Actinides AMS at CIRCE in Caserta (Italy)

Nuclear Instruments and Methods in Physics Research B 268 (2010) 779–783

Contents lists available at ScienceDirect

Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

Actinides AMS at CIRCE in Caserta (Italy) M. De Cesare a,c,*, L. Gialanella c, D. Rogalla a,c, A. Petraglia a, Y. Guan a,d,g, N. De Cesare b,c, A. D’Onofrio a,c, F. Quinto a,e, V. Roca c,f, C. Sabbarese a, F. Terrasi a,c a

CIRCE, INNOVA, and Dipartimento di Scienze Ambientali, Seconda Università di Napoli, Caserta, Italy CIRCE, INNOVA, and Dipartimento di Scienze della Vita, Seconda Università di Napoli, Caserta, Italy c INFN Sezione di Napoli, Napoli, Italy d College of Physical Science and Engineering Technology, Guangxi University, Nanning 530004, China e Faculty of Physics, University of Vienna, Vienna, Austria f Dipartimento di Scienze Fisiche, Università Federico II, Napoli, Italy g ICTP, Trieste, Italy b

a r t i c l e

i n f o

Article history: Available online 7 October 2009 Keywords: Uranium AMS Optical simulation TOF-E

a b s t r a c t The operation of Nuclear Power Plants and atmospheric tests of nuclear weapons performed in the past, together with production, transport and reprocessing of nuclear fuel, lead to the release into the environment of a wide range of radioactive nuclides, such as uranium, plutonium, fission and activation products. These nuclides are present in the environment at ultra trace levels. Their detection requires sensitive techniques like AMS (Accelerator Mass Spectrometry). In order to perform isotopic ratio measurements of the longer-lived actinides, e.g., of 236U relative to the primary 238U and various Pu isotopes relative to 239Pu, an upgrade of the CIRCE accelerator (Center for Isotopic Research on Cultural and Environmental Heritage) in Caserta, Italy, is underway. In this paper we report on the results of simulations aiming to define the best ion optics and to understand the origin of possible measurement background. The design of a high resolution TOF-E (Time of Flight-Energy) detector system is described, which will be used to identify the rare isotopes among interfering background signals. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Nuclear Power Plants (NPPs) convert nuclear energy into electric power, while nuclear weapons use the destructive effects of supercritical chain reactions; both are based on the release of energy due to fission of U and Pu. Release from different sources containing U or Pu might affect the environment on a large (atmospheric weapons test fallout and Chernobyl accident) and on a local (around NPPs) scale. The accumulating concentrations of the burn-up products like plutonium and uranium isotopes, depend on the specific composition of the source material and on its subsequent irradiation history. These sources do not contribute equally to the Pu and U signature at a given site. In turn, isotopic analysis of environmental samples can be used to identify the source of an observed contamination. Useful tools to disentangle different sources are the isotopic ratios 236U/238U, 240Pu/239Pu, 241 Pu/239Pu, 242Pu/239Pu and 238Pu/239+240Pu (238Pu cannot be measured by AMS but by a-spectrometry technique due to interference

from natural 238U). The long-lived radionuclides 236U and Pu isotopes (e.g., the half life of 236U is 2.3  107 y and of 244Pu is 8.1  107 y) are present in environmental samples at ultra trace levels (the 236U concentration is quoted to be in the order of pg/ kg or fg/kg and, e.g., around fg/kg for 239Pu) [1]. Measurement of these isotopic ratios requires mass spectrometric techniques, but only AMS offers the sensitivity needed to measure 236U/238U at natural levels (109 to 1013) because of its capability to suppress background induced by isobaric molecules such as 235UH. However, the typical concentration of 238U in environmental samples is several orders of magnitude higher than that of 238Pu, and no chemical procedure is efficient enough to separate the plutonium fraction from uranium at levels required to allow a unambiguous mass spectrometric measurement of 238Pu. To summarize, AMS is well suited for quantifying 236U and Pu isotopes; only for the measurement of 238Pu concentrations, AMS is not suitable and alphaspectroscopy remains the appropriate technique [2]. 2. Uranium measurements at CIRCE

* Corresponding author. Address: CIRCE Laboratory and Department of Environmental Sciences, Second University of Naples, Viale Carlo III 153, San Nicola la Strada, Italy and Via Vivaldi 43, Caserta, Italy. Tel.: +39 0823 274814/0823 274631; fax: +39 0823 274814/0823 274605. E-mail address: [email protected] (M. De Cesare). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.10.029

CIRCE is a dedicated AMS facility based on a 3MV-tandem accelerator. Different from many nuclear physics applications, the pretreated sample material (a few mg is put into the ion source) itself

780

M. De Cesare et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 779–783

is analyzed by two mass spectrometers (called injector and analyzer) which are coupled to the tandem accelerator. A schematic layout of the CIRCE facility is shown in Fig. 1. The system, originally equipped for Radiocarbon-AMS [3], will be upgraded to perform AMS measurements with actinides. CIRCE

is equipped with an injection magnet being able to bend all the elements up to the heaviest masses in the periodic table, and an analyzing magnet (ME/q2 = 176 MeV amu/e2) which allows to bend, e.g., 238U5+ at E = 18 MeV. Fig. 2 shows the scan of the injection magnet while measuring negative ion currents in the Faraday

Circe Accelerator Injection Magnet ME/q2= 15 MeV amu/e2 r= 0.457 m

Multi beam switcher

FC02

x/y steerer

Gas stripper

FC03

Electrostatic quadrupole triplet

Electrostatic Analyzer E/q= 90 keV/e r= 0.300 m

Slit system

Analyzing Magnet ME/q2= 176 MeV amu/e2 r= 1.270 m

Offset FC and Stable Isotope Measurement

Beam profile monitor

y steerer Focus FC04

FCS1

Heavy Isotope Detection

Sample material

E-detection

Magnetic quadrupole doublet

Electrostatic Analyzer E/q= 5.1 MeV/e r= 2.540 m

TOF-E detector 14 C detection

FC05

ERNA separator

Fig. 1. Schematic layout of the CIRCE accelerator and of CIRCE accelerator upgrade with the new heavy isotope detection layout: the switching magnet, the magnetic quadrupole doublet, the start and the stop TOF-E detector. FC denotes Faraday Cup; arrows indicate a slits system. ERNA is the acronym of European Recoil separator for Nuclear Astrophysics.

1.0E-06

238

1.0E-09

1.0E-10 230

Positive ion Current (A) - FC03

238

U16O3-

U16OU16O-+ 238U14N- + 235U17O-

1.0E-08

U-

235

1.0E-08

1.0E-07

236

1.0E-07

Negative ion Current (A) - FC02

238

238

U16O2-

U16O-

1.0E-06

1.0E-09

240

250

260

270

280

1.0E-10 290

Injected mass (amu) Fig. 2. Negative ion (dashed line, left side of the ordinate) and positive ion currents (solid line, at the right side of the y-axis) versus injected mass. The 238U16O peak (mass 254) yields the highest current. For 236U measurements the injected mass is 252 (236U16O). The strongest contribution to the observed peak is due to 238U14N.

781

M. De Cesare et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 779–783

Cup FC02 (see Fig. 1). Also currents from positive ions measured in the Faraday Cup FC03 (located right after the exit of the tandem) are plotted in Fig. 2. A U3O8 sample was used for these measurements. The main difficulty measuring 236U is the intense 238U ion-beam. When all the accelerator parameters are set to count 236 U, most of the 238U ions are filtered by the injection magnet, analyzing magnet and the electrostatic analyzer. Still, a small fraction of 238U ions is transported to the final detector due to charge exchange in the residual gas in the beamline [4,5], and they interfere with the 236U detection [6]. In Fig. 3 the scans of the high-energy magnet while injecting 238U16O and 236U16O, respectively, are shown. The currents were measured in the Faraday Cup FC04, located after the analyzing magnet. The ordinate on the left side of the figure corresponds to 238U16O injecting mass. Besides the peaks, which correspond to charge states from 5+ to 12+, one clearly observes measurable currents originating from 238U ions which underwent charge exchange reactions, mainly due to the bad vacuum before analyzing magnet, 3.5  107 Torr. The y-axis on the right side of Fig. 3 shows the current in FC04 when mass 252 was injected (e.g., 236U16O or 238U14N). The arrow indicates the position of 236U5+ when 236U16O is injected. A tuning of the transport elements up to the final detector is required in order to maximize the ion optical transmission. The number of events of 236U5+ is measured in the final detector and 238U5+ is measured as current in the high energy side. The target material uranium oxide (U3O8), obtained from uranyl nitrate (sample label ‘‘VERA-KkU”, 236U/238U = (6.98 ± 0.32)  1011) [7], was measured at the 0° beamline by a 60 mm diameter silicon detector. The preliminary result indicates that the 236U/238U background level is about 3  109.

3. Upgrade of the CIRCE AMS facility This section focuses on the planned upgrade of the CIRCE setup, which is needed to achieve the necessary sensitivity of

236

U/238U  1012 to 1013. The first step of the upgrade was to add a switching magnet (SM), which provides both, the option for several dedicated beamlines and it needed as an additional purification stage. In Fig. 4 the COSY infinity magnetic optics simulation [8] is shown, where the development of two beams has been analyzed, starting from the waist of the high-energy magnet with a relative energy difference of DE/E = 0.001 (corresponds to the resolution of the ESA). The adopted beam profiles are approximately Gaussian, with a halfwidth of 0.15 cm. A maximum divergence of 3 mrad was assumed. The position of the elements has been varied in such a way to maximize the quantity M = D/L, the ratio between the distance D of the center of the two beams, 238U and 236U5+, and the sum of the half-widths of the two beam intensity distributions (L) in the x-plane (horizontal). Simulations were performed for different geometric configurations. The distance ESA-SM (energy electrostatic analyzer-switching magnet, respectively) is denoted as (a), SM-MQD (switching magnet-magnetic quadrupole doublet) as (b) and (c) = MQD-FP (Focal Plane = the doublet focusing position). For a given distance of (a) and (b), M is weakly dependent on the distance (c), since, if the focal distance increases, there will be also an increase of both, the distance of the two beam centers and of the width of the beam intensity distribution. For fixed values of (a) and (c) one also cannot find significant changes in M while varying (b): e.g., increasing (b) is correlated with the beams becoming more focused and less spaced, since the beams get more spread in the MQD element and the fields have to be higher. If one compares the parameter set obtained with fixed (b) and (c) while varying (a), one notes an increase of M values with a decreasing (a). The maximum for the quantity M was obtained for the values: ESA-SM = 0.5 m, SM-MQD = 3.04 m and MQD-FP = 2.54 m. In Fig. 1 the future CIRCE upgrade is shown, where the switching magnet has been already installed (Bmax = 1.3 T, r = 1.760 m and ME/q2 = 252.5 MeV amu/e2 at the 20° exit). The first step, in the heavy isotope beamline, will be to include, a position sensitive silicon detector, with an active area of 58  58 mm2, mounted just

3.0E-10

1.0E-07

Injected

236

U160

-

5+

238

U

Injected

U

238

3+ 238

U

5+ 238

U

4+

U

U

238

2.6E-10

8+

3+ 238

2.4E-10

U

1+ 238

U

4+ 238

U

2+

U

238

238

U

5+

U

238

9+

U

238

10+

U

238

11+

U

12+

U

238

238

1.0E-09

4+

U

238

6+ 238

1.0E-08

Positive Ion current (A)- FC04

2.8E-10

238

7+

16

14

Positive ion current (A)- FC04

N

1+

O

U

1+

6+

238

U160-

2.2E-10

1.0E-10 6400

7400

8400

9400

10400

11400

12400

13400

14400

2.0E-10 15400

High energy magnet ( G) Fig. 3. Positive ion current versus high energy magnetic field. The scan of the high-energy magnet was monitored with FC04. The dashed line corresponds to the injection of 238 16  U O (left y-axis, logarithmic scale), the solid line to 236U16O (right y-axis, linear scale), respectively. The terminal voltage was set to TV = 2.875 MV.

782

M. De Cesare et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 779–783

Fig. 4. Simulation with Cosy Infinity of the transport of 236U5+ and 238U ions with a energy of 17 MeV. ESA corresponds to Electrostatic Analyzer, SM to Switching Magnet, MQD to Magnetic Quadrupole Doublet and the two planes indicate the position of start (SD1) and stop (SD2) measurement of the TOF-E detector. The position of the focal plane (FP) is the waist of the magnetic quadrupole doublet.

after the waist of ESA. The second step will be to install a TOF-E (Time of Flight-Energy) detector system with a path length of 1.5 m; the start detector will be positioned at the same position of the mentioned silicon detector. The last step will be to install a magnetic quadrupole doublet, after the switching magnet, which is needed for focusing the ions in the Focal Plane, between start and stop detector of the TOF-E system. At the end of this line a TOF-E detector will be placed, with a path of 3 m, that should allow to reach further suppression of the still interfering 238U and 235U. 4. TOF-E detection system In Fig. 5a calculation of the performance of the TOF-E detector is shown for 236U5+ and 238U ions, respectively. In our artificial set-up

820

δΕ = 200 keV

L =3m

δΤ = 0.8 ns

810

U

T (ns)

238

800

790 16.5

236

the start signal for the TOF measurement is given by a MCP (MicroChannel Plate), while the energy information is given by a silicon detector, which also provides the timing for the stop signal. Briefly, ions pass through a thin carbon foil placed orthogonal to the beam axis and are detected by a silicon detector that is located some meters downstream to the start detector. Electrons are stripped from the carbon foil and are accelerated by an acceleration grid that is parallel to the carbon foil. Since most of the electrons are stripped from the carbon foil with an energy of a few eV [9], after acceleration, they have approximately the same energy and their velocity direction is almost parallel to the beam axis, e.g., for the electrons that exit parallel to the foil (h = 90°) with energy of Eelectron = 10 eV and the potential difference between the carbon foil and the electrons acceleration grid DV(Vfoil  Vaccl.) = 1.1 kV, we calculate (1) that the exit angle from the acceleration grid (b) is 95 mrad and so after 10 cm they will have a shift of 9.5 mm:

5+

U

17

1.75 E (MeV)

Fig. 5. Time-of-flight T (ns) correlation with energy E (MeV) for 236U5+ ions with 17 MeV particle energy and for 238U ions. The calculation shows that the two isotopes are well separated (1r) for a flight path of 3 m (see text).

b¼

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4 sin h  Eelectron 2

2

sin h  cos2 h  Eelectron þ DV  sin h

:

ð1Þ

The electrons are then bent, by means of an electrostatic mirror, towards the MCP which is placed parallel to the beam axis [10]. The mirror consists of two parallel grids placed at 45° to the beam axis. The electrostatic field between these two grids bends the electrons through 90° towards the MCP. As calculated from (1), there are no problems to collect the electrons with MCP of 25 mm diameter. If the distance between start and stop detectors (L) is known, measuring T, the time between start and stop signals, (that is equivalent to L/v, v is the velocity of the ion) and knowing the relation between the velocity and the kinetic energy (E) of the ion, one obtains:

T ¼ L  ðm=2EÞ1=2 ; where m is the mass in the non-relativistic approximation.

ð2Þ

M. De Cesare et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 779–783

This means that if T as a function of the energy of the ions is plotted, ions with different masses will arrange on different curves. In Fig. 5 we show a calculation for 236U5+ and 238U for a flight path L = 3 m, dE = 200 keV and dT = 0.8 ns resolutions. For L = 3 m the time difference DT(236U5+  235U) = 1.7 ns and DT(238U  236U5+) = 3.4 ns for the same energy. For half of the flight path, i.e. L = 1.5 m, DT(236U5+  235U) becomes 0.8 ns and DT(238U  236U5+) = 1.7 ns. Because of this overlap, it is not possible to separate 235U sufficiently from 236U5+. A MCP start detector in electrostatic mirror configuration, positioned 1 m after the MQD (magnetic quadrupole doublet), and a silicon detector providing the stop signal will be used in our upgrade. The later is positioned after 3 m flight path. The divergence of the beam for particles passing a carbon foil of 0.6 lg/cm2 thickness, was calculated with SRIM [11]. For 90% of particles scattering angles less then 2 mrad were calculated; with 9% probability angles between 2 and 10 mrad and for 1% of the particles more then 10 mrad was found. The relatively small angular straggling indicates that the carbon foil should have a negligible effect on the optics of the beam and consequently, on the separation of adjacent masses. Under these assumptions the trajectories between ESA and stop detector of 236U5+ and 238U were calculated, as shown in Fig. 4. A more detailed investigation of the effect of angular straggling induced by the carbon foils is in progress, which will incorporate the results of the SRIM calculations into the optical simulations (COSY Infinity). 5. Summary In this work the planned upgrade layout of CIRCE for AMS measurements on actinides has been described and the envisaged one,

783

aiming for a sensitivity of 1012 to 1013 in uranium isotopic ratio measurements, has been discussed. The preliminary result of 236 U/238U background ratio level was larger then 1  109 at the 0° beamline. The main upgrade so far has been the addition of a switching magnet placed 50 cm after the exit of the high-energy ESA. This magnet offers the possibility to use a dedicated beamline, and it provides a supplementary dispersive analysis tool. In addition the magnetic quadrupole doublet and the TOF-E system will be installed according to the COSY simulations. Acknowledgements VERA (Vienna Environmental Research Accelerator) Laboratory kindly provided the measured samples. This work was supported by SOGIN, SOcietà Gestione Impianti Nucleari, and by ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy. References [1] F. Quinto, Ph.d. Thesis 2004–2007, Assessment of contamination of river sediments by liquid releases from the Garigliano Nuclear Power Plant. [2] L.K. Fifield, Quatern. Geochronol. 3 (2008) 276–290. [3] F. Terrasi et al., Nucl. Instr. Meth. Phys. Res. B 259 (2007) 14–17. [4] H.D. Betz et al., Rev. Modern Phys. 44 (1972) 465–539. [5] C. Vockenhuber et al., Int. J. Mass Spectrom. 223–224 (2003) 713–732. [6] AMS 11 conference, contribution: E3_I1, Peter Steier: Analysis and application of heavy isotopes in the environment. [7] P. Steier et al., Nucl. Instr. Meth. Phys. Res. B 266 (2008) 2246–2250. [8] K. Makino et al., Nucl. Instr. Meth. Phys. Res. A 427 (1999) 338–343. [9] H. Rothard et al., Nucl. Instr. Meth. Phys. Res. B 258 (2007) 91–95. [10] M. De Cesare et al., Mem. S.A.It. 78 (2007) 458–464. [11] J.F. Ziegler et al., SRIM program, 2006, .