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A recoil separator for the measurement of radiative capture reactions
Nuclear Instruments and Methods in Physics Rehearch A 376 ( 1996) 174-184
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH
i-= tizk9 ELSEVIER
SecrlonA
A recoil separator for the measurement
reactions
of radiative capture
72
L. Gialanella”“, F. Striedera, K. Brandh, L. Campajolab, A. D’Onofrio’, U. Greife”, E. Hutteld, F. Petrazzuolog, V. Rocab, C. Rolfs”‘“, M. Romanob, M. Romoli’, S. Schmidt”, W.H. Schulte”, F. Terrasi’, H.P. Trautvetter”, D. Zahnowa “Institut
fiir
hDipartimento
E.xperimentalphysik Fisiche.
Universitci
di Fisica
Teorica
SMSA, Universitir
‘Dipartimento
‘Forscl~ur~gs~entruin ‘Istituto ‘Facoltci
111, Ruhr-Universitiit
di Scienzr
Karlsnthe,
Federico
Hauptabteilung
Nazionale
di Fisica
Nucleare,
di Scienze Ambietltali,
Seconda
Universitci
“ENEA, “D~nanritron-Tanden
INN-ART, Laboratorium,
Bochum, II, Napoli
Bochum, and INFN.
Germany Napoli.
di Salerno
and INFN,
Napoli,
Zyklotrort,
Karlsruhe,
German?
Sezione di Napoli. di Napoli,
CR Casacciu,
Napoli.
Italy Ital)
Italy
Caserta and INFN,
Napoli,
Italy
Roma, ltaly
Ruhr-Universitiil,
&chum,
Germany
Abstract A recoil separator in combination with a windowless gas target has been designed for the measurement of the radiative capture reaction p(‘Be, y)‘B. The separator consists of momentum and velocity filters and a A&E detector telescope. The setup was tested quantitatively using the p( “C, y)“N reaction at the effective energy E,, = 841 keV. Projectile fluxes were measured directly with Faraday cups and indirectly with elastic backscattering into Si detectors, while the “N recoil flux was measured directly with the A.!-E telescope. A suppression of the “C beam particles by a factor 2 X IO-“’ was observed when the system was tuned for the recoil “N’s, Special emphasis was given to the charge state probabilities of the llN recoils.,Possible improvements of the system for the measurement of other capture reactions are discussed.
1. Introduction A comparison of the observed solar neutrino fluxes in GALLEX/SAGE, HOMESTAKE, and KAMIOKANDE provides no unique picture of the microscopic processes in the sun ([I] and references therein). A solution of this “solar neutrino puzzle” can possibly be found in the areas of neutrino physics, solar physics (models), or nuclear physics. In the area of nuclear physics, there is a large uncertainty in the absolute cross section c(E) of the reaction ‘Be(p, y)‘B (Q = 0.14 MeV). Using a radioactive ‘Be target (T,,, = 53 d), the measurement of this cross section via the direct detection of the capture y-rays or of the residual ‘B nuclides is difficult and has to date not been carried out. Instead, the cross section was derived from the P-delayed a-decay of ‘B. The work of several investigators ([2] and references therein) led to a fairly consistent picture of the energy dependence of a(E) -or equivalently of the astrophysical S(E) factor - but not on .’ Supported in part by the Deutsche Forschungsgemeinschaft (Ro429/21-4) and VIGONI/CRUI. *Corresponding author. Tel. +39 81 7253111, fax +39 81 2394508, e-mail infn@vaxnal .decnet. 0168.9002/96/$15.00 PII
Copyright
SO 168.9002(96)00264-
0 1996 Elsevier Science 1
the absolute value: the extrapolated absolute S(0) factor ranges from 16 to 45 eV b (see also [3]). The discrepancy is most likely to be found in the complicated target stoichiometry of the radioactive ‘Be target (produced via hot chemistry). It is the aim of a project at the 3 MV tandem accelerator in Naples to provide an improved ‘Be(p, y)‘B cross section value in the nonresonant region, i.e. at the centerof-mass energy EC:, = 1.0 MeV. The reaction will be studied in inverse kinematics, p(‘Be, y)‘B, i.e. a radioactive ‘Be ion beam of Q,, = 8.0 MeV (current about I pnA) is guided into a windowless gas target system filled with H, gas (pressure pJHL) = 5 mbar). thus avoiding the above problems of target stoichiometry. Furthermore, the setup should allow to observe both the capture y-rays and the kinematically forward-focussed ‘B recoils; as a third (previously used) method, the ‘B recoils are implanted into a sheet and detected by their P-delayed a-decay. All 3 methods require a high transmission of the ‘Be ion beam between the sputter ion source and the center of the gas target system. The direct observation of the ‘B recoils requires an efficient recoil separator ([4] and references therein) to filter out the radioactive ‘Be beam particles @‘(‘Be) = I pnA = 6 X 10’ s- ’ ) from the ‘B recoils
B.V. All rights reserved
L. Gialanella et al. I Nucl.
Instr. and Meth. in Phys. Res. A 376 (1996) 174-184
(A’(‘B) = 0.06 s- ’ for u = 1 pb and a gas density of N(HZ) = 1 X lOI atoms/cm”). The recoil separator must also filter out beam contaminants. small-angle elastic scattering products, and background events (e.g. from multiple scattering processes). If the filtering of the recoil separator is sufficiently effective (i.e. of order N(‘B)I N(7Be) = 1 X IO-“), the ‘B recoils can be counted directly in a AE-E detector telescope (allowing for particle identification) placed in the beam line. The recoil separator must not only have a high filtering power but also a high transmission of the ‘B recoils (for the selected charge state 4) between the gas target chamber and the AGE telescope. For an optimum ‘B transmission, the design of the downstream apertures in the gas target system include a cone of 0,( ‘B) = 0.20” opening angle, within which the ‘B recoils emerge from the target zone, due to the emission of the I. 14 MeV capture y-rays. The feasibility of the separation of projectiles and recoils with a mass difference of
17.5
and the subsequent recoil detection lamutolpartinl0”, in a A&E telescope, is the focus of the present studies utilizing - as a simulating test case - the p( “C, y)lZN reaction (Q = 1.94 MeV) at the effective energy Ecm = 841 keV (incident energy E,,,( “C) = 11.O MeV), with a(E) = 0.3820.04 pb [.5] and a cone of 0,( “N) = 0.32”.
2. Equipment
and setup
A schematic diagram of the 3 MV TTT-3 tandem accelerator at Naples. the beam transport system, the windowless gas target, and the recoil separator is shown in Fig. I. The setup has been used for accelerator-massspectrometry (AMS) of ‘“C and technical details were described elsewhere [6,7]. Thus, we recall here briefly relevant features and focus mainly on modifications necessary to match the requirements of the present studies.
s mm
SL FCI
3 W Tr-r-3 TANDEb
S FC2 I\IQPD
RECOIL SEPARATOR
I
POST-STRIPPER FOIL WINDOWLESS
I SWITCHING blc\CNET
ANALYZING I\lACNET
SL
s FC3
FC-l MQPD
FCG Fig. 1. Schematic diagram of the setup at the 3 MV tandem accelerator in Naples including a windowless gas target and a recoil separator (S = X-Y steerers, SL = slits, FC = Faraday cup, MQPD = magnetic quadrupole doublet, MQPT = magnetic quadrupole triplet).
slits (Fig. I ) to change the charge state y of the accelerated ion beam (e.g. to fully stripped ions). The windowless (differentially pumped) gas target system [9] has three pumping stages on both sides of the target chamber (Fig. 3), which consist of Roots blowers (e.g. WS2000, pumping speed = 2000 m ‘Ih), turbo pumps (TV360, pumping speed = 360 I/s), and roughing pumps (e.g. D65B. pumping speed = 65 m’/h). The beam entered the target chamber through three electrically insulated apertures of high gas-flow impedance fC to A; with diameters 0, lengths 1. and distances d given in Fig. 2) and left the chamber through a similar set of apertures (Section 3.1): these apertures defined the ion beam axis to better than 0.5”. The target chamber was filled with the target gas - via a needle valve - to a pressure p,,. The pressure was measured with a Baratron capacitance manometer to an accuracy of 4%; the measurement is absolute and independent of the gas used. The pressure at several other locations in the target system was determined by thermocouple and ionization manometers. For H, gas of p,, = 5.0 mbar. the three-stage pumping system reduced the upstream pressures to pi = 0. I9 mbar, p1 = 6.3 x IO-+ mbar, and p3 = I .6 X IO-’ mbar in the regions between the apertures A and B. B and C, and outside aperture C, respectively; a similar pressure reduction was observed downstream from the target chamber. Due to the low ion beam intensities used in the present work. their
A new cesium sputter ion source (model 300 from Kingston Scientific [S]) was installed operating at 25 kV potential. The conically shaped ionizer allows to sputter the elemental materials nearly homogeneously over an area of up to 5 mm diameter (Section 3.6). The negative ion beam of interest is selected by a 35” injection magnet (mass resolution m/Am = 30), focused by a gridded lens. and accelerated to the terminal voltage U of the tandem. After electron stripping in a C foil (3 kg/cm’ thick). the positive ions of selected charge state q emerging from the accelerator are focused by a magnetic quadrupole doublet on the object slits of a 90” analysing magnet. The doublefocusing analysing magnet (mass-energy product = 45) focuses the beam on the image slits and a downstream magnetic quadrupole doublet focuses the beam on the center of the gas target system. The subsequent recoil separator includes [6.7] a magnetic quadrupole triplet, a 30” switching magnet, a magnetic quadrupole doublet. a Wien filter, and a conventional A&E ionisation chamber, tilled with isobutane at a pressure of 9 mbar. A rectangular Ta aperture (5 mm high, 8 mm wide) is placed in front of the rectangular entrance window (mylar foil; 20 mm high, 50 mm wide) of the AE-E detector telescope. Faraday cups and X-Y steerers are placed along the beam transport system (Fig. I ) to monitor and optimize the beam transmission, respectively. Finally, a post-stripper C foil (3 pg/cm’ thick) can be placed in front of the object
_
d=252
1_
d=440
/
d=248
,
d=440
,_
d=252
GASINLET
0 0
TV360
TV360 0
TV360 0 Q
t D25B 8
D8B 0
Fig. 2. Schematic diagram of the two-sided differentially pumped gas target. The quoted values of lengths (I). diameters (0), and distances the diameter of the downstream aperture A’ (and B’) was (d) are in mm. Due to the 0.32” cone of the “N recoils from p(“C.y)“N, increased to 6.7 mm (and 9.0 mm) in the present studies.
L. Gialanella et al. I Nucl. Imtr. and Meth. in Phys. Res. A 376 (1996) 174-184
influence on the gas density is negligible
(less than 0.05%
1101). The disc-shaped target chamber (Fig. 3) has a central ion-beam pipe of 12 mm diameter, which houses the apertures A and A’. The chamber is connected with the large pumping crosses of the first pumping stages by tubes of 10 mm inner diameter and 74 mm length. The chamber has several ports radiating from the center of the chamber, which is at a distance of z = 1245 1 mm from the center of the apertures A and A’. The ports are used for several purposes: gas inlet. adapter for the Baratron manometer, and installation of collimated particle detectors at the scattering angles B,ab=45” and -45”. The detectors (Si: active area a = 1.50 mm”, thickness r = 300 km, energy resolution AE = I5 keV at E_ = 5.5 MeV) were collimated by a slit-hole combination installed at the ends of a tube (length f= 1132 1 mm): the horizontal slit (width s = 2.00?0.05 mm) faced the center of the target chamber and
117
the circular collimator (radius r = 5.00?0.05 mm) faced the detector. The distance from the circular collimator to the center of the target chamber was d = 173k2 mm. This geometry defined the effective target length seen by the detector along the beam axis [ 1 l] as 1cttp = sd(fsin
0,a,)m’ ,
(1)
corresponding to a range of scattering angles, B,,,I!zAB,~,, and solid angles. &?A&,. At S,ab = %45” with &, = m*/d’ = 2.62 msr one calculates lrttp = 4.30 mm, AH,ab= 0.5”, AG,a, = 0.047 msr. and lztfpfililb = 11.3?0.4 mm msr. The absolute values of the scattering angles, O,ah= 44.87”?0.06” and -44.94”iO.O6”, have been measured in a way as described previously [9].
3. Experimental
procedures
and results
The number of 17N nuclides of p(“C, y)“N produced in the gas target chamber per unit of time, IFJ, is given by the equation
SIDE VIEW
where N, is the number of “C projectiles per unit of time. N, is the density of the HZ target gas, &,, is the effective target length along the beam axis producing ‘?N nuclides, and c(Ee,,) is the absolute cross sect;zn of p( “C,r)“N at the effective energy EC,, (Section 3.3). For the selected charge state q of the I ‘N recoils, the number of ’ 'N recoils observed in the AE-E telescope is
TOP VIEW
GAS iNLET
where Gq is the probability of the “N recoils emerging from the gas target with charge state 4 and F, represents the transmission of the “N”+ recoils in the recoil separator. The elastic scattering yield. I,, observed at B,ab= 245”. is given by 1. = N,N,l,,,,~,,,rr,,,.s(~.
E,,, KLlfl.
I& ’
(4)
where (T_,,(0. Ee,,) is the elastic scattering cross section at the associated angle e, and Oc,/rZ,,, is the ratio of center-of-mass solid angle to laboratory solid angle. The ratio of the observed yields I,, and I, then leads to the expression
Fig. 3. Disc-shaped target chamber in side view (top) and top view (bottom). The chamber has several ports radiating from the center of the chamber, which are used for various purposes, such as the installation of collimated particle detectors at several scattering angles (present studies: f?,,, = 245”). The Pb-collimated 2 X 2 in. BGO crystal was moved parallel to the beam axis (z-axis) and the observed y-flux from a nuclear resonance reaction was used to measure the gas density along the beam axis (Fig. 4).
(5) which is independent of ion beam current and beaminduced heating of the target gas, and - in a way - also of gas pressure. Thus, the absolute capture cross section CT(E~,,) can be determined with high precision, if the critical input parameters of Eq. 5 are measured with sufficient precision. The parameters l,,,,fl,a, and & (and
L. Gialar~ella
178
et al.
I Nucl. Instr.
and Meth.
for the detected thus Q,l.n,,>, = 4 CDS B,sh = 2.833 t0.003 hydrogen recoils) have been discussed in Section 2. The other parameters are investigated in what follows.
2. I.
Pressure
profile
o_f the H,
gas
In the extended gas target chamber (Fig. 3) the H2 gas pressure between the apertures A and A’ is expected to be nearly constant and unaffected by the gas flow through these apertures. The major pressure drop should occur across the length of these apertures, followed by a further pressure drop along the tubes connecting the chamber with the first pumping stages, where one observes a pressure ratio p, lp,, = 0.038. To measure this expected pressure profile, the E,,,,, = 3.073 MeV (f,,,,, = 85 keV) resonance
in the p(‘Li, y)‘B capture reaction [12] was used as a probe. Details of these measurements have been reported [9]. Briefly, the flux of capture transitions was observed in a 2 X 2 in. BGO crystal (Fig. 3. bottom) collimated - on the beam facing side - by a 10 cm thick lead shield with a vertical slit of 10 mm viewing a Icrtr = 44.6 mm target length at the beam axis (z-axis). This length corresponds to a target thickness of d,a, = 25 keV for a pressure of p,,(H,) = 6.8 mbar. Thus, the observed y-flux, N,,, represents a thin-target-yield (to within 82%) and an excitation function of this flux for a given distance z of the BGO crystal (i.e. varying the incident beam energy at : = constant) reflects a Breit-Wigner shape of this resonance: the resonance is “moved” along the beam axis within the target chamber. The maximum resonance yield, Ny,m,x(~). at a given BGO : position was measured in this way and the total curve of Ny,max(~) (i.e. moving the BGO setup parallel to the beam axis) is proportional to the H2 gas density (pressure) as a function of Z. The resulting HZ pressure profile supported [9] the above expectations: a constant pressure plateau at the central region of the target chamber and a relatively sharp pressure drop near the apertures A and A’, with a full-width-at-half-maximum of zFWHM= 28228 mm and a mean width of ZMW= 34428 mm. In order to adapt the downstream apertures to the 0.32” cone of the ‘?N recoils, the diameter of the downstream aperture A’ was enlarged from 5.0 to 6.7 mm and that of B’ from 8.0 to 9.0mm. Assuming a homogeneous ‘“N production over a cylindrical beam-target volume with diameter 5.0 mm (aperture A) and height z,, = IcnN = 344 mm, 98.6% of all “N recoils can emerge from the gas target system. Since the incident “C beam profile at aperture A has most likely a profile with an effective diameter smaller than 5.0 mm and since angle straggling of the 13N recoils in the gas volume is small (less than 0.16”). we assumed a 100% emergence of the “N recoils produced. Due to the increased aperture diameters of A’ and B’, the HZ pressure profile is expected to extend somewhat
in Phys. Res. A 376 (1996)
174-184
further downstream with increased values for z,,,, and L,,. Applying again the above p(‘Li, y)‘B resonance method, the resulting HZ profile for the maximum pressure of p,,(H,) = 5 mbar (Fig. 4) verifies a constant pressure plateau at the central region of the target chamber, a pressure drop near the apertures A and A’, and a further downstream extension of the pressure profile. The magnitudes of the shoulders on both sides of the pressure drop are in the ratio of the areas of apertures A and A’: the lengths are larger, as expected: zFWHM= 330?8 mm and zM,,, = Ic,,., = 376?8 mm.
3.2.
Calibration
of the analysing
magnet
The analysing magnet was calibrated using the p( “F. c.ry)lhO resonance at E, = 6416.8-+0.8 keV [ 131, which was investigated with different charge states q of the “F ions. The 6.13 MeV y-ray flux was observed with the collimated BGO crystal (Fig. 3) placed at the central region of the target chamber (7 = 0). For a given charge state of the “F ions, excitation curves were obtained for p,,(HZ) = 5.0 mbar and 0.5 mbar; a gas mixture of H2 and Ar (in a ratio H, :Ar = 10: 1) was used, where the “F on Ar elastic scattering yield - observed at O,.,h= +45” served as a monitor in these measurements. The observed centroids of the resonance yield curves were extrapolated to zero gas pressure, whereby the deduced NMR frequency, A,,(p,= 0), of the magnetic held of the analysing magnet is independent of uncertainties in stopping
Fig. 4. Pressure profile of HI gas (for PC,= 5.0 mbar) along the beam axis obtained in the setup of Fig. 3 (bottom). The zero point of the z-axis is at the center of the target chamber. Also shown are the locations of the apertures A and A’ and the connecting tubes to the first pumping stages (WS2000 Roots blowers). Due to the different diameters of the apertures A and A’, the pressure profile is asymmetric extending somewhat further at the downstream side. The data observed in the regions of the WS2000 pumping crosses are consistent with the observed pressure ratios p, /p,, = 0.038 and pi/p0 = 0.062. The curve through the data points represents a fit using the combination of Fermi-functions and arctan-functions.
L. Giulanella et al. I Nucl. Insrr. and Meth. in Phys. Res. A 376 (1996) 174-184 Table I Calibration
of the 90” analysing
Reaction
p( “F, cry)‘“0
“Al(p,
r)“Si
179
magnet
Energy E, [keV]
Charge
Mass
Frequency”
4
MP [amul
f,,,
6416.8Z0.8
3+ 5+ 6+ 7+
18.9968 18.9957 18.995 1 18.9946
34 1402 10 20 47625 17067-cS 14 6281-6
214.904?0.076 214.826rO.066 214.876-cO.076 214.866+0.101
9269~4
214.784?0.100
991.9+0.04
I .00728
I+
Calibration factor K [kHz/MeV]
fkHz1
214.854+0.036”
Weighted mean ’ Extrapolated value for a gas pressure p,, = 0. h Internal error (external error = 0.020).
power values. The resulting values off,,,( p. = 0) for the charge states q = 3, 5, 6, and 7 are summarized in Table I. The E, = 991.90-+0.04 keV resonance in “Al(p, r)‘“Si [14] was also studied using an 8 keV thick Al target on a I mm thick Ta backing (here: charge normalisation of incident protons): the resulting f,,, value is also given in Table I. Finally, the calibration factor K of the analysing magnet was derived from the equation
214.854-CO.036 kHz/MeV. The f,,, range covers the appropriate region of interest for the p( ‘Be, JJ)~B and p( “C, y)“N studies, where the above error in K leads to a negligible error in the beam energy and thus in the cross section (e.g. Au/u = 20.03% for p(7Be, y)‘B at E,,, = 8.0 MeV). The proton beam energy spread was found to be 0.8 keV at the 992 keV “Al(p, y)“Si resonance.
K = q f,,,(2M,c’E,
3.3. Effective beam energy
+ E;))“‘,
(f-3)
where M, is the exact isotopic mass of the projectile (in amu) corrected for ionic charge state (Table I ). The resulting K values (Table I and Fig. 5) are independent of f NMR(and. thus of the magnetic field strength) over the range investigated, with a weighted mean value of K =
The effective beam energy, EC,,. at the center of the target chamber is - in first approximation - the relevant energy associated with the deduced absolute cross section. This energy is calculated from the incident energy Eldh( “C) (Section 3.2) and the energy loss of the ‘*C
---_ 20000
NMR
30000
40000
f [kHz]
Fig. 5. Results for the magnetic calibration factor K plotted as a function of NMR frequency of the field of the 90” analysing magnet. The solid line through the data points represents the weighted average for a field independent K. The arrows indicate the NMR frequencies applied to a ‘Be’+ beam (8.0 MeV) and “C’+ and “C”+ beams ( I1 .O MeV).
L. Giultrnrllu et cd. I Nrrcl. Instr. md Meth. in Phy.\. Res. A 376 (1996)
180
projectiles chamber:
174-184
in the target gas up to the center of the target
EC,, = E,,,( “C) ---p,,(zMW12)(dEld?) ,
(7)
where dE/dz is the energy loss in the gas. which was obtained from the program TRiM6-95 [ 151 with an assumed uncertainty of 57r (for experimental tests of dE/dz, see Ref. [S]). For Elilh( ‘*C) = 1 I .O MeV and P,, = 5.0 mbar, one finds EC,, = LO.862 MeV or E,,,,,f, = 0.841 MeV. 3.4. Elastic stuttering
cross section
of p +
“C
The p “‘C elastic scattering process has been investigated f 16,171 in the energy region E_,, = 0.27 to 4.02 MeV at the scattering angles eL,, = 89.1” to 169.2”. Interpolating the data to &,, = 90” at the relevant energy EILlb(“C) = 1 I .O MeV (E,., = 0.853 MeV), one finds a,,,(& E) = 410 mbisr. In order to test this inte~olation, we used a gas mixture of H, and Ar (in a ratio H2 :Ar = 100: 1) with a total pressure of pli = 5.0 mbar and observed the scattering yields on both target nuclides at B,ah= It45” and E,,,, = 2.0, 3.0, 4.0 and 1 I .O MeV. For the “C on Ar scattering system, the projectile energies are far below the height of the Coulomb barrier E,-, EC,,,/& = 0.080 to 0.44, suggesting the validity of the Rutherford scattering law at all energies. For the “C on p scattering system, it is experimentally known that cr,,,,,(B, E) follows the Rutherford scattering law at E,a, 54.0 MeV [5]. Sample spectra obtained at E,a, = 2.0 and 1 I .O MeV are shown in Fig. 6: since the intensity of the proton recoil nuclides of the “C + p system, I,, and that of the scattered “C projectiles of the “C + Ar system, I,,, can be measured in the same detector, their intensity ratio is given by:
where the constant N contains the density ratio of the two gases and solid-angle transformations. The resulting ratios R* at Eta,, = 2.0, 3.0, and 4.0 MeV are identical within experimental uncertainty, with a weighted mean value of R*(i?& = 2-4 MeV) = 12.34+0.07, in comparison with R = R*(E,,,, = thus, R*(E,,, = 11MeV) = 19.4520.24; 1I MeV)/R*(E,~~ = 2-4 MeV) = s,,,(S, ~)/~~(~, E) = 1.576t:0.021 and n,,,(@, E) = 42257 mb/sr at the effective energy E,, = 841 keV (the quoted error includes the uncertainty in &. Section 2) in good agreement with the inte~oiated value quoted above. 3.5. Calibrution
qf the AE-E telescope
The AE-E telescope was calibrated using incident ion beams of ‘H, ‘Li, ‘Be, “B, “C, 14N, and ‘“0 at energies in the range of interest for the present studies. A given ion beam was first focused on the Faraday cup (FC6) in front of the telescope (Fig. I); subsequently the ion beam current was reduced to below 1000 per second by closing
Fig. 6. Sample spectra obtained at H,ab= 45” in the setup of Fig. 3 for the elastic scattering of “C on a HZ/Ar gas mixture at &,,( “C) = 2.0 and 11 .O MeV. The observed structures are identified. The low energy background arises predominantly from lnultipie scattering of the projectiles on the wails of the target
The dotted curves represent the assumed background under the relevant peaks.
chamber.
the slits in the ion beam transport system and the centroid in the AE-E matrix was recorded. The results are illustrated in Fig. 7 in form of the centroid channel numbers for the AE and E detectors: the different ions are well separated from each other. The ‘Li ions exhibit an inversion in the matrix near 6 MeV, due to incomplete stopping in the E detector above this energy.
Since the ‘Be nuclides are produced in a ‘Liz0 matrix (via the ‘Li(p, n)‘Be reaction at E, = 13 MeV, at the Karlsruhe cyclotron, within a cylindrical volume of 5 mm diameter and I.2 mm depth), they will form most likely an oxide (‘BeO) in the matrix. Thus, it appears best to extract ‘Be from the sputter source as a ‘BeO- molecule, which will be accompanied by ‘LiO molecules at the exit of the 3.5” injection magnet, with an expected current ratio R( 7Be0m /‘LiO -) = 0.06. After acceleration and stripping in the tandem at a terminal voltage of II = 2.42 MV, 8.OMeV ions of 7Be’+ and ‘Li”+ emerge from the accelerator in a current ratio R( 7Beis /‘Li” ) = 0.04.
L. Gialarlella et al. / Nd.
Irrstr. and Meth. in Phw. Res. A 376 (1996) 174-183
Fig. 7. Observed matrix of the AE-E telescope for ions between p and ‘“0 at the energies quoted (in units of MeV). The solid curves through the data points are to guide the eye only. Isotopes of a given element lie on the same curve shifted in energy by their mass ratios
Inserting the post-stripper C foil (Fig. 1), completely stripped ‘Be4+ ions are produced with a 4A = 80% probability [ 181.These ‘Be’+ eonsare then deflected by the 90” analysing magnet, while other ion beams, such as ‘Lil+, are filtered. Thus. a pure ‘Be ion beam should emerge if the background (“leaky”) beams with the same rigidity are negligible by comparison. Due to the high voltage stability (better than 20.5 kV. GVM stabilisation), a constant transmission by the beam transport system has been observed in AMS experiments [6.7], for low ion beam currents as in the ‘Be case. Finally, the fully stripped ‘B5’ recoils of p(‘Be, T)~B emerging from the gas target will be focused on the AE-E telescope. In the present simulation studies an atomic “C beam was injected into the accelerator at U = 2.20 MV, stripped to the q = 4+ charge state ($a = 28% [ 18]), accelerated to &h> = 1 I .O MeV. post-stripped to q = 5+ with 4, ^- 48% (or to q = 6+ with 4e = 1.5%). and deflected by the 90” analysing magnet. The observed currents at the Faraday cups FCI and FC3 (Fig. I ). corrected for their charge states and the charge state probabilities 4q and +5 (or d+,), yield the transmission E, through the accelerator: E, = 5&, consistent with the observations for other atomic ions injected into the accelerator. The transmission Ed through the gas target system, defined by the observed currents at FC3 and FC4, was found to be about 50%. Finally, the transmission Ed through the recoil separator, defined by the observed currents at FC4 and FC6. was observed to be 100% within experimental uncertainty (3%). for different ions and charge states. In contrast, the transmissions E, and E? were lower by a factor 2, when molecular ions such as ‘LiO- or ‘BeO- were injected and accelerated, due to effects of the Coulomb explosion of the molecules in the terminal stripper foil [19]. After tuning the “C beam with, say, q = 6+ through
181
the recoil separator, hydrogen gas of p,,(H,) = 5.0 mbar was filled into the gas target system and the “Ch’ beam was retuned through the separator (taking into account the energy loss in the gas) leading to a set of optimum values for the magnetic field of the switching magnet, (BT)SM = the Wien filter. (BA’),, = 8/v ( f5 = constant (Bb’),,, electric field strength, u = velocity of tuned particles), and the magnetic quadrupoles, (BA’),,,,. The fully stripped ’ 'N" recoils of p( “C, y)’ ‘N emerging from the gas target have the same momentum as the “C projectiles except for their charge states; thus, the magnetic fields of the switching magnet and magnetic quadrupoles must be scaled down by 6/7, while the magnetic field of the Wien filter must be scaled up by the ratio of masses, i.e. 13/ 12. For a fine tuning of the switching magnet and the magnetic quadrupoles, we used a “pilot” “C”’ beam (at E,,, = (6/7)‘E( “C) = 8.08 MeV) of the same rigidity as 12N7’. whereby the Wien filter was set at (Bp,,,rt)WF = (7/ 6)(BA’),,. After this fine tuning, only the Wien filter had to be reset at the field strength (B:‘),, = (13/12)(6/ 7 )(B p,,~,,)wF (for further experimental tests. see Section 3.8 ).
3.7. Charge
.stnte probability
of
’ 'N
recoils
For the “C projectile energy of Eluh( “C) = 1I .O MeV, the “N recoils of p( “C, y)“N emerge from the gas target with an&energy E,,,(‘3N) = (12/13)E,~,(“C) = 10.15 MeV and in various charge states q. As noted above, the fully stripped “N recoils, “N”. are tuned in the recoil separator, since for this charge state the intensity of the leaky “C beams should be minimized. Their charge state probability 4, in the H, gas at p,,(H,) = 5.0 mbar was measured using a lJN ion beam at the same energy per amu, i.e. Elah( 14N) = (l4/ 13)&,( “N) = 10.93 MeV. The incident “N’+ beam was first focused on the Faraday cup (FC6) in front of the AE-E telescope. After inserting the Hz gas, each charge state q was retuned through the recoil separator. The observed currents were corrected for their charge states and normalized to the sum of currents of all charge states. The c#~ results are shown in the top part of Fig. 8 as a function of Hz pressure. At low pressure the 5+ charge state dominates approaching an intensity equal to the incident beam, while at a pressure above 4 mbar the charge states approach their equilibrium values. The experiment was repeated with “Nh’ projectiles; the results (bottom part of Fig. 8) show similar features as just discussed, except that here & approaches unity for zero pressure equal to the incident beam. Note that the equilibrium values are independent from the charge state of the incident ions: & = 27%. & = 62%, and 4, = 11% (4d = 1.5% - not shown -has been neglected in the analysis). If the “N recoils from p( “C, y)“N are created near the upstream part of the gas target, they pass the full target length and the charge state probability 4, corresponds to
L. Giulutwllrc et (I/. I Nucl. lnstr. md Mrrh. in Phys. Res. A 376 (1996) 174-184
182
LEAkY
PRO-ZNS
J
E
(‘4N)
Q In =
o~‘,“““,“‘l”“““““‘.i 0 2
3
1
P&H,)
=
10 93
MeV
Fig. 9. Two-dimensional density plot of the AE-E telescope with the recoil separator tuned to the “N’+ nuclides from p( “C,y)“N. The observed structures are identified.
i
6’
4
5
6
[rborl
elastic scattering process in the H2 gas target (for calibration/identification, see Fig. 7). The leaky “C beams correspond to a suppression factor of 2 X IO- I” for the incident “C projectiles. For a test of the tuning procedures of the recoil separator, the yield of the “N recoils was measured as a function of the magnetic field strength B of the switching magnet and the Wien filter (Fig. 1). The results are displayed in Fig. IO: the observed center of the yield curve (C for Center) for the switching magnet is higher by 0.65%
Fig. 8. Charge state distributions of 10.93 MeV “N projectiles in Hz gas. as a function of gas pressure. The top (or bottom) figure shows the results for an incident charge state q,, = 5+ (or q,, = 6+). The curves through the data points are fits using exponential functions.
the value observed at p(, = 5.0 mbar (Fig. 8). If they are created in the middle of the target gas, they pass one-half of the full target length and qS, is equivalent to the value at p. = 2.5 mbar. Finally, if they are created near the downstream part of the gas target, 4, = 0 at p. = 0 mbar. Thus, the observed 12N7’ yield corresponds to the mean value 4: over the range p,,(H,) = 0 to 5.0 mbar. For the incident 4T(q,, = 5+) = charge state q,, = 5+ one calculates one finds @(qi, =6+)= 6.6?0.2% and for q,, =6+ 9.6?0.3%. If these results can be applied to the p( “C, Y)‘~N reaction, i.e. if the created ‘?N nuclides the initial charge state q,, of the “C projec“memorise” tiles, one expects a “N yield ratio I,,(q,, = 5+)/Z,,(qln = 6+) = (b$(q;. = 5+)l@(q,, =6+) = 0.6920.03. 3.8. Acceptance
of the recoil separator
With the recoil separator tuned to the 13N7’ recoils (Section 3.6), Fig. 9 shows a two-dimensional density plot of the counts observed in the A&E telescope: the counts from the llN recoils are well resolved from the band of counts due to “leaky ” “C beams: at low channel numbers protons created by the “C + p one observes “leaky”
3050
SWITCHNG
3100
MAGNET
3150
B
[G]
Fig. 10. Fine-tuning of the recoil separator. i.e. of the magnetic fields of the switching magnet and the Wien filter, using the “N yield from p( “C,y)“N observed in the AE-E telescope (Fig. 1). The tuning values deduced from the “pilot” beam (T for Tuning) are systematically different by 0.6% from the centres (C) of the yield curves (see text). Also shown are the observed acceptances of the switching magnet and the Wien filter. which are larger than the emittance of the “N recoils from p( “C.y)“N. The curves through the data points represent fits using Fermi-functions at the edges.
L. Gialanella
et al. I Nucl.
Instr. and Meth. in Phys. Res. A 376 (1996)
compared to the value deduced from the “pilot” beam (T for Tuning), while C is lower by 0.55% compared to T in the case of the Wien filter. Both results indicate that the momentum p of the “N recoils is higher by about 0.60% compared to that of the pilot “C projectiles. The above tuning procedure assumed - in fact - an equal energy loss AE for both beams. Using the TRIM6-95 program [ 151 one finds AE, = 304 keV for the 8.08 MeV pilot “C projectiles (for zMW= 376 mm and p,,(HZ) = 5.0 mbar) and AE, = 182 keV for the 10.13 MeV “N recoils and AE, = 138 keV for the I I .O MeV ‘*C projectiles (both for z,,r = :,,/2 = 188 mm and p,,(H,) = 5.0 mbar); this leads to a field ratio of the switching magnet of B,IB, = 1.004, which is of the order observed. Fig. IO also shows the observed acceptance of the switching magnet, (Applp),, =I 1.9%, and that of the Wien filter, (Au/u),, = (Applp),, = 2.6%. Both values are significantly larger than the emittance, (A~/P)~ = 1.1%. of the “N recoils from p(“C, y)“N due to the isotropic emission of the 2.79 MeV capture y-rays at E,,,( “C) = I I .O MeV The calculated acceptance of the 30” switching magnet is (Apip),, = 2.6% for a radius of 802 mm, a path length of 500 mm, and a slit width of 10 mm; the observed value is smaller due to the subsequent limitations imposed by the Wien filter in combination with the slits in front of the AE-E telescope (Fig. 1).
3.9. Absolute cross section of p( ‘?C, yji9N In the case of ‘*C” projectiles, the ‘jN yield (I,,) normalised to the elastic scattering yield (I,) - is found to be I,,lZ,(q,, = 5+) = (0.71?0.03) X 10e7, and for ‘*C”+ projectiles it is I,,ll,(q,, = 6+) = (0.92?0.07) X lo-‘. Thus, the ratio of the two “N yields is I,,(qln =5+)/ I,,(q,, = 6+) = 0.77?0.07, in good agreement with the observed ratio of charge state probabilities @(q,, = 5+)/ dT(q,,, = 6+) = 0.69kO.03 (Section 3.7). The result supports the suggestion that the ‘“N nuclides - created by p( “C, y)‘jN in the Hz gas target - “memorise” the initial charge state q,, of the “C projectiles. The weighted mean value of the ratios (I,,ll~)(q,.)l~T(q,,) for q,, = 5+ and h- is (1.04?0.05) X IO-*. Using this result and Eq. (5) with the parameters I,,,,Qa, = I 1.3 20.4 mm msr (Section Q.mlL’,~, = 2.833t0.003 (Section 2), F~,,, = 2). 42227 mb/sr (Section 3.4), z,, = I,,,N = 376?8 mm (Section 3.1), and &1= 1.00+0.03 (Section 3.6), one finds a value of LT= 0.37kO.03 pb for the absolute cross section of p( “C, y)“N at E,,,,L, = 841 keV, in excellent agreement with the reported value r = 0.38?0.04 p_b [5].
4. Discussion A recoil separator in combination with a windowless gas target has been designed for the measurement of the
174-184
I83
radiative capture reaction p(‘Be, y)*B. The setup was tested quantitatively using the p( “C, y)“N reaction, whereby a suppression of the ‘*C beam particles by a factor 2 X lo-“’ was achieved. The setup can be used to measure an absolute capture cross section with high if the critical experimental parameters are precision, known with sufficient accuracy. In a recoil separator. it is of course necessary to make a charge state selection of the recoil nuclides, causing a reduction in the number of recoils transmitted through the separator. However, since there is usually a charge state representing about 50% of the total recoils produced, this reduction is not too serious. To achieve the above beam suppression in the present recoil separator, the selected charge state of the recoils had to be higher than the maximum possible charge state of the projectiles, i.e. the choice had to be fully stripped recoils. In the p( ‘“C, Y)‘~N studies at E,a,( “C) = I I .O MeV, the “N’+ recoils were detected with 4: = 6.6% and 9.6%, and in the planned studies of p(‘Be, y)‘B at E,ab(7Be) = 8.0 MeV the ‘B” recoils have an observed probability 4: = 64.2%. Although these probabilities are comfortably high for the present studies, at other projectile energies the charge state probability for fully stripped recoils can be quite low requiring an improvement in the setup. Such an improvement could be an energy filter (with an associated slit system) added at the end of the present recoil separator, which would filter sufficiently the “leaky” projectiles allowing for a free and optimum choice of the selected charge state of the recoils. The recoils emerge from the gas target within a cone with an opening angle 0, given by the relation tan 0, = E.,/(~~c*E,~~)“~,
(9)
where m and E,a, are the mass and the energy of the projectiles and E., is the energy of the capture y-rays. For most capture reactions this angle is a few degrees (e.g. 0, = 1.56” for “He( “C, y)lhO at Elab( “C) = 4.0 MeV), while in the present studies of p( “C. y)“N at E,*,( “C) = 1 l.OMeV and p(‘Be, v)~B at E,=,(‘Be) = 8.0 MeV it is 0, = 0.32” and 0.20”, respectively. The present setup of the gas target system restricts 13, to less than 0.40”; it was designed for a concurrent observation of the capture y-rays in p(‘Be, y)‘B (Figs. 2 and 3). Without such a restriction, the distances between the downstream apertures can be reduced significantly and - if more powerful pumps are installed-the diameter of the apertures can also be increased substantially. Both improvements should allow for a 100% emergence of the recoils having an opening angle of a few degrees. If the recoil nuclides are stable, such as in 4He(‘2C. y)lhO and p(gBe, y)‘“B, leaky beams of the recoil type can hamper seriously the measurements. These leaky beams emerging from the accelerator with a “white” spectrum of energies are many orders of mag-
1x4
L. Gicdme/la rt ul. I Nucl. Irutr. md Meth. in Phys. Rex. A 376 (1996) 17&184
nitude weaker than the beam of interest (and thus difficult to observe directly by current measurements), but they could be more intense than the typical intensity ratio IO- ” between the capture recoils and the projectiles of interest. Consider the case of p(“Be. y)“)B with q,“(‘Be) = 4+ and q,( “‘B) = 5+: the momentum filters of the accelerator are tuned to the rigidity R(‘Be) = m,v,/4, while the momentum filters of the separator are tuned to R( “‘B) = m ,,,v ,,,/.5 and the Wien filter is tuned to u,,, = (n~~lnr,,,)u,; thus, a leaky “‘B beam with q,(“‘B) = 4+ and v, = (m,lm,,,)v, will pass the analysing magnet and - if its charge state is changed to q, ( “‘B) = 5+ in the gas target - it also passes the recoil separator. This experiment was carried out and the AE-E telescope revealed “‘B particles at the energy corresponding to the capture reaction, whose intensity was however several orders of magnitude higher than expected from the known capture cross section. This problem can be eliminated to a large extend, if a Wien filter is installed before the analysis magnet (Fig. 1) purifying the projectiles emerging from this magnet. Of course, if the recoils are instable, this problem does not exist. In summary, the present recoil separator in combination with the windowless gas target and the technical improvements just discussed appear to make such a system a powerful tool to study radiative capture reactions over a wide range of energies. Such a system may be of particular interest to study the wide field of radiative capture reactions (predominantly of the type (p,y) and (o,y)) involving short-lived radioactive ion beams, needed for the field of nuclear astrophysics [4.l I].
Acknowledgements The Bochum members thank INFN and the University Federico II of Naples for the hospitality and other support given.
References [II J.N. Bahcall and M.H. Pinsonneault.
Rev. Mod. Phys. 64
( 1992) 88.5. 121 B.W. Filippone, Ann. Rev. Nucl. Part. Sci. 36 (1986) 717. 131 T. Motobayashi et al.. Phys. Rev. Lett. 73 (1994) 2680. [41 MS. Smith. C. Rolfs and C.A. Barnes, Nucl. Instr. and Meth. A 306 (1991) 233. r51 C. Rolfs and R.E. Azuma, Nucl. Phys. A 227 ( 1974) 291. 161 L. Campajola, A. Brondi, A. D’Onofrio, G. Gialanella. M. Romano, F. Terrasi. C. Tuniz, C. Azzi and S. Improta. Nucl. Instr. and Meth. B 29 ( 1987) 129. A. Brondi. M. Cipriano. A. 171 F. Terrasi. L. Campajola. D’Onofrio. E. Fioretto, M. Romano, C. Azzi, F. Bella and C. Tuniz. Nucl. Instr. and Meth. B 52 (1990) 259. G.D. Alton, G.D. Mills and J. Dellwo, Rev. Sci. Instr. 65 ( 1994) 2006. 191 F. Strieder et al., Z. Phys. A, in press. 1101 J. Goerres, K.U. Kettner, H. Kraewinkel and C. Rolfs, Nucl. Instr. and Meth. 177 (1980) 295. in the Cosmos [Ill C. Rolfs and W.S. Rodney, Cauldrons (University of Chicago Press, 1988). F. Ajzenberg-Selove, Nucl. Phys. A 490 ( 1988) I. F. Ajzenberg-Selove. Nucl. Phys. A 475 ( 1987) 1. P.M. Endt, Nucl. Phys. A 521 (1990) 1. TRIM6-95, The Transport of Ions in Matter (Handbook of version 95.vi. New York, 1995). H.L. Jackson, AI. Galonsky. F.J. Eppling. R.W. Hill, E. Goldberg and J.R. Cameron, Phys. Rev. 89 (19.53) 365. H.O. Meyer, G.R. Plattner and I. Sick, 2. Phys. A 279 (1976) I. J.B. Marion and F.C. Young. Nuclear Reaction Analysis (North-Holland. Amsterdam, 1968). J. Klein, R. Middleton and H. Tang, Nucl. Instr. and Meth. 193 (1982) 601.
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH
i-= tizk9 ELSEVIER
SecrlonA
A recoil separator for the measurement
reactions
of radiative capture
72
L. Gialanella”“, F. Striedera, K. Brandh, L. Campajolab, A. D’Onofrio’, U. Greife”, E. Hutteld, F. Petrazzuolog, V. Rocab, C. Rolfs”‘“, M. Romanob, M. Romoli’, S. Schmidt”, W.H. Schulte”, F. Terrasi’, H.P. Trautvetter”, D. Zahnowa “Institut
fiir
hDipartimento
E.xperimentalphysik Fisiche.
Universitci
di Fisica
Teorica
SMSA, Universitir
‘Dipartimento
‘Forscl~ur~gs~entruin ‘Istituto ‘Facoltci
111, Ruhr-Universitiit
di Scienzr
Karlsnthe,
Federico
Hauptabteilung
Nazionale
di Fisica
Nucleare,
di Scienze Ambietltali,
Seconda
Universitci
“ENEA, “D~nanritron-Tanden
INN-ART, Laboratorium,
Bochum, II, Napoli
Bochum, and INFN.
Germany Napoli.
di Salerno
and INFN,
Napoli,
Zyklotrort,
Karlsruhe,
German?
Sezione di Napoli. di Napoli,
CR Casacciu,
Napoli.
Italy Ital)
Italy
Caserta and INFN,
Napoli,
Italy
Roma, ltaly
Ruhr-Universitiil,
&chum,
Germany
Abstract A recoil separator in combination with a windowless gas target has been designed for the measurement of the radiative capture reaction p(‘Be, y)‘B. The separator consists of momentum and velocity filters and a A&E detector telescope. The setup was tested quantitatively using the p( “C, y)“N reaction at the effective energy E,, = 841 keV. Projectile fluxes were measured directly with Faraday cups and indirectly with elastic backscattering into Si detectors, while the “N recoil flux was measured directly with the A.!-E telescope. A suppression of the “C beam particles by a factor 2 X IO-“’ was observed when the system was tuned for the recoil “N’s, Special emphasis was given to the charge state probabilities of the llN recoils.,Possible improvements of the system for the measurement of other capture reactions are discussed.
1. Introduction A comparison of the observed solar neutrino fluxes in GALLEX/SAGE, HOMESTAKE, and KAMIOKANDE provides no unique picture of the microscopic processes in the sun ([I] and references therein). A solution of this “solar neutrino puzzle” can possibly be found in the areas of neutrino physics, solar physics (models), or nuclear physics. In the area of nuclear physics, there is a large uncertainty in the absolute cross section c(E) of the reaction ‘Be(p, y)‘B (Q = 0.14 MeV). Using a radioactive ‘Be target (T,,, = 53 d), the measurement of this cross section via the direct detection of the capture y-rays or of the residual ‘B nuclides is difficult and has to date not been carried out. Instead, the cross section was derived from the P-delayed a-decay of ‘B. The work of several investigators ([2] and references therein) led to a fairly consistent picture of the energy dependence of a(E) -or equivalently of the astrophysical S(E) factor - but not on .’ Supported in part by the Deutsche Forschungsgemeinschaft (Ro429/21-4) and VIGONI/CRUI. *Corresponding author. Tel. +39 81 7253111, fax +39 81 2394508, e-mail infn@vaxnal .decnet. 0168.9002/96/$15.00 PII
Copyright
SO 168.9002(96)00264-
0 1996 Elsevier Science 1
the absolute value: the extrapolated absolute S(0) factor ranges from 16 to 45 eV b (see also [3]). The discrepancy is most likely to be found in the complicated target stoichiometry of the radioactive ‘Be target (produced via hot chemistry). It is the aim of a project at the 3 MV tandem accelerator in Naples to provide an improved ‘Be(p, y)‘B cross section value in the nonresonant region, i.e. at the centerof-mass energy EC:, = 1.0 MeV. The reaction will be studied in inverse kinematics, p(‘Be, y)‘B, i.e. a radioactive ‘Be ion beam of Q,, = 8.0 MeV (current about I pnA) is guided into a windowless gas target system filled with H, gas (pressure pJHL) = 5 mbar). thus avoiding the above problems of target stoichiometry. Furthermore, the setup should allow to observe both the capture y-rays and the kinematically forward-focussed ‘B recoils; as a third (previously used) method, the ‘B recoils are implanted into a sheet and detected by their P-delayed a-decay. All 3 methods require a high transmission of the ‘Be ion beam between the sputter ion source and the center of the gas target system. The direct observation of the ‘B recoils requires an efficient recoil separator ([4] and references therein) to filter out the radioactive ‘Be beam particles @‘(‘Be) = I pnA = 6 X 10’ s- ’ ) from the ‘B recoils
B.V. All rights reserved
L. Gialanella et al. I Nucl.
Instr. and Meth. in Phys. Res. A 376 (1996) 174-184
(A’(‘B) = 0.06 s- ’ for u = 1 pb and a gas density of N(HZ) = 1 X lOI atoms/cm”). The recoil separator must also filter out beam contaminants. small-angle elastic scattering products, and background events (e.g. from multiple scattering processes). If the filtering of the recoil separator is sufficiently effective (i.e. of order N(‘B)I N(7Be) = 1 X IO-“), the ‘B recoils can be counted directly in a AE-E detector telescope (allowing for particle identification) placed in the beam line. The recoil separator must not only have a high filtering power but also a high transmission of the ‘B recoils (for the selected charge state 4) between the gas target chamber and the AGE telescope. For an optimum ‘B transmission, the design of the downstream apertures in the gas target system include a cone of 0,( ‘B) = 0.20” opening angle, within which the ‘B recoils emerge from the target zone, due to the emission of the I. 14 MeV capture y-rays. The feasibility of the separation of projectiles and recoils with a mass difference of
17.5
and the subsequent recoil detection lamutolpartinl0”, in a A&E telescope, is the focus of the present studies utilizing - as a simulating test case - the p( “C, y)lZN reaction (Q = 1.94 MeV) at the effective energy Ecm = 841 keV (incident energy E,,,( “C) = 11.O MeV), with a(E) = 0.3820.04 pb [.5] and a cone of 0,( “N) = 0.32”.
2. Equipment
and setup
A schematic diagram of the 3 MV TTT-3 tandem accelerator at Naples. the beam transport system, the windowless gas target, and the recoil separator is shown in Fig. I. The setup has been used for accelerator-massspectrometry (AMS) of ‘“C and technical details were described elsewhere [6,7]. Thus, we recall here briefly relevant features and focus mainly on modifications necessary to match the requirements of the present studies.
s mm
SL FCI
3 W Tr-r-3 TANDEb
S FC2 I\IQPD
RECOIL SEPARATOR
I
POST-STRIPPER FOIL WINDOWLESS
I SWITCHING blc\CNET
ANALYZING I\lACNET
SL
s FC3
FC-l MQPD
FCG Fig. 1. Schematic diagram of the setup at the 3 MV tandem accelerator in Naples including a windowless gas target and a recoil separator (S = X-Y steerers, SL = slits, FC = Faraday cup, MQPD = magnetic quadrupole doublet, MQPT = magnetic quadrupole triplet).
slits (Fig. I ) to change the charge state y of the accelerated ion beam (e.g. to fully stripped ions). The windowless (differentially pumped) gas target system [9] has three pumping stages on both sides of the target chamber (Fig. 3), which consist of Roots blowers (e.g. WS2000, pumping speed = 2000 m ‘Ih), turbo pumps (TV360, pumping speed = 360 I/s), and roughing pumps (e.g. D65B. pumping speed = 65 m’/h). The beam entered the target chamber through three electrically insulated apertures of high gas-flow impedance fC to A; with diameters 0, lengths 1. and distances d given in Fig. 2) and left the chamber through a similar set of apertures (Section 3.1): these apertures defined the ion beam axis to better than 0.5”. The target chamber was filled with the target gas - via a needle valve - to a pressure p,,. The pressure was measured with a Baratron capacitance manometer to an accuracy of 4%; the measurement is absolute and independent of the gas used. The pressure at several other locations in the target system was determined by thermocouple and ionization manometers. For H, gas of p,, = 5.0 mbar. the three-stage pumping system reduced the upstream pressures to pi = 0. I9 mbar, p1 = 6.3 x IO-+ mbar, and p3 = I .6 X IO-’ mbar in the regions between the apertures A and B. B and C, and outside aperture C, respectively; a similar pressure reduction was observed downstream from the target chamber. Due to the low ion beam intensities used in the present work. their
A new cesium sputter ion source (model 300 from Kingston Scientific [S]) was installed operating at 25 kV potential. The conically shaped ionizer allows to sputter the elemental materials nearly homogeneously over an area of up to 5 mm diameter (Section 3.6). The negative ion beam of interest is selected by a 35” injection magnet (mass resolution m/Am = 30), focused by a gridded lens. and accelerated to the terminal voltage U of the tandem. After electron stripping in a C foil (3 kg/cm’ thick). the positive ions of selected charge state q emerging from the accelerator are focused by a magnetic quadrupole doublet on the object slits of a 90” analysing magnet. The doublefocusing analysing magnet (mass-energy product = 45) focuses the beam on the image slits and a downstream magnetic quadrupole doublet focuses the beam on the center of the gas target system. The subsequent recoil separator includes [6.7] a magnetic quadrupole triplet, a 30” switching magnet, a magnetic quadrupole doublet. a Wien filter, and a conventional A&E ionisation chamber, tilled with isobutane at a pressure of 9 mbar. A rectangular Ta aperture (5 mm high, 8 mm wide) is placed in front of the rectangular entrance window (mylar foil; 20 mm high, 50 mm wide) of the AE-E detector telescope. Faraday cups and X-Y steerers are placed along the beam transport system (Fig. I ) to monitor and optimize the beam transmission, respectively. Finally, a post-stripper C foil (3 pg/cm’ thick) can be placed in front of the object
_
d=252
1_
d=440
/
d=248
,
d=440
,_
d=252
GASINLET
0 0
TV360
TV360 0
TV360 0 Q
t D25B 8
D8B 0
Fig. 2. Schematic diagram of the two-sided differentially pumped gas target. The quoted values of lengths (I). diameters (0), and distances the diameter of the downstream aperture A’ (and B’) was (d) are in mm. Due to the 0.32” cone of the “N recoils from p(“C.y)“N, increased to 6.7 mm (and 9.0 mm) in the present studies.
L. Gialanella et al. I Nucl. Imtr. and Meth. in Phys. Res. A 376 (1996) 174-184
influence on the gas density is negligible
(less than 0.05%
1101). The disc-shaped target chamber (Fig. 3) has a central ion-beam pipe of 12 mm diameter, which houses the apertures A and A’. The chamber is connected with the large pumping crosses of the first pumping stages by tubes of 10 mm inner diameter and 74 mm length. The chamber has several ports radiating from the center of the chamber, which is at a distance of z = 1245 1 mm from the center of the apertures A and A’. The ports are used for several purposes: gas inlet. adapter for the Baratron manometer, and installation of collimated particle detectors at the scattering angles B,ab=45” and -45”. The detectors (Si: active area a = 1.50 mm”, thickness r = 300 km, energy resolution AE = I5 keV at E_ = 5.5 MeV) were collimated by a slit-hole combination installed at the ends of a tube (length f= 1132 1 mm): the horizontal slit (width s = 2.00?0.05 mm) faced the center of the target chamber and
117
the circular collimator (radius r = 5.00?0.05 mm) faced the detector. The distance from the circular collimator to the center of the target chamber was d = 173k2 mm. This geometry defined the effective target length seen by the detector along the beam axis [ 1 l] as 1cttp = sd(fsin
0,a,)m’ ,
(1)
corresponding to a range of scattering angles, B,,,I!zAB,~,, and solid angles. &?A&,. At S,ab = %45” with &, = m*/d’ = 2.62 msr one calculates lrttp = 4.30 mm, AH,ab= 0.5”, AG,a, = 0.047 msr. and lztfpfililb = 11.3?0.4 mm msr. The absolute values of the scattering angles, O,ah= 44.87”?0.06” and -44.94”iO.O6”, have been measured in a way as described previously [9].
3. Experimental
procedures
and results
The number of 17N nuclides of p(“C, y)“N produced in the gas target chamber per unit of time, IFJ, is given by the equation
SIDE VIEW
where N, is the number of “C projectiles per unit of time. N, is the density of the HZ target gas, &,, is the effective target length along the beam axis producing ‘?N nuclides, and c(Ee,,) is the absolute cross sect;zn of p( “C,r)“N at the effective energy EC,, (Section 3.3). For the selected charge state q of the I ‘N recoils, the number of ’ 'N recoils observed in the AE-E telescope is
TOP VIEW
GAS iNLET
where Gq is the probability of the “N recoils emerging from the gas target with charge state 4 and F, represents the transmission of the “N”+ recoils in the recoil separator. The elastic scattering yield. I,, observed at B,ab= 245”. is given by 1. = N,N,l,,,,~,,,rr,,,.s(~.
E,,, KLlfl.
I& ’
(4)
where (T_,,(0. Ee,,) is the elastic scattering cross section at the associated angle e, and Oc,/rZ,,, is the ratio of center-of-mass solid angle to laboratory solid angle. The ratio of the observed yields I,, and I, then leads to the expression
Fig. 3. Disc-shaped target chamber in side view (top) and top view (bottom). The chamber has several ports radiating from the center of the chamber, which are used for various purposes, such as the installation of collimated particle detectors at several scattering angles (present studies: f?,,, = 245”). The Pb-collimated 2 X 2 in. BGO crystal was moved parallel to the beam axis (z-axis) and the observed y-flux from a nuclear resonance reaction was used to measure the gas density along the beam axis (Fig. 4).
(5) which is independent of ion beam current and beaminduced heating of the target gas, and - in a way - also of gas pressure. Thus, the absolute capture cross section CT(E~,,) can be determined with high precision, if the critical input parameters of Eq. 5 are measured with sufficient precision. The parameters l,,,,fl,a, and & (and
L. Gialar~ella
178
et al.
I Nucl. Instr.
and Meth.
for the detected thus Q,l.n,,>, = 4 CDS B,sh = 2.833 t0.003 hydrogen recoils) have been discussed in Section 2. The other parameters are investigated in what follows.
2. I.
Pressure
profile
o_f the H,
gas
In the extended gas target chamber (Fig. 3) the H2 gas pressure between the apertures A and A’ is expected to be nearly constant and unaffected by the gas flow through these apertures. The major pressure drop should occur across the length of these apertures, followed by a further pressure drop along the tubes connecting the chamber with the first pumping stages, where one observes a pressure ratio p, lp,, = 0.038. To measure this expected pressure profile, the E,,,,, = 3.073 MeV (f,,,,, = 85 keV) resonance
in the p(‘Li, y)‘B capture reaction [12] was used as a probe. Details of these measurements have been reported [9]. Briefly, the flux of capture transitions was observed in a 2 X 2 in. BGO crystal (Fig. 3. bottom) collimated - on the beam facing side - by a 10 cm thick lead shield with a vertical slit of 10 mm viewing a Icrtr = 44.6 mm target length at the beam axis (z-axis). This length corresponds to a target thickness of d,a, = 25 keV for a pressure of p,,(H,) = 6.8 mbar. Thus, the observed y-flux, N,,, represents a thin-target-yield (to within 82%) and an excitation function of this flux for a given distance z of the BGO crystal (i.e. varying the incident beam energy at : = constant) reflects a Breit-Wigner shape of this resonance: the resonance is “moved” along the beam axis within the target chamber. The maximum resonance yield, Ny,m,x(~). at a given BGO : position was measured in this way and the total curve of Ny,max(~) (i.e. moving the BGO setup parallel to the beam axis) is proportional to the H2 gas density (pressure) as a function of Z. The resulting HZ pressure profile supported [9] the above expectations: a constant pressure plateau at the central region of the target chamber and a relatively sharp pressure drop near the apertures A and A’, with a full-width-at-half-maximum of zFWHM= 28228 mm and a mean width of ZMW= 34428 mm. In order to adapt the downstream apertures to the 0.32” cone of the ‘?N recoils, the diameter of the downstream aperture A’ was enlarged from 5.0 to 6.7 mm and that of B’ from 8.0 to 9.0mm. Assuming a homogeneous ‘“N production over a cylindrical beam-target volume with diameter 5.0 mm (aperture A) and height z,, = IcnN = 344 mm, 98.6% of all “N recoils can emerge from the gas target system. Since the incident “C beam profile at aperture A has most likely a profile with an effective diameter smaller than 5.0 mm and since angle straggling of the 13N recoils in the gas volume is small (less than 0.16”). we assumed a 100% emergence of the “N recoils produced. Due to the increased aperture diameters of A’ and B’, the HZ pressure profile is expected to extend somewhat
in Phys. Res. A 376 (1996)
174-184
further downstream with increased values for z,,,, and L,,. Applying again the above p(‘Li, y)‘B resonance method, the resulting HZ profile for the maximum pressure of p,,(H,) = 5 mbar (Fig. 4) verifies a constant pressure plateau at the central region of the target chamber, a pressure drop near the apertures A and A’, and a further downstream extension of the pressure profile. The magnitudes of the shoulders on both sides of the pressure drop are in the ratio of the areas of apertures A and A’: the lengths are larger, as expected: zFWHM= 330?8 mm and zM,,, = Ic,,., = 376?8 mm.
3.2.
Calibration
of the analysing
magnet
The analysing magnet was calibrated using the p( “F. c.ry)lhO resonance at E, = 6416.8-+0.8 keV [ 131, which was investigated with different charge states q of the “F ions. The 6.13 MeV y-ray flux was observed with the collimated BGO crystal (Fig. 3) placed at the central region of the target chamber (7 = 0). For a given charge state of the “F ions, excitation curves were obtained for p,,(HZ) = 5.0 mbar and 0.5 mbar; a gas mixture of H2 and Ar (in a ratio H, :Ar = 10: 1) was used, where the “F on Ar elastic scattering yield - observed at O,.,h= +45” served as a monitor in these measurements. The observed centroids of the resonance yield curves were extrapolated to zero gas pressure, whereby the deduced NMR frequency, A,,(p,= 0), of the magnetic held of the analysing magnet is independent of uncertainties in stopping
Fig. 4. Pressure profile of HI gas (for PC,= 5.0 mbar) along the beam axis obtained in the setup of Fig. 3 (bottom). The zero point of the z-axis is at the center of the target chamber. Also shown are the locations of the apertures A and A’ and the connecting tubes to the first pumping stages (WS2000 Roots blowers). Due to the different diameters of the apertures A and A’, the pressure profile is asymmetric extending somewhat further at the downstream side. The data observed in the regions of the WS2000 pumping crosses are consistent with the observed pressure ratios p, /p,, = 0.038 and pi/p0 = 0.062. The curve through the data points represents a fit using the combination of Fermi-functions and arctan-functions.
L. Giulanella et al. I Nucl. Insrr. and Meth. in Phys. Res. A 376 (1996) 174-184 Table I Calibration
of the 90” analysing
Reaction
p( “F, cry)‘“0
“Al(p,
r)“Si
179
magnet
Energy E, [keV]
Charge
Mass
Frequency”
4
MP [amul
f,,,
6416.8Z0.8
3+ 5+ 6+ 7+
18.9968 18.9957 18.995 1 18.9946
34 1402 10 20 47625 17067-cS 14 6281-6
214.904?0.076 214.826rO.066 214.876-cO.076 214.866+0.101
9269~4
214.784?0.100
991.9+0.04
I .00728
I+
Calibration factor K [kHz/MeV]
fkHz1
214.854+0.036”
Weighted mean ’ Extrapolated value for a gas pressure p,, = 0. h Internal error (external error = 0.020).
power values. The resulting values off,,,( p. = 0) for the charge states q = 3, 5, 6, and 7 are summarized in Table I. The E, = 991.90-+0.04 keV resonance in “Al(p, r)‘“Si [14] was also studied using an 8 keV thick Al target on a I mm thick Ta backing (here: charge normalisation of incident protons): the resulting f,,, value is also given in Table I. Finally, the calibration factor K of the analysing magnet was derived from the equation
214.854-CO.036 kHz/MeV. The f,,, range covers the appropriate region of interest for the p( ‘Be, JJ)~B and p( “C, y)“N studies, where the above error in K leads to a negligible error in the beam energy and thus in the cross section (e.g. Au/u = 20.03% for p(7Be, y)‘B at E,,, = 8.0 MeV). The proton beam energy spread was found to be 0.8 keV at the 992 keV “Al(p, y)“Si resonance.
K = q f,,,(2M,c’E,
3.3. Effective beam energy
+ E;))“‘,
(f-3)
where M, is the exact isotopic mass of the projectile (in amu) corrected for ionic charge state (Table I ). The resulting K values (Table I and Fig. 5) are independent of f NMR(and. thus of the magnetic field strength) over the range investigated, with a weighted mean value of K =
The effective beam energy, EC,,. at the center of the target chamber is - in first approximation - the relevant energy associated with the deduced absolute cross section. This energy is calculated from the incident energy Eldh( “C) (Section 3.2) and the energy loss of the ‘*C
---_ 20000
NMR
30000
40000
f [kHz]
Fig. 5. Results for the magnetic calibration factor K plotted as a function of NMR frequency of the field of the 90” analysing magnet. The solid line through the data points represents the weighted average for a field independent K. The arrows indicate the NMR frequencies applied to a ‘Be’+ beam (8.0 MeV) and “C’+ and “C”+ beams ( I1 .O MeV).
L. Giultrnrllu et cd. I Nrrcl. Instr. md Meth. in Phy.\. Res. A 376 (1996)
180
projectiles chamber:
174-184
in the target gas up to the center of the target
EC,, = E,,,( “C) ---p,,(zMW12)(dEld?) ,
(7)
where dE/dz is the energy loss in the gas. which was obtained from the program TRiM6-95 [ 151 with an assumed uncertainty of 57r (for experimental tests of dE/dz, see Ref. [S]). For Elilh( ‘*C) = 1 I .O MeV and P,, = 5.0 mbar, one finds EC,, = LO.862 MeV or E,,,,,f, = 0.841 MeV. 3.4. Elastic stuttering
cross section
of p +
“C
The p “‘C elastic scattering process has been investigated f 16,171 in the energy region E_,, = 0.27 to 4.02 MeV at the scattering angles eL,, = 89.1” to 169.2”. Interpolating the data to &,, = 90” at the relevant energy EILlb(“C) = 1 I .O MeV (E,., = 0.853 MeV), one finds a,,,(& E) = 410 mbisr. In order to test this inte~olation, we used a gas mixture of H, and Ar (in a ratio H2 :Ar = 100: 1) with a total pressure of pli = 5.0 mbar and observed the scattering yields on both target nuclides at B,ah= It45” and E,,,, = 2.0, 3.0, 4.0 and 1 I .O MeV. For the “C on Ar scattering system, the projectile energies are far below the height of the Coulomb barrier E,-, EC,,,/& = 0.080 to 0.44, suggesting the validity of the Rutherford scattering law at all energies. For the “C on p scattering system, it is experimentally known that cr,,,,,(B, E) follows the Rutherford scattering law at E,a, 54.0 MeV [5]. Sample spectra obtained at E,a, = 2.0 and 1 I .O MeV are shown in Fig. 6: since the intensity of the proton recoil nuclides of the “C + p system, I,, and that of the scattered “C projectiles of the “C + Ar system, I,,, can be measured in the same detector, their intensity ratio is given by:
where the constant N contains the density ratio of the two gases and solid-angle transformations. The resulting ratios R* at Eta,, = 2.0, 3.0, and 4.0 MeV are identical within experimental uncertainty, with a weighted mean value of R*(i?& = 2-4 MeV) = 12.34+0.07, in comparison with R = R*(E,,,, = thus, R*(E,,, = 11MeV) = 19.4520.24; 1I MeV)/R*(E,~~ = 2-4 MeV) = s,,,(S, ~)/~~(~, E) = 1.576t:0.021 and n,,,(@, E) = 42257 mb/sr at the effective energy E,, = 841 keV (the quoted error includes the uncertainty in &. Section 2) in good agreement with the inte~oiated value quoted above. 3.5. Calibrution
qf the AE-E telescope
The AE-E telescope was calibrated using incident ion beams of ‘H, ‘Li, ‘Be, “B, “C, 14N, and ‘“0 at energies in the range of interest for the present studies. A given ion beam was first focused on the Faraday cup (FC6) in front of the telescope (Fig. I); subsequently the ion beam current was reduced to below 1000 per second by closing
Fig. 6. Sample spectra obtained at H,ab= 45” in the setup of Fig. 3 for the elastic scattering of “C on a HZ/Ar gas mixture at &,,( “C) = 2.0 and 11 .O MeV. The observed structures are identified. The low energy background arises predominantly from lnultipie scattering of the projectiles on the wails of the target
The dotted curves represent the assumed background under the relevant peaks.
chamber.
the slits in the ion beam transport system and the centroid in the AE-E matrix was recorded. The results are illustrated in Fig. 7 in form of the centroid channel numbers for the AE and E detectors: the different ions are well separated from each other. The ‘Li ions exhibit an inversion in the matrix near 6 MeV, due to incomplete stopping in the E detector above this energy.
Since the ‘Be nuclides are produced in a ‘Liz0 matrix (via the ‘Li(p, n)‘Be reaction at E, = 13 MeV, at the Karlsruhe cyclotron, within a cylindrical volume of 5 mm diameter and I.2 mm depth), they will form most likely an oxide (‘BeO) in the matrix. Thus, it appears best to extract ‘Be from the sputter source as a ‘BeO- molecule, which will be accompanied by ‘LiO molecules at the exit of the 3.5” injection magnet, with an expected current ratio R( 7Be0m /‘LiO -) = 0.06. After acceleration and stripping in the tandem at a terminal voltage of II = 2.42 MV, 8.OMeV ions of 7Be’+ and ‘Li”+ emerge from the accelerator in a current ratio R( 7Beis /‘Li” ) = 0.04.
L. Gialarlella et al. / Nd.
Irrstr. and Meth. in Phw. Res. A 376 (1996) 174-183
Fig. 7. Observed matrix of the AE-E telescope for ions between p and ‘“0 at the energies quoted (in units of MeV). The solid curves through the data points are to guide the eye only. Isotopes of a given element lie on the same curve shifted in energy by their mass ratios
Inserting the post-stripper C foil (Fig. 1), completely stripped ‘Be4+ ions are produced with a 4A = 80% probability [ 181.These ‘Be’+ eonsare then deflected by the 90” analysing magnet, while other ion beams, such as ‘Lil+, are filtered. Thus. a pure ‘Be ion beam should emerge if the background (“leaky”) beams with the same rigidity are negligible by comparison. Due to the high voltage stability (better than 20.5 kV. GVM stabilisation), a constant transmission by the beam transport system has been observed in AMS experiments [6.7], for low ion beam currents as in the ‘Be case. Finally, the fully stripped ‘B5’ recoils of p(‘Be, T)~B emerging from the gas target will be focused on the AE-E telescope. In the present simulation studies an atomic “C beam was injected into the accelerator at U = 2.20 MV, stripped to the q = 4+ charge state ($a = 28% [ 18]), accelerated to &h> = 1 I .O MeV. post-stripped to q = 5+ with 4, ^- 48% (or to q = 6+ with 4e = 1.5%). and deflected by the 90” analysing magnet. The observed currents at the Faraday cups FCI and FC3 (Fig. I ). corrected for their charge states and the charge state probabilities 4q and +5 (or d+,), yield the transmission E, through the accelerator: E, = 5&, consistent with the observations for other atomic ions injected into the accelerator. The transmission Ed through the gas target system, defined by the observed currents at FC3 and FC4, was found to be about 50%. Finally, the transmission Ed through the recoil separator, defined by the observed currents at FC4 and FC6. was observed to be 100% within experimental uncertainty (3%). for different ions and charge states. In contrast, the transmissions E, and E? were lower by a factor 2, when molecular ions such as ‘LiO- or ‘BeO- were injected and accelerated, due to effects of the Coulomb explosion of the molecules in the terminal stripper foil [19]. After tuning the “C beam with, say, q = 6+ through
181
the recoil separator, hydrogen gas of p,,(H,) = 5.0 mbar was filled into the gas target system and the “Ch’ beam was retuned through the separator (taking into account the energy loss in the gas) leading to a set of optimum values for the magnetic field of the switching magnet, (BT)SM = the Wien filter. (BA’),, = 8/v ( f5 = constant (Bb’),,, electric field strength, u = velocity of tuned particles), and the magnetic quadrupoles, (BA’),,,,. The fully stripped ’ 'N" recoils of p( “C, y)’ ‘N emerging from the gas target have the same momentum as the “C projectiles except for their charge states; thus, the magnetic fields of the switching magnet and magnetic quadrupoles must be scaled down by 6/7, while the magnetic field of the Wien filter must be scaled up by the ratio of masses, i.e. 13/ 12. For a fine tuning of the switching magnet and the magnetic quadrupoles, we used a “pilot” “C”’ beam (at E,,, = (6/7)‘E( “C) = 8.08 MeV) of the same rigidity as 12N7’. whereby the Wien filter was set at (Bp,,,rt)WF = (7/ 6)(BA’),,. After this fine tuning, only the Wien filter had to be reset at the field strength (B:‘),, = (13/12)(6/ 7 )(B p,,~,,)wF (for further experimental tests. see Section 3.8 ).
3.7. Charge
.stnte probability
of
’ 'N
recoils
For the “C projectile energy of Eluh( “C) = 1I .O MeV, the “N recoils of p( “C, y)“N emerge from the gas target with an&energy E,,,(‘3N) = (12/13)E,~,(“C) = 10.15 MeV and in various charge states q. As noted above, the fully stripped “N recoils, “N”. are tuned in the recoil separator, since for this charge state the intensity of the leaky “C beams should be minimized. Their charge state probability 4, in the H, gas at p,,(H,) = 5.0 mbar was measured using a lJN ion beam at the same energy per amu, i.e. Elah( 14N) = (l4/ 13)&,( “N) = 10.93 MeV. The incident “N’+ beam was first focused on the Faraday cup (FC6) in front of the AE-E telescope. After inserting the Hz gas, each charge state q was retuned through the recoil separator. The observed currents were corrected for their charge states and normalized to the sum of currents of all charge states. The c#~ results are shown in the top part of Fig. 8 as a function of Hz pressure. At low pressure the 5+ charge state dominates approaching an intensity equal to the incident beam, while at a pressure above 4 mbar the charge states approach their equilibrium values. The experiment was repeated with “Nh’ projectiles; the results (bottom part of Fig. 8) show similar features as just discussed, except that here & approaches unity for zero pressure equal to the incident beam. Note that the equilibrium values are independent from the charge state of the incident ions: & = 27%. & = 62%, and 4, = 11% (4d = 1.5% - not shown -has been neglected in the analysis). If the “N recoils from p( “C, y)“N are created near the upstream part of the gas target, they pass the full target length and the charge state probability 4, corresponds to
L. Giulutwllrc et (I/. I Nucl. lnstr. md Mrrh. in Phys. Res. A 376 (1996) 174-184
182
LEAkY
PRO-ZNS
J
E
(‘4N)
Q In =
o~‘,“““,“‘l”“““““‘.i 0 2
3
1
P&H,)
=
10 93
MeV
Fig. 9. Two-dimensional density plot of the AE-E telescope with the recoil separator tuned to the “N’+ nuclides from p( “C,y)“N. The observed structures are identified.
i
6’
4
5
6
[rborl
elastic scattering process in the H2 gas target (for calibration/identification, see Fig. 7). The leaky “C beams correspond to a suppression factor of 2 X IO- I” for the incident “C projectiles. For a test of the tuning procedures of the recoil separator, the yield of the “N recoils was measured as a function of the magnetic field strength B of the switching magnet and the Wien filter (Fig. 1). The results are displayed in Fig. IO: the observed center of the yield curve (C for Center) for the switching magnet is higher by 0.65%
Fig. 8. Charge state distributions of 10.93 MeV “N projectiles in Hz gas. as a function of gas pressure. The top (or bottom) figure shows the results for an incident charge state q,, = 5+ (or q,, = 6+). The curves through the data points are fits using exponential functions.
the value observed at p(, = 5.0 mbar (Fig. 8). If they are created in the middle of the target gas, they pass one-half of the full target length and qS, is equivalent to the value at p. = 2.5 mbar. Finally, if they are created near the downstream part of the gas target, 4, = 0 at p. = 0 mbar. Thus, the observed 12N7’ yield corresponds to the mean value 4: over the range p,,(H,) = 0 to 5.0 mbar. For the incident 4T(q,, = 5+) = charge state q,, = 5+ one calculates one finds @(qi, =6+)= 6.6?0.2% and for q,, =6+ 9.6?0.3%. If these results can be applied to the p( “C, Y)‘~N reaction, i.e. if the created ‘?N nuclides the initial charge state q,, of the “C projec“memorise” tiles, one expects a “N yield ratio I,,(q,, = 5+)/Z,,(qln = 6+) = (b$(q;. = 5+)l@(q,, =6+) = 0.6920.03. 3.8. Acceptance
of the recoil separator
With the recoil separator tuned to the 13N7’ recoils (Section 3.6), Fig. 9 shows a two-dimensional density plot of the counts observed in the A&E telescope: the counts from the llN recoils are well resolved from the band of counts due to “leaky ” “C beams: at low channel numbers protons created by the “C + p one observes “leaky”
3050
SWITCHNG
3100
MAGNET
3150
B
[G]
Fig. 10. Fine-tuning of the recoil separator. i.e. of the magnetic fields of the switching magnet and the Wien filter, using the “N yield from p( “C,y)“N observed in the AE-E telescope (Fig. 1). The tuning values deduced from the “pilot” beam (T for Tuning) are systematically different by 0.6% from the centres (C) of the yield curves (see text). Also shown are the observed acceptances of the switching magnet and the Wien filter. which are larger than the emittance of the “N recoils from p( “C.y)“N. The curves through the data points represent fits using Fermi-functions at the edges.
L. Gialanella
et al. I Nucl.
Instr. and Meth. in Phys. Res. A 376 (1996)
compared to the value deduced from the “pilot” beam (T for Tuning), while C is lower by 0.55% compared to T in the case of the Wien filter. Both results indicate that the momentum p of the “N recoils is higher by about 0.60% compared to that of the pilot “C projectiles. The above tuning procedure assumed - in fact - an equal energy loss AE for both beams. Using the TRIM6-95 program [ 151 one finds AE, = 304 keV for the 8.08 MeV pilot “C projectiles (for zMW= 376 mm and p,,(HZ) = 5.0 mbar) and AE, = 182 keV for the 10.13 MeV “N recoils and AE, = 138 keV for the I I .O MeV ‘*C projectiles (both for z,,r = :,,/2 = 188 mm and p,,(H,) = 5.0 mbar); this leads to a field ratio of the switching magnet of B,IB, = 1.004, which is of the order observed. Fig. IO also shows the observed acceptance of the switching magnet, (Applp),, =I 1.9%, and that of the Wien filter, (Au/u),, = (Applp),, = 2.6%. Both values are significantly larger than the emittance, (A~/P)~ = 1.1%. of the “N recoils from p(“C, y)“N due to the isotropic emission of the 2.79 MeV capture y-rays at E,,,( “C) = I I .O MeV The calculated acceptance of the 30” switching magnet is (Apip),, = 2.6% for a radius of 802 mm, a path length of 500 mm, and a slit width of 10 mm; the observed value is smaller due to the subsequent limitations imposed by the Wien filter in combination with the slits in front of the AE-E telescope (Fig. 1).
3.9. Absolute cross section of p( ‘?C, yji9N In the case of ‘*C” projectiles, the ‘jN yield (I,,) normalised to the elastic scattering yield (I,) - is found to be I,,lZ,(q,, = 5+) = (0.71?0.03) X 10e7, and for ‘*C”+ projectiles it is I,,ll,(q,, = 6+) = (0.92?0.07) X lo-‘. Thus, the ratio of the two “N yields is I,,(qln =5+)/ I,,(q,, = 6+) = 0.77?0.07, in good agreement with the observed ratio of charge state probabilities @(q,, = 5+)/ dT(q,,, = 6+) = 0.69kO.03 (Section 3.7). The result supports the suggestion that the ‘“N nuclides - created by p( “C, y)‘jN in the Hz gas target - “memorise” the initial charge state q,, of the “C projectiles. The weighted mean value of the ratios (I,,ll~)(q,.)l~T(q,,) for q,, = 5+ and h- is (1.04?0.05) X IO-*. Using this result and Eq. (5) with the parameters I,,,,Qa, = I 1.3 20.4 mm msr (Section Q.mlL’,~, = 2.833t0.003 (Section 2), F~,,, = 2). 42227 mb/sr (Section 3.4), z,, = I,,,N = 376?8 mm (Section 3.1), and &1= 1.00+0.03 (Section 3.6), one finds a value of LT= 0.37kO.03 pb for the absolute cross section of p( “C, y)“N at E,,,,L, = 841 keV, in excellent agreement with the reported value r = 0.38?0.04 p_b [5].
4. Discussion A recoil separator in combination with a windowless gas target has been designed for the measurement of the
174-184
I83
radiative capture reaction p(‘Be, y)*B. The setup was tested quantitatively using the p( “C, y)“N reaction, whereby a suppression of the ‘*C beam particles by a factor 2 X lo-“’ was achieved. The setup can be used to measure an absolute capture cross section with high if the critical experimental parameters are precision, known with sufficient accuracy. In a recoil separator. it is of course necessary to make a charge state selection of the recoil nuclides, causing a reduction in the number of recoils transmitted through the separator. However, since there is usually a charge state representing about 50% of the total recoils produced, this reduction is not too serious. To achieve the above beam suppression in the present recoil separator, the selected charge state of the recoils had to be higher than the maximum possible charge state of the projectiles, i.e. the choice had to be fully stripped recoils. In the p( ‘“C, Y)‘~N studies at E,a,( “C) = I I .O MeV, the “N’+ recoils were detected with 4: = 6.6% and 9.6%, and in the planned studies of p(‘Be, y)‘B at E,ab(7Be) = 8.0 MeV the ‘B” recoils have an observed probability 4: = 64.2%. Although these probabilities are comfortably high for the present studies, at other projectile energies the charge state probability for fully stripped recoils can be quite low requiring an improvement in the setup. Such an improvement could be an energy filter (with an associated slit system) added at the end of the present recoil separator, which would filter sufficiently the “leaky” projectiles allowing for a free and optimum choice of the selected charge state of the recoils. The recoils emerge from the gas target within a cone with an opening angle 0, given by the relation tan 0, = E.,/(~~c*E,~~)“~,
(9)
where m and E,a, are the mass and the energy of the projectiles and E., is the energy of the capture y-rays. For most capture reactions this angle is a few degrees (e.g. 0, = 1.56” for “He( “C, y)lhO at Elab( “C) = 4.0 MeV), while in the present studies of p( “C. y)“N at E,*,( “C) = 1 l.OMeV and p(‘Be, v)~B at E,=,(‘Be) = 8.0 MeV it is 0, = 0.32” and 0.20”, respectively. The present setup of the gas target system restricts 13, to less than 0.40”; it was designed for a concurrent observation of the capture y-rays in p(‘Be, y)‘B (Figs. 2 and 3). Without such a restriction, the distances between the downstream apertures can be reduced significantly and - if more powerful pumps are installed-the diameter of the apertures can also be increased substantially. Both improvements should allow for a 100% emergence of the recoils having an opening angle of a few degrees. If the recoil nuclides are stable, such as in 4He(‘2C. y)lhO and p(gBe, y)‘“B, leaky beams of the recoil type can hamper seriously the measurements. These leaky beams emerging from the accelerator with a “white” spectrum of energies are many orders of mag-
1x4
L. Gicdme/la rt ul. I Nucl. Irutr. md Meth. in Phys. Rex. A 376 (1996) 17&184
nitude weaker than the beam of interest (and thus difficult to observe directly by current measurements), but they could be more intense than the typical intensity ratio IO- ” between the capture recoils and the projectiles of interest. Consider the case of p(“Be. y)“)B with q,“(‘Be) = 4+ and q,( “‘B) = 5+: the momentum filters of the accelerator are tuned to the rigidity R(‘Be) = m,v,/4, while the momentum filters of the separator are tuned to R( “‘B) = m ,,,v ,,,/.5 and the Wien filter is tuned to u,,, = (n~~lnr,,,)u,; thus, a leaky “‘B beam with q,(“‘B) = 4+ and v, = (m,lm,,,)v, will pass the analysing magnet and - if its charge state is changed to q, ( “‘B) = 5+ in the gas target - it also passes the recoil separator. This experiment was carried out and the AE-E telescope revealed “‘B particles at the energy corresponding to the capture reaction, whose intensity was however several orders of magnitude higher than expected from the known capture cross section. This problem can be eliminated to a large extend, if a Wien filter is installed before the analysis magnet (Fig. 1) purifying the projectiles emerging from this magnet. Of course, if the recoils are instable, this problem does not exist. In summary, the present recoil separator in combination with the windowless gas target and the technical improvements just discussed appear to make such a system a powerful tool to study radiative capture reactions over a wide range of energies. Such a system may be of particular interest to study the wide field of radiative capture reactions (predominantly of the type (p,y) and (o,y)) involving short-lived radioactive ion beams, needed for the field of nuclear astrophysics [4.l I].
Acknowledgements The Bochum members thank INFN and the University Federico II of Naples for the hospitality and other support given.
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