the monitor spectra and could be easily eliminated. These background reactions pose no problem when the particles of interest are unstable, e.g. in the study of the inverse p(F,Ne)c or p(N,O)c [6] reactions. Other reactions involving inverse reaction kinematics e.g. p(N,O)c or p(Ne,Na)c, resulting in stable reaction products which might be present (e.g. as water or from the parting agent) as trace elements in the target require special considerations. 2.2. Charge-state corrections The collimator installed after the first electric dipole of the FMA restricts the transmission to the focal plane to a single-charge state. In order to obtain the total cross section this yield has to be divided by the corresponding charge-state fraction. We have performed two measurements of the charge-state distribution for F ions with energies of &16 MeV. The first used F recoil ions produced by bombarding a CaF target with an O beam and detecting the mass 19 recoil particles at h "13°. The second experiment measured the F yield from the p(O,F)c reaction at the
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Fig. 4. Charge-state distribution measured for F ions at E "16 MeV. The lines are taken from calculations given in Refs. [19,20].
1/2> resonance (E "8.791 MeV) in F. Fig. 4 V gives the result of these measurements compared with the theoretical predictions from Ref. [19] (solid line) and Ref. [20] (dashed line). Similar to the results obtained in Ref. [11], we observe that the F (p,c) yield at the maximum of the chargestate distribution is slightly higher than predicted by the theoretical models. The deviations for the most probable charge state are, however, less than 10%. This agreement justifies the use of calculated charge-state fractions in cases where a full chargestate distribution has not been measured. In addition to the charge-state fractionation the transport efficiency to the FMA has to be considered. This efficiency was calculated with the code GIOS [16] taking into account the acceptance limitations imposed by the 1.7 cm wide collimator installed after the first electric dipole. An overall efficiency for detecting F ions in the focal plane of the FMA of 30$5% is obtained including the efficiency of the PGAC and the IC [13].
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target thicknesses. In our experiment we have used solid polypropylene (CH ) targets with thicknesses L between 60—150 lg/cm. While targets of polyimid [(C H N O ) ] or Mylar [(C H O ) ] show L L better stability under heavy-ion bombardment, the high (2:1) H:C ratio and the lack of other elements in its chemical composition, which can cause background reactions with the incident beam, led to the choice of polypropylene as target. However, traces of contaminants, e.g. oxygen from water and fluorine from the target production technique, were present in small amounts in the material. The targets were produced by stretching 2.3 mg/cm thick polypropylene material over a heated teflon piston to thicknesses down to 60 lg/cm. The absolute target thickness was determined through an energy-loss measurement using 5.4 MeV a particles. The homogeneity of the target area (diameter 12 mm) was found to be better than 10%. The main difficulty with these targets is their tendency to lose hydrogen at beam currents above 0.5—1 pnA of O. The hydrogen content of the targets was therefore continuously monitored by detecting H recoil ions in two monitor detectors mounted at $30°. Fig. 5 shows an energy spectrum measured in a Si detector at 30° with a CH
3. Hydrogen target The low bombarding energies (600—700 keV/u) of the experiments prevented the use of a hydrogen gas cell with windows. Windowless gas targets, on the other hand, can achieve only relatively low
Fig. 5. Energy spectrum measured with the monitor detector at h "30° for a 13.4 MeV O beam bombarding a 90 lg/cm CH target.
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target and an O beam at E "13.4 MeV. The various groups are caused by elastically scattered O ions, by H and C recoils and by alphas from the H(O,a)N reaction and a weak summing peak at channel number 1200. The hydrogen content of the CH target can be determined from the ratio of the hydrogen and carbon recoil peaks. The H:C ratio of the target can be calculated from the proton and C recoil yields N(H) and N(C) and the elastic scattering cross sections dp! and dp& , respectively, by N(H)dp! , (1) H/C" N(C)dp& which for pure Rutherford scattering can be simplified to
N(H) Z M (M #M ) N(H) ! & ! " ) 0.631. H/C" N(C) Z M (M #M ) N(C) & ! & (2) It should be noted that Eq. (2) is independent of the scattering angle. Because in our case, at the energies of interest, the p#O system shows strong resonances in the excitation function for elastic scattering [21], the actual cross sections for dp& were used for determining the H:C ratio. The elastic dp! cross sections were taken as the Rutherford values. For this the actual scattering angle of the monitor detector has to be known. It can be determined from the ratio of the oxygen and carbon peaks in the monitor spectrum. Because at these low energies the cross section for elastic scattering of O on C is given by the Rutherford value, the ratio of the yields between the O and C peaks is independent of the incident energy and depends only on the scattering angle h (in the laboratory system) via N(O) k cosh " N(C) 4 sin(h#sin\(k sin h)/2) ;
(cos h#(1/k!sinh) (1/k!sinh
(3)
with M(O) k" . M(C)
(4)
At a scattering angle of 30° this ratio changes by &15% for a change of 1° in h. Since the statistical
Fig. 6. Change in the hydrogen to carbon ratio observed over a time period of 12 h when bombarding a 140 lg/cm CH target with 1—3 enA O beams.
uncertainties allow the determination of this ratio to about 1%, the scattering angle can be determined with an accuracy of about 0.1°. To keep the hydrogen losses in the polypropylene targets to a minimum we have used a target wobbler which distributed the beam spot (typical diameter 2 mm) over a circle with a radius of &5 mm. The target thickness was scanned before and after the experiment with an alpha source. The maximum change in thickness observed in these experiments was 20%. For a 140 lg/cm CH tar get this uncertainty in thickness translates into an uncertainty in the center-of-mass energy for the p(FNe)c reaction of about 2 keV. The change in composition of a 140 lg/cm polypropylene target which was bombarded with an O beam is shown in Fig. 6. The H/C ratio decreases from the initial value of 2 to about 1.5 over a period of about 12 h. The absolute energy of the incident beam was determined by scattering O ions from two Au targets with thicknesses of 200 and 90 lg/cm and detecting the O particles in the focal plane of the Enge Split Pole spectrograph which was calibrated with a Th a source. Extrapolating to zero target thickness the energy of the incident O beam was found to agree to within 42 keV with the value of 13 400 keV predicted from the 90° bending magnet setting. In the center-of-mass frame for the O#p system this translates into an uncertainty of 2.2 keV.
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4. Beam monitoring While a silicon detector can be used to monitor the hydrogen content of the CH target it cannot differentiate between the incident O and F particles. In the F(p,a)O experiment [22] Z-identification of the mass 18 particles was achieved using the method of a gas-filled magnet [23]. Since this was not possible for the (p,c) experiment, a circular Al ring was installed about 150 mm after the CH target covering the angular range from 3.6° to 9.7°. Elastically scattered F were implanted into this ring and the accumulated 511 keV activity was measured with a calibrated Ge detector for each F run. The time dependence of the F beam was monitored with a Ge detector located in close vicinity of the collimator after the first electric dipole where most of the incident beam was intercepted. Fig. 7 shows the time dependence of the 511 keV annihilation radiation measured with this detector for one F sample. As will be shown below, the total number of incident F ions can be obtained from a measurement of the time dependence of the 511 keV annihilation and from a measurement of the F activity measured at the end of the experi-
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ment in the catcher ring. Also shown is the time dependence of the monitor counting rate, which mainly reflects the intensity of the isobar impurity O. The time unit in Fig. 7 corresponds to 1 min. One observes a general exponential decrease caused by the b> decay of F and by sputtering losses of F material in the ion source. The arrows indicate the times when the sample in the ion source was rotated in order to reach a fresh spot for the sputtering process [14]. For a calculation of the total number N of F ions incident on the target in a time interval ranging from t Pt we consider the number of ions which are scattered onto the catcher ring, N , and the collimator after the electric dipole, N , respec tively. They are related via geometrical efficiency factors to the total number of incident F ions. While the efficiency at the collimator requires a detailed knowledge of the transport properties of the FMA and the charge-fractionation effects, the efficiency for collecting F ions on the catcher ring, e , can be calculated from the geometry of the circular catcher and the Rutherford cross section. Since the collimator and the catcher are exposed to the same (time-dependent) beam of F, the
Fig. 7. (top) Time dependence of the 511 keV annihilation radiation observed with a Ge detector located close to the collimator after the first electric dipole of the FMA. (bottom) Time dependence of the count rate measured in the monitor detector. The arrows indicate times when the sample in the ion source was rotated.
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activities on the collimator and the catcher ring A and A , respectively, measured at the end of the experiment (t"t ) are related to the total num ber of accumulated F ions via A (t ) N " . A (t )!A (t ) e\HR\R N The total number of incident particles N is M N A (t ) . N " ) e A (t )!A (t ) e\HR\R P
(5)
(6)
By solving the differential equation for production and decay of F ions which are collected on the collimator, one obtains
R 1 A (t) dt# [A (t )!A (t )], (7) j R where the integral gives the number of ions which decayed during the accumulation time between t and t , A (t )/j is the number of particles which decay from t"t PR, and A (t )/j is the num ber of atoms present at the beginning of the experiment. All parameters in Eqs. (5) and (6) are measured quantities and thus N can be calculated. Together with the number of O particles from the monitor detector a time-averaged ratio of O/F of 2500:1 has been determined. This value agrees well with the results measured in our (p,a) studies [7] where the time dependence of O/F ratio could be determined from a direct measurement of elastically scattered O and F particles. N "
5. Experimental results with a F beam The technique used for the production of the F samples is described in detail in Ref. [14]. F was produced by bombarding O water with a 11.4 MeV proton beam from the medical cyclotron at the University of Wisconsin. After chemical separation, the F material was deposited on a cathode insert for a negative ion source (SNICS from the National Electrostatic Corporation), flown to Argonne and installed in the ion source of the tandem accelerator which is one of the injectors
of the ATLAS accelerator. The choice of solid F material compared to gaseous fluorine compounds (e.g. CF H) was based on the observation that the chemically very active fluorine can be kept at a well defined spot while gaseous material is distributed over the whole interior of the source. The extraction efficiency measured with a solid test sample of F was in the range 0.1—1% while with gaseous CF H only 3;10\% has been observed [22]. Five samples of F were prepared and measured at an energy of E "670 keV (calculated for the center of the 90 lg/cm target foil) with the setup and the techniques described above. The energy loss of a F beam in the target is 70 keV and thus includes the dominant part of the 3/2> resonance in Ne at E "652 keV. The *E—E spectrum sum 0 med over these five samples representing a total integrated charge of 2.8 pnC is shown in Fig. 8. Although this energy is off-resonance for the p(O,F)c reaction a considerable yield of F is observed in this summed spectrum. The origin of this F background was discussed in Section 2.1. While the average energy of Ne ions is identical to the F energy, the area in the *E—E spectrum 0 is shifted to lower E energies by about 1 MeV. The 0 shape and size of the region shown by the ellipse in Fig. 8a is based on the p(O,F)c reaction, shown in Fig. 3a, where the ellipse contains more than 95% of the F events. The solid lines in Fig. 8a are the expected *E—E curves calculated for the present detector geometry. Due to the considerably higher yields for F and O there are three events located in the area of interest which show *E values intermediate between Z"9 and 10 particles. However, an independent measurement of the energy loss *E in the PGAC (see Fig. 8b) shows that all three events coincide with the maximum of the F distribution. We, therefore, conclude that none of these events has a clear signature for being caused by a Ne particle produced via the F(p,c) reaction, but rather seem to be events from the tail of the energy loss distribution of F ions. From these events an upper limit for the F(p,c)Ne reaction of 42 lb has been calculated. The implication of this result for the breakout from the hot CNO cycle has been discussed in Ref. [13].
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some cases be difficult. For the radiative capture reaction p(F,Ne)c a recoil mass separator was used as a detection device. A suppression factor for the incident beam relative to the (p,c) reaction products of 10 was achieved. Since the F beams had a considerable contamination from the neighbouring stable isobar O, a Z identification was required in order to determine the integrated incoming beam. In the experiment this was accomplished by collecting scattered F particles on a catcher ring and measuring the accumulated c activity after the run. With a low-intensity F beam (typically &10 F/s) it was possible to set an upper limit for the p(F,Ne)c reaction of about 40 lb at the 3/2> resonance at E "652 keV in Ne.
Acknowledgements We want to thank J. Greene for his help with the target production and the thickness measurements. This work was supported by the U.S. Department of Energy, Nuclear Physics Division under Contract No. W-31-109-ENG-38, the National Science Foundation and by a University of Chicago/Argonne National Laboratory Collaborative Grant.
Fig. 8. (a) *E—E spectrum measured in the focal plane of the 0 FMA for the p(F,Ne)c reaction at E "13.4 MeV with a beam that contains a considerable O contamination. The area where Ne events are expected is encircled. The solid lines are the result of simulations of *E versus E for different ions. (b) 0 *E signals measured in the focal plane counter of the FMA for the three events inside the Ne region in comparison with measured F and O events. The dashed line is the expected Ne distribution extrapolated from the O and F data.
6. Summary Experiments with radioactive ion beams in inverse kinematics require the development of new detection systems. While these studies have the benefit of increased detection efficiency due to the c.m. to lab conversion factor, the separation of the reaction products from the incident beam can in
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Study of radiative capture reactions with radioactive ion beams K.E. Rehm *, C.L. Jiang , M. Paul, D. Blumenthal , L.A. Daniel, C.N. Davids , P. Decrock , S.M. Fischer , D. Henderson , C. Lister , J. Nickles, J. Nolen , R.C. Pardo , J.P. Schiffer , D. Seweryniak , R.E. Segel Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA Hebrew University, Jerusalem, 91904 Israel University of Wisconsin, Madison, WI 53706, USA Northwestern University, Evanston, IL 60208, USA Received 9 January 1998; received in revised form 8 June 1998
Abstract A technique for separating and identifying reaction products from radiative capture reactions induced by radioactive ion beams has been developed. The F(p,c)Ne and O(p,c)F reactions have been measured in inverse kinematics by identifying the F and Ne reaction products with respect to mass and charge at forward angles in the Fragment Mass Analyzer. A total detection efficiency of 30% and a suppression factor for the incident beam relative to the (p,c) reaction products of better than 10:1 was achieved. Details of the detection technique, the beam monitoring procedure as well as experience in the use of polypropylene targets in experiments with radioactive beams are discussed. 1998 Published by Elsevier Science B.V. All rights reserved. PACS: 25.40.Lw; 25.60.-t; 29.30.-h Keywords: Radiative capture reactions; Radioactive ion beams; FMA
1. Introduction Radiative capture reactions play an important role in the synthesis of elements in stars [1]. In the CNO cycle, which is a sequence of (p,c), (p,a) reactions and b> decays, [C(p,c) N(b>)C(p,c)N(p,c) O(b>)N(p,a)C] providing the energy in the burning of second generation stars with masses
* Corresponding author. Tel.: #1 630 252 4073; #1 630 252 6210; e-mail: [email protected].
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heavier than the mass of the sun, the three (p,c) reactions contribute more than 60% to the total energy production. At higher temperatures (p,c) and (a,c) reactions e.g. Ne(p,c)Na or O(a, c)Ne, provide a path for the production of heavier elements via the so-called rapid-proton (rp) process [2] which occurs in nova and supernova explosions and in X-ray bursts [3]. Finally, in the r-process, a series of (n,c) reactions followed by b\ decays, the heavier elements are formed [1]. Experimental studies of radiative capture reactions induced by charged particles have been
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studied extensively in the past, mainly using gamma detectors. The low efficiency of this technique combined with the small cross sections, especially at low bombarding energies, requires the use of high-intensity proton or a-beams. As shown in various network calculations [3] the majority of the radiative capture reactions of interest to astrophysics involves nuclei with half-lifes that are too short for the production of suitable targets which could be used in proton or a-induced reactions. Some of these unstable nuclei, however, are now available as beams at radioactive beam facilities [4] which opens up the possibility of using Coulomb breakup [5] or reactions with inverse kinematics [p(X,c)] on hydrogen targets [6] for radiative capture studies. Because at the current generation of radioactive beam facilities these secondary beams are typically 3—6 orders of magnitude weaker than stable ion beams, new detection techniques with higher efficiencies have to be developed. An additional difficulty in experiments with radioactive beams is the presence of isobaric beam impurities which sometimes can be much more intense than the ion beam of interest [7] and can lead to considerable background. In a recent experiment [8] studying the p(Ne,Na)c reaction at Louvain-la-Neuve, a specific property of the residual Na nuclei was utilized to increase the detection efficiency. Some of the excited states in Ne populated in the b> decay of Na are unstable with respect to a emission into O. By detecting these a particles a detection efficiency of &1% was achieved. Other techniques which have been investigated employ a solenoidal field for separating the positrons from the decay of the beam particles and the (p,c) reaction products, respectively [9]. In this contribution we describe the use of a recoil mass separator for the detection of the reaction products produced in a radiative capture reaction studied in inverse kinematics. Such recoil mass separators [10] which consist of a combination of electric and magnetic fields have been used in many experiments for measurements of processes with small cross sections, ranging from proton decay studies to the search for superheavy nuclei. Similar separators consisting of a Wien filter and a magnetic dipole have been used for radiative capture
studies involving stable C beams [11,12]. The experiment described below studied the p(F, Ne)c reaction at energies around 650 keV/u. It is the first study where a recoil separator has been used in radiative capture experiments involving low-intensity radioactive beams [13]. The beam production technique has been described previously in Ref. [14].
2. Description of the technique Because of the small momentum of the outgoing c-ray, the products of inverse-kinematics capture reactions are emitted within a narrow forward cone which makes separation from the incident beam very difficult. For a 13.4 MeV F beam the opening cone for the Ne ions produced via the p(F,Ne)c reaction is &0.59°, while the halfangle for small-angle scattering in a 100 lg/cm CH target is &0.21°. Since the momentum p of the Ne reaction products is identical to that of the incident F beam, a magnetic field which separates the particles according to p/q cannot be used for separating the particles of interest. For an incident energy of 13.4 MeV the energy and velocity of the Ne reaction products after the target (E"11.2$0.2 MeV, v/c"0.0355$0.0003), however, are sufficiently different from the corresponding values of the F particles (E"12.0 MeV, v/c"0.0378) that an electric dipole or a Wien filter [11] can be used to spatially separate the radiative capture products from the primary beam. In our experiment an electric dipole has been used. Because of the low probability for the Ne> charge state at these energies the technique of using fully stripped ions cannot be applied for particle separation. The reaction products were analyzed by the Fragment Mass Analyzer (FMA) [15] installed at the superconducting heavy ion accelerator ATLAS. The FMA consists of two magnetic quadrupole doublets, two electric dipoles and one magnetic dipole as shown in Fig. 1. With an angular acceptance of $2.3° and an energy acceptance of $20%, it completely covers the energy and angle range expected for the Ne recoil products from the F(p,c) experiment.
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Table 1 Energies, kinematic opening angles, small angle scattering angles and energy widths used as input parameters in the GIOS [16] calculations Fig. 1. Schematic layout of the ion-optical elements of the FMA used for a measurement of the inverse F(p,c)Ne reaction. (TGT: target station, Q: quadrupole, ED: electric dipole, MD: magnetic dipole, DET: focal plane detector. The overall length is 8.2 m.)
Particle
E (MeV)
H (°)
H (°)
H (°)
*E (keV)
O F
13.40 11.38
0.0 0.68
0.18 0.21
0.18 0.71
27 160
2.1. Test measurements of the p(O,F)c reaction Before the actual experiment with F beams, test measurements with stable O beams were performed. The separation of the reaction products from the incident beam particles was first studied with the ion-optics code GIOS [16]. This code was modified to include energy and small-angle straggling as well as the effects of a finite beam spot size. The transport calculations were performed for an O beam of E "13.4 MeV bombarding a 100 lg/cm CH target. The input parameters for the GIOS calculations are summarized in Table 1. The centroids of the trajectories for O> and F> ions are separated after the first electric dipole by 26 mm (Fig. 2). Installing a collimator at this location will stop the dominant fraction of the primary beam before it reaches the magnetic dipole. The other charge states (also shown schematically in Fig. 2) are deflected to larger (8>) or smaller (6>) angles and are stopped either at the collimator or at the anode plate of the first electric dipole. At the focal plane of the FMA the particles are dispersed according to m/q and are detected by an xy-position sensitive parallel grid avalanche counter (PGAC) [15] followed by a large-volume ionization chamber (IC) equipped with a 5;5 cm Si detector. This setup provides x—y position signals for m/q determination, two *E signals (from the PGAC and the IC, respectively) and an E sig0 nal for Z-identification. Fig. 3a shows a plot of energy loss versus residual energy, *E—E , mea0 sured with the ionization chamber for an incident O beam at E "16 MeV which corresponds to an energy slightly above the 1/2> resonance at E "8.971 MeV in F. The solid lines are simulaV tions for the expected *E—E curves for O and 0
Fig. 2. Envelopes for the trajectories of O> > > and F> ions in the first part of the FMA calculated with the ion-optics code GIOS [16] using the input parameters summarized in Table 1. The horizontal dimensions in this figure are strongly compressed.
F, respectively, taking the dimensions of the IC and the energy loss in the two entrance foils into account. The F reaction products from the p(O,F)c reaction are well separated from the O background. These O events are caused by the scattering of the incident O beam at various locations in the FMA (e.g., the anode plate of the first electric dipole). The suppression factor for the incident beam relative to the F reaction products measured in these experiments was better than 10:1. Changing the incident energy to E "13.4 MeV results in a *E—E spectrum which is shown in 0 Fig. 3b. The yield for the F events is considerably reduced but is still clearly visible. The main contribution to the yield at this energy is attributed to a small fluorine contamination in the targets which were produced by stretching thin polyethylene foils
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Fig. 3. a) *E—E spectrum measured at the focal plane of the 0 FMA for the O(p,c)F reaction at E "16 MeV. This en ergy is close to the 1/2> resonance at E "8.791 MeV in F. V The solid lines are the result of detector simulations of *E versus E for different ions. (b) *E—E spectrum measured at the 0 0 focal plane of the FMA for the O(p,c)F reaction at E "13.4 MeV.
over a heated teflon piston which contains fluorine material. Fluorine impurities have also been observed in measurements of other (p,c) reactions [17]. The recoil F ions from the fluorine impurities in the angular range h"0—2.3° (produced by backward-scattered O particles) have an integrated cross section of &3 mb which is considerably larger than typical (p,c) cross sections which are of the order of tens of lb. Thus, even small amounts of impurities can influence low cross section measurements. This is a general problem for radiative capture measurements in inverse kinematics when the reaction product is stable and might be present as an impurity in the target. Ions of F can also be produced via the O(d,n)F reaction on the small (0.015%)
deuterium content in the CH target. With typical cross sections of &10 mb/sr [18] for this reaction one calculates background yields corresponding to a cross section of &2 lb. This is about a factor of 10 smaller than the background caused by the F recoil ions caused by the fluorine impurity in the target. Another source of F background that was discovered in these experiments originates from impurities in the incident beam. An electrical discharge in the ion source of the tandem accelerator which is set nominally for injection of O\ particles can cause F\ ions to be transmitted for a short time through the injection magnet. Through charge-exchange processes with the residual gases in the high-energy section of the tandem, F particles with the same magnetic rigidity, i.e. an energy of ;E(O) can be produced. Since this is the exact energy of the (p,c) reaction products, the F particles are transmitted with high efficiency to the focal plane of the FMA and identified as F in the ionization chamber. This effect was visible through an increased count rate at the appropriate energy in the monitor spectra and could be easily eliminated. These background reactions pose no problem when the particles of interest are unstable, e.g. in the study of the inverse p(F,Ne)c or p(N,O)c [6] reactions. Other reactions involving inverse reaction kinematics e.g. p(N,O)c or p(Ne,Na)c, resulting in stable reaction products which might be present (e.g. as water or from the parting agent) as trace elements in the target require special considerations. 2.2. Charge-state corrections The collimator installed after the first electric dipole of the FMA restricts the transmission to the focal plane to a single-charge state. In order to obtain the total cross section this yield has to be divided by the corresponding charge-state fraction. We have performed two measurements of the charge-state distribution for F ions with energies of &16 MeV. The first used F recoil ions produced by bombarding a CaF target with an O beam and detecting the mass 19 recoil particles at h "13°. The second experiment measured the F yield from the p(O,F)c reaction at the
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Fig. 4. Charge-state distribution measured for F ions at E "16 MeV. The lines are taken from calculations given in Refs. [19,20].
1/2> resonance (E "8.791 MeV) in F. Fig. 4 V gives the result of these measurements compared with the theoretical predictions from Ref. [19] (solid line) and Ref. [20] (dashed line). Similar to the results obtained in Ref. [11], we observe that the F (p,c) yield at the maximum of the chargestate distribution is slightly higher than predicted by the theoretical models. The deviations for the most probable charge state are, however, less than 10%. This agreement justifies the use of calculated charge-state fractions in cases where a full chargestate distribution has not been measured. In addition to the charge-state fractionation the transport efficiency to the FMA has to be considered. This efficiency was calculated with the code GIOS [16] taking into account the acceptance limitations imposed by the 1.7 cm wide collimator installed after the first electric dipole. An overall efficiency for detecting F ions in the focal plane of the FMA of 30$5% is obtained including the efficiency of the PGAC and the IC [13].
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target thicknesses. In our experiment we have used solid polypropylene (CH ) targets with thicknesses L between 60—150 lg/cm. While targets of polyimid [(C H N O ) ] or Mylar [(C H O ) ] show L L better stability under heavy-ion bombardment, the high (2:1) H:C ratio and the lack of other elements in its chemical composition, which can cause background reactions with the incident beam, led to the choice of polypropylene as target. However, traces of contaminants, e.g. oxygen from water and fluorine from the target production technique, were present in small amounts in the material. The targets were produced by stretching 2.3 mg/cm thick polypropylene material over a heated teflon piston to thicknesses down to 60 lg/cm. The absolute target thickness was determined through an energy-loss measurement using 5.4 MeV a particles. The homogeneity of the target area (diameter 12 mm) was found to be better than 10%. The main difficulty with these targets is their tendency to lose hydrogen at beam currents above 0.5—1 pnA of O. The hydrogen content of the targets was therefore continuously monitored by detecting H recoil ions in two monitor detectors mounted at $30°. Fig. 5 shows an energy spectrum measured in a Si detector at 30° with a CH
3. Hydrogen target The low bombarding energies (600—700 keV/u) of the experiments prevented the use of a hydrogen gas cell with windows. Windowless gas targets, on the other hand, can achieve only relatively low
Fig. 5. Energy spectrum measured with the monitor detector at h "30° for a 13.4 MeV O beam bombarding a 90 lg/cm CH target.
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target and an O beam at E "13.4 MeV. The various groups are caused by elastically scattered O ions, by H and C recoils and by alphas from the H(O,a)N reaction and a weak summing peak at channel number 1200. The hydrogen content of the CH target can be determined from the ratio of the hydrogen and carbon recoil peaks. The H:C ratio of the target can be calculated from the proton and C recoil yields N(H) and N(C) and the elastic scattering cross sections dp! and dp& , respectively, by N(H)dp! , (1) H/C" N(C)dp& which for pure Rutherford scattering can be simplified to
N(H) Z M (M #M ) N(H) ! & ! " ) 0.631. H/C" N(C) Z M (M #M ) N(C) & ! & (2) It should be noted that Eq. (2) is independent of the scattering angle. Because in our case, at the energies of interest, the p#O system shows strong resonances in the excitation function for elastic scattering [21], the actual cross sections for dp& were used for determining the H:C ratio. The elastic dp! cross sections were taken as the Rutherford values. For this the actual scattering angle of the monitor detector has to be known. It can be determined from the ratio of the oxygen and carbon peaks in the monitor spectrum. Because at these low energies the cross section for elastic scattering of O on C is given by the Rutherford value, the ratio of the yields between the O and C peaks is independent of the incident energy and depends only on the scattering angle h (in the laboratory system) via N(O) k cosh " N(C) 4 sin(h#sin\(k sin h)/2) ;
(cos h#(1/k!sinh) (1/k!sinh
(3)
with M(O) k" . M(C)
(4)
At a scattering angle of 30° this ratio changes by &15% for a change of 1° in h. Since the statistical
Fig. 6. Change in the hydrogen to carbon ratio observed over a time period of 12 h when bombarding a 140 lg/cm CH target with 1—3 enA O beams.
uncertainties allow the determination of this ratio to about 1%, the scattering angle can be determined with an accuracy of about 0.1°. To keep the hydrogen losses in the polypropylene targets to a minimum we have used a target wobbler which distributed the beam spot (typical diameter 2 mm) over a circle with a radius of &5 mm. The target thickness was scanned before and after the experiment with an alpha source. The maximum change in thickness observed in these experiments was 20%. For a 140 lg/cm CH tar get this uncertainty in thickness translates into an uncertainty in the center-of-mass energy for the p(FNe)c reaction of about 2 keV. The change in composition of a 140 lg/cm polypropylene target which was bombarded with an O beam is shown in Fig. 6. The H/C ratio decreases from the initial value of 2 to about 1.5 over a period of about 12 h. The absolute energy of the incident beam was determined by scattering O ions from two Au targets with thicknesses of 200 and 90 lg/cm and detecting the O particles in the focal plane of the Enge Split Pole spectrograph which was calibrated with a Th a source. Extrapolating to zero target thickness the energy of the incident O beam was found to agree to within 42 keV with the value of 13 400 keV predicted from the 90° bending magnet setting. In the center-of-mass frame for the O#p system this translates into an uncertainty of 2.2 keV.
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4. Beam monitoring While a silicon detector can be used to monitor the hydrogen content of the CH target it cannot differentiate between the incident O and F particles. In the F(p,a)O experiment [22] Z-identification of the mass 18 particles was achieved using the method of a gas-filled magnet [23]. Since this was not possible for the (p,c) experiment, a circular Al ring was installed about 150 mm after the CH target covering the angular range from 3.6° to 9.7°. Elastically scattered F were implanted into this ring and the accumulated 511 keV activity was measured with a calibrated Ge detector for each F run. The time dependence of the F beam was monitored with a Ge detector located in close vicinity of the collimator after the first electric dipole where most of the incident beam was intercepted. Fig. 7 shows the time dependence of the 511 keV annihilation radiation measured with this detector for one F sample. As will be shown below, the total number of incident F ions can be obtained from a measurement of the time dependence of the 511 keV annihilation and from a measurement of the F activity measured at the end of the experi-
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ment in the catcher ring. Also shown is the time dependence of the monitor counting rate, which mainly reflects the intensity of the isobar impurity O. The time unit in Fig. 7 corresponds to 1 min. One observes a general exponential decrease caused by the b> decay of F and by sputtering losses of F material in the ion source. The arrows indicate the times when the sample in the ion source was rotated in order to reach a fresh spot for the sputtering process [14]. For a calculation of the total number N of F ions incident on the target in a time interval ranging from t Pt we consider the number of ions which are scattered onto the catcher ring, N , and the collimator after the electric dipole, N , respec tively. They are related via geometrical efficiency factors to the total number of incident F ions. While the efficiency at the collimator requires a detailed knowledge of the transport properties of the FMA and the charge-fractionation effects, the efficiency for collecting F ions on the catcher ring, e , can be calculated from the geometry of the circular catcher and the Rutherford cross section. Since the collimator and the catcher are exposed to the same (time-dependent) beam of F, the
Fig. 7. (top) Time dependence of the 511 keV annihilation radiation observed with a Ge detector located close to the collimator after the first electric dipole of the FMA. (bottom) Time dependence of the count rate measured in the monitor detector. The arrows indicate times when the sample in the ion source was rotated.
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activities on the collimator and the catcher ring A and A , respectively, measured at the end of the experiment (t"t ) are related to the total num ber of accumulated F ions via A (t ) N " . A (t )!A (t ) e\HR\R N The total number of incident particles N is M N A (t ) . N " ) e A (t )!A (t ) e\HR\R P
(5)
(6)
By solving the differential equation for production and decay of F ions which are collected on the collimator, one obtains
R 1 A (t) dt# [A (t )!A (t )], (7) j R where the integral gives the number of ions which decayed during the accumulation time between t and t , A (t )/j is the number of particles which decay from t"t PR, and A (t )/j is the num ber of atoms present at the beginning of the experiment. All parameters in Eqs. (5) and (6) are measured quantities and thus N can be calculated. Together with the number of O particles from the monitor detector a time-averaged ratio of O/F of 2500:1 has been determined. This value agrees well with the results measured in our (p,a) studies [7] where the time dependence of O/F ratio could be determined from a direct measurement of elastically scattered O and F particles. N "
5. Experimental results with a F beam The technique used for the production of the F samples is described in detail in Ref. [14]. F was produced by bombarding O water with a 11.4 MeV proton beam from the medical cyclotron at the University of Wisconsin. After chemical separation, the F material was deposited on a cathode insert for a negative ion source (SNICS from the National Electrostatic Corporation), flown to Argonne and installed in the ion source of the tandem accelerator which is one of the injectors
of the ATLAS accelerator. The choice of solid F material compared to gaseous fluorine compounds (e.g. CF H) was based on the observation that the chemically very active fluorine can be kept at a well defined spot while gaseous material is distributed over the whole interior of the source. The extraction efficiency measured with a solid test sample of F was in the range 0.1—1% while with gaseous CF H only 3;10\% has been observed [22]. Five samples of F were prepared and measured at an energy of E "670 keV (calculated for the center of the 90 lg/cm target foil) with the setup and the techniques described above. The energy loss of a F beam in the target is 70 keV and thus includes the dominant part of the 3/2> resonance in Ne at E "652 keV. The *E—E spectrum sum 0 med over these five samples representing a total integrated charge of 2.8 pnC is shown in Fig. 8. Although this energy is off-resonance for the p(O,F)c reaction a considerable yield of F is observed in this summed spectrum. The origin of this F background was discussed in Section 2.1. While the average energy of Ne ions is identical to the F energy, the area in the *E—E spectrum 0 is shifted to lower E energies by about 1 MeV. The 0 shape and size of the region shown by the ellipse in Fig. 8a is based on the p(O,F)c reaction, shown in Fig. 3a, where the ellipse contains more than 95% of the F events. The solid lines in Fig. 8a are the expected *E—E curves calculated for the present detector geometry. Due to the considerably higher yields for F and O there are three events located in the area of interest which show *E values intermediate between Z"9 and 10 particles. However, an independent measurement of the energy loss *E in the PGAC (see Fig. 8b) shows that all three events coincide with the maximum of the F distribution. We, therefore, conclude that none of these events has a clear signature for being caused by a Ne particle produced via the F(p,c) reaction, but rather seem to be events from the tail of the energy loss distribution of F ions. From these events an upper limit for the F(p,c)Ne reaction of 42 lb has been calculated. The implication of this result for the breakout from the hot CNO cycle has been discussed in Ref. [13].
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some cases be difficult. For the radiative capture reaction p(F,Ne)c a recoil mass separator was used as a detection device. A suppression factor for the incident beam relative to the (p,c) reaction products of 10 was achieved. Since the F beams had a considerable contamination from the neighbouring stable isobar O, a Z identification was required in order to determine the integrated incoming beam. In the experiment this was accomplished by collecting scattered F particles on a catcher ring and measuring the accumulated c activity after the run. With a low-intensity F beam (typically &10 F/s) it was possible to set an upper limit for the p(F,Ne)c reaction of about 40 lb at the 3/2> resonance at E "652 keV in Ne.
Acknowledgements We want to thank J. Greene for his help with the target production and the thickness measurements. This work was supported by the U.S. Department of Energy, Nuclear Physics Division under Contract No. W-31-109-ENG-38, the National Science Foundation and by a University of Chicago/Argonne National Laboratory Collaborative Grant.
Fig. 8. (a) *E—E spectrum measured in the focal plane of the 0 FMA for the p(F,Ne)c reaction at E "13.4 MeV with a beam that contains a considerable O contamination. The area where Ne events are expected is encircled. The solid lines are the result of simulations of *E versus E for different ions. (b) 0 *E signals measured in the focal plane counter of the FMA for the three events inside the Ne region in comparison with measured F and O events. The dashed line is the expected Ne distribution extrapolated from the O and F data.
6. Summary Experiments with radioactive ion beams in inverse kinematics require the development of new detection systems. While these studies have the benefit of increased detection efficiency due to the c.m. to lab conversion factor, the separation of the reaction products from the incident beam can in
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