c Pleiades Publishing, Ltd., 2014. ISSN 1063-7788, Physics of Atomic Nuclei, 2014, Vol. 77, No. 7, pp. 817–823.  c Yu.M. Sereda, S.M. Lukyanov, A.G. Artukh, A.N. Vorontsov, E.I. Voskoboynik, M.P. Ivanov, D.A. Kyslukha, S.A. Klygin, G.A. Kononenko, V.A. Maslov, Original Russian Text  T.I. Mikhailova, Yu.E. Penionzhkevich, B. Erdemchimeg, 2014, published in Yadernaya Fizika, 2014, Vol. 77, No. 7, pp. 864–870.

NUCLEI Experiment

Investigation of the Fragmentation of 20 Ne and at the COMBAS Setup

40

Ar Ions

Yu. M. Sereda1), 2) , S. M. Lukyanov1), 2) , A. G. Artukh1), 2)* , A. N. Vorontsov1), 2) , E. I. Voskoboynik1), M. P. Ivanov1) , D. A. Kyslukha1), 3) , S. A. Klygin1) , G. A. Kononenko1) , V. A. Maslov1), T. I. Mikhailova1), Yu. E. Penionzhkevich1) , and B. Erdemchimeg1), 4) Received October 12, 2013

Abstract—Properties of the COMBAS fragment separator are compared with respective properties of similar setups. Results of experiments aimed at obtaining products of one-proton-stripping reactions induced by a beam of 40 Ar ions with an energy of 35 MeV/A and two-neutron-stripping reactions induced by a beam of 20 Ne ions with an energy of 52 MeV/A are presented. A high resolution of the fragment separator in obtaining secondary neutron-rich 39 Cl and neutron-deficient 18 Ne ion beams is demonstrated. DOI: 10.1134/S1063778814060131

1. INTRODUCTION Beams of accelerated radioactive nuclei make it possible to obtain and study neutron-rich and protonrich nuclei. This permits making considerable advances along traditional lines of research in nuclear physics such as the synthesis of new nuclei and investigation of their properties, which, as even the first experiments with radioactive beams showed, may differ substantially those that are already known and which were predicted earlier. The application of projectile nuclei characterized by an anomalous value of the ratio N /Z may furnish radically new information about nuclear-reaction mechanisms. Transfer and fragmentation reactions, which are reaction types that are the most widespread as a means for producing exotic nuclei, have efficiently been used worldwide for many years in large research centers such as GANIL (France), GSI (Germany), MSU NSCL (USA), and RIKEN (Japan). The objective of the present study was to explore, in forward-angle measurements at the COMBAS fragment separator, the yields and separation of the isotopes 39 Cl and 18 Ne produced in, respectively, the reaction of stripping of one proton off a 40 Ar projectile 1)

Joint Institute for Nuclear Research, ul. Joliot-Curie 6, Dubna, Moscow oblast, 141980 Russia. 2) Institute for Nuclear Research, National Academy of Sciences Ukraine, pr. Nauki 47, Kyiv, 03680 Ukraine. 3) Karazin Kharkiv National University, pl. Svobody 4, Kharkiv, 61000 Ukraine. 4) Nuclear Research Center, National University of Mongolia, Ulaanbaatar 210646, Mongolia. * E-mail: [email protected]

nucleus and the reaction of stripping of two neutrons off a 20 Ne projectile nucleus. It is well known that momentum distributions of products of few-nucleontransfer reactions do not show a significant deviation from the projectile momentum; moreover, the momentum distributions of products originating from such nuclear reactions in the case of employing thick targets may overlap substantially the projectile momentum. Products of few-nucleon-transfer reactions are the most interesting from the point of view of obtaining high-intensity secondary beams of radioactive nuclei since the cross sections for their production is maximal. 2. EXPERIMENTAL CONDITIONS The experiment described in the present article employs the high-luminosity and high-resolution COMBAS fragment separator [1], which ensures an efficient collection and transportation of high-energy (up to 150 MeV/A) product nuclei over a the broad charge-number range of 2  Z < 30. In creating this separator, the strong-focusing principle was used for the first time in the world. Owing to this, we were able to obtain record parameters of the setup, simultaneously minimizing higher order aberrations. The COMBAS magnetic structure (see Fig. 1a) is based on the use of wide-aperture multipole magnets, where quadrupole, sextupole, and octupole components, which are required for minimizing high-order aberrations, are generated. The use of such multipole magnets made it possible to eliminate individual quadrupole, sextupole, and octupole lenses, whose 817

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(a)

M2

M5 – M8 Second selection by Bρ M3

M4 Fd M5 M6

M7

M1

M8 D AC1

Beam stopper

F0 T0 Primary beam

Fa

Dispersion focus

Production target

Correlation chamber

AC2 AC3

(b)

1 ΔE1 1 ΔE2

Fa

1E Telescope

Fig. 1. Schematic view of the experimental setup at the COMBAS fragment separator. It contains (a) an Fo M1 M2 M3 M4 Fd M5 M6 M7 M8 Fa magnetic-optical scheme of the fragment separator with degrader D and avalanche counter AC1 at the dispersive focus Fd and (b) a correlation chamber with telescope detectors (ΔE1 , ΔE2 , E) and a pair of coordinate avalanche counters AC2 and AC3 for measurements of track particles. The AC1 and AC2 avalanche counters provide TOF measurements. The telescope detectors consist of two Si 32-strip detectors, ΔE1 (X coordinate) of thickness 380 μm and ΔE2 (Y coordinate) of thickness 1000 μm and a scintillation (CsI(Tl)) detector for residual-energy measurement (E).

apertures basically constrain the aperture of the separating channel. The COMBAS fragment separator (Fig. 1a) involves eight (M1 –M8 ) multipole magnets forming a wide-aperture separator that implements triple focusing of particles at the exit focus Fa (in energy and in the horizontal and vertical directions). The separator is designed in the form of two identical sections (M1 –M4 and M5 –M8 ) with a symmetry plane in the middle part (dispersion focus Fd ). The first analyzing section (M1 –M4 ) plays the role of a filter for high-energy particles in momentum, rejecting the primary beam, while the second section (M5 –M8 ) compensates for the dispersion in the first one and minimizes effects of aberrations at the exit achromatic focus Fa . The presence of symmetry between the two sections of the separator makes it possible to use, without violating optics, a penetrate foil (degrader) at the position Fd . The degrader provides an additional

isotope separation in the second section because the energy loss of particles by ionization in the foil is different for separated and satellite nuclear-reaction products. Moreover, the flight base of the second section, which is free from an intense primary-particle beam, may efficiently be used for time-of-flight (TOF) measurements. Such measurements are required for implementing an additional separation of isotopes in A and Z in the case where their momentum distributions overlap one another or in the case where a substantial contribution comes from charged states of transported ions. The table presents a comparison of basic features of the COMBAS wide-aperture fragment separator and similar fragment separators currently operating at research centers in France, USA, Japan, and Germany. From the table, one can see that the COMBAS is superior to them in the momentum and angular acceptances (by, respectively, a factor of 4 to 10 and PHYSICS OF ATOMIC NUCLEI Vol. 77

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Comparison of parameters of existing fragment separators Separator

Ω, msr

Δp/p, %

Bρ, T m

Resolution

LISE (France) [2]

1.0

5.0

3.2

800

A1200 (USA) [3]

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3.0

5.4

700–1500

RIPS (Japan) [4]

5.0

6.0

5.76

1500

0.7–2.5

2.0

9–18

240–1500

6.4

20

4.5

4300

FRS (Germany) [5] COMBAS (Russia) [1]

a factor of 1.5 to 6.4), which are quantities of importance for improving the collection of secondary beams of radioactive nuclei. In order to detect efficiently the whole variety of nuclear-reaction products transported to the exit focus Fa , the detecting system (see Fig. 1b) is designed in the form of a telescope containing three detectors. This is necessary for simultaneously detecting the whole spectrum of particles from long-range light reaction products, in which case all three detectors are involved to short-range particles producing a strong ionization and having large atomic numbers, in which case use is basically made only of the first two detectors along the beam. The telescope consists of three detectors (ΔE1 , ΔE2 , and E). The ΔE1 detector is a 32-strip silicon X detector 380 μm in thickness and 64 × 64 mm2 in area for measuring the particle coordinate along the horizontal direction. The ΔE2 detector is a 32strip silicon Y detector 1000 μm in thickness and 64 × 64 mm2 in area for measuring the particle coordinate along the vertical direction. The E detector is a granular assembly of nine full-absorption scintillation detectors 20 mm thick, the total area of the assembly being 64 × 64 mm2 . The strip structure of the penetrate ΔE1 and ΔE2 detectors also played an important role in tuning beam products to the exit focus Fa with minimal losses in the telescope aperture. In the course of the exposure, the monitoring functions of the strip detectors made it possible to correct the fragment-beam axis in X and Y toward the Fa focus. A 9 Ве target 15 mg/cm2 thick was irradiated with a primary beam of 40 Ar nuclei with an energy of 35 MeV/A (or 20 Ne nuclei with en energy of 52 MeV/A). The primary-beam spot at the target was collimated by means of a diaphragm with an aperture diameter of 6 mm. The intensity of the primary beam incident to the target was measured by a current detector in μA units. The measurements of the current at the target were then used to normalize the yields of reaction products to the number of beam particles in order to determine their production cross sections. PHYSICS OF ATOMIC NUCLEI

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The momentum distributions of reaction products were determined by analyzing reaction-product yields at the exit focus Fа versus the magnetic rigidity Bρ, which were normalized to the primary-beam current measured in each exposure by the current detector in μA units. In order to increase the counting rate, the momentum spectra of reaction products were scanned with a 2% momentum window whose width was determined by the collimator slit in the dispersion plane Fd . In order to study the degrader separating ability for isotopes—for example, 39 Cl (or 18 Ne)—the magnetic rigidity Bρ was set to the values corresponding to the maxima of the momentum distributions for the measured isotopes. The degree of purification of the isotope 39 Cl [in the 40 Ar (35 MeV/A) + 9 Ве reaction] and the isotope 18 Ne [in the 20 Ne (52 MeV/A) + 9 Ве reaction] from satellite reaction products was determined by means of a comparison for the fraction of counts associated with the isotope 39 Cl (or 18 Ne) in the total sum of isotopes detected first in the course of a degraderfree exposure in the dispersion plane Fd . After the insertion of the degrader at Fd , the exposure was repeated in order to determine the fraction of 39 Cl (or 18 Ne) counts in the new total sum of detected products. Moreover, the magnetic rigidity Bρ of the M5 –M8 (second) separator section must be reduced by the value of the energy loss of the isotope 39 Cl (or 18 Ne) throughout the thickness of the aluminum-foil degrader. 3. SEPARATION AND IDENTIFICATION OF FRAGMENTS The ion charge q, mass number m, and atomic number Z were determined by the formula  2 mv 2 Z = dE + Er , , mv ∼ qBR, dE ∼ v 2 where dE is the energy loss of a particle in a thin ΔE detector and Er is its energy loss in the fullabsorption E detector.

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dE

dE

(a)

(b)

15O

80

15O

14

80

N

14

N

13C

13C

60

60

40

40 10B

20

20

7

Be

20

40

60

80

0

20

40

60

80 TOF

Er

Fig. 2. Identification matrices of detected fragments at Fa versus the measured quantities: (a) dE and Er and (b) dE and time of flight. These diagrams were obtained for the 20 Ne (52 MeV/nucleon) + 9 Be reaction. All values are indicated in the channels.

We performed experiments aimed at testing the possibility of obtaining secondary radioactive beams formed in reactions involving the fragmentation of 20 Ne (52 MeV/A) and 40 Ar (35 MeV/A) ions. Figure 2 shows examples of experimental twodimensional matrices of events detected in the 20 Ne (52 MeV/A) + 9 Ве reaction—namely, the distribution of events in the plane spanned by the variables dE and Er (Fig. 2a) and the distribution of events in the plane spanned by the variable dE and the time of flight (Fig. 2b). From these diagrams, one can see that an apparatus combining a fragment separator and a detecting system has a high resolution and makes it possible to determine the absolute values of m, Z, and q. Figure 3 presents two-dimensional diagrams in the form of the product yield as a function of the specific loss dE and the residual energy Er . The diagram in Fig. 3a was obtained for the reaction in which 40 Ar beam ions of energy 35 MeV/A undergo fragmentation on a 9 Be target 80 μm thick. The diagram in Fig. 3b was obtained for the same reaction in the case of employing an aluminum absorber (wedge) in the dispersion plane Fd of the separator. One can see that a high dispersion of the separator makes it possible to separate quite efficiently, among a multitude of product fragments, a rather clean monoisotopic beam (78%) of 39 Cl+17 ions in the Fa focal plane of the separator. The admixture of 37 S+16 ions is about 10%, the total contribution of all other ions, including 38,40 Cl and 41 Ar, being less that 12%. The total counting rate in the semiconductor telescope used to identify fragments was about 2000 particles per second at the 40 Ar primary-beam current of about 100 nA.

The possibility of obtaining light proton-rich isotopes was demonstrated by using the primary beam of 20 Ne nuclei with an energy of 52 MeV/A. For this purpose, we have chosen 18 Ne ions produced in the reaction where two neutrons are stripped off one of the ions of a 52-MeV/A 20 Ne ion beam incident to a 9 Be target 200 μm thick. The respective two-dimensional diagram in the form of the product yield as a function of the specific energy loss dE and the residual energy Er is given in Fig. 4. The diagrams presented here show that a high dispersion of the separator makes it possible to separate efficiently, from a great many product fragments, a rather clean monoisotope beam of 18 Ne+10 ions (60%) in the separator focal plane Fa . The admixture of 17 F+9 ions is 20%, the total contribution of the remaining ions, including 7 Be, 10 B, 12 C, and 13 N, being less than 15%. The total counting rate in the semiconductor telescope used to identify fragments is 2000 particles per second at the 20 Ne primary-beam current of about 100 nA. 4. CROSS SECTIONS FOR FRAGMENT PRODUCTION AND INVARIANCE OF THE FRAGMENTATION PROCESS WITH RESPECT TO ENERGY An extremely wide variety of heavy-ion reactions involving a large number of possible projectile–target nucleus combinations opens good prospects for obtaining as-yet-unknown isotopes of known elements, isotopes featuring a large excess or deficit of neutrons and lying in the vicinity of or even beyond the nucleon drip line. The search for reactions that have the largest cross sections for the production of the exotic PHYSICS OF ATOMIC NUCLEI Vol. 77

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dE 18Ne

Q=Z–1 40

80

+16

Ar

17

F

39K+18

60 36Ar+17

60

15O

34

Cl+16

40 S

Si+13

13

40

32 +15

31

N

12

C

30 +14

P

20

28Si+13

10B

20

7Be

(a) 0

0

30

40

50

60

70

Cl+17

40

(b) 20

40

60

80

Er

Fig. 3. Two-dimensional diagrams in the form of the product yield as a function of the specific loss dE and the residual energy Er for (a) the reaction in which projectile 40 Ar ions of energy 35 MeV/A undergo fragmentation on a 9 Be target 80 μm thick and (b) the case where use was made of a 400-μm aluminum absorber (wedge) in the separator dispersion plane Fd . All values are indicated in the channels.

nuclei of interest is very important for obtaining the maximum yield of exotic products. Figure 5 gives experimental values of cross sections for isotope production in 40 Ar+9 Be fragmentation reactions versus the isotope mass at various energies of 40 Ar ions. The dependences presented in Fig. 5 show that the fragmentation cross section is severalfold smaller at the energy of 57 MeV/A [7] than at higher projectileion energies. One possible explanation for the observed distinctions between fragmentation cross sections assumes different systematic errors in the measurements [6–9]. The transmission value used to calculate the fragmentation cross section is the main source of the systematic error. As a matter of fact, PHYSICS OF ATOMIC NUCLEI

60

80

Fig. 4. Two-dimensional diagrams in the form of the product yield as a function of the specific energy loss dE and the residual energy Er in the reaction where two neutrons are stripped off a nucleus from a 52-MeV/A 20 Ne ion beam interacting with a 9 Be target 200 μm thick. This data set was obtained in the case of employing a 400-μm aluminum absorber (wedge) in the dispersion plane Fd of the separator. All values are indicated in the channels.

39

0

40

Er

60

20

20

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the values quoted for the fragmentation cross sections are semiempirical since one takes into account the transmission values obtained from the calculations of the angular and momentum acceptances of the fragment separator used. An alternative explanation is also possible [10]: a high energy may give rise to cascade processes and, hence, lead to an increase in the contribution of products of secondary reactions. It follows that, at high beam energies, a secondary process of the fragmentation of fragments that have already arisen may proceed in a target material. This process should lead to enhanced production of lighter isotopes, as one can see from the dependences quoted here: the cross sections for the production of, for example, neon to magnesium isotopes at the energies of 57 and 120 MeV/A differ by a factor of three to five, but there is no such difference for neutron-rich phosphorous isotopes. By and large, we conclude that the observed difference in the values of the fragmentation cross sections is insignificant and does not lead to substantial discrepancies between the estimates of the fragmentation-product yields. On the basis of the dependences presented in Fig. 5, we conclude that the

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102 O

F

Ne

Na

Mg

Al

Si

P

100 10–2 10–4

σ, mb

10–6 102 100 10–2 10–4 10–6

15 20 25 30

20 25 30 35 20 25 30 35 20 25 30 35 40 A

Fig. 5. Cross sections for isotope production in 40 Ar+9 Be fragmentation reactions versus the isotope mass the 40 Ar-ion energies per nucleon of (open boxes) 1000 MeV [6], (open inverted triangles) 57 MeV [7], (open circles) 120 MeV [8], (closed circles) 90 MeV [9], and (closed stars) 35 MeV (our present study).

fragmentation cross sections remain invariant with respect to the projectile energy. 5. CONCLUSIONS The properties of the COMBAS fragment separator are compared with the respective properties of similar setups. By virtue of the quoted comparative ion–optical characteristics and the technical implementation of the setup, the wide-aperture kinematical separator COMBAS is currently one of the best fragment separators existing in research centers worldwide. The procedure used for the detection, identification, and isotope separation of reaction products has been described. The results of experiments devoted to measuring the yields of Z = 4–23 isotopes produced in the fragmentation of a primary beam of 40 Ar ions with an energy of 35 MeV/A on a 9 Ве target has demonstrated a high “in-flight” separation ability of the COMBAS separator. The resolution of products in Z and A is illustrated in Fig. 3 for the example of separation of the isotope 39 Cl produced in the oneproton-stripping reaction. It is well known that the momentum distributions of products originating from few-nucleon-transfer reactions deviate insignificantly from the projectile momentum and may even overlap it in the case of employing thick targets. Such nuclear products are of greatest interest from the point of

view of the production of high-intensity secondary beams of radioactive nuclei since their production cross sections are maximal. By using an unshaped aluminum degrader in the Fd dispersion plane, a high degree of purification of the beam of 39 Cl neutronrich nuclei (about 78%) from the admixture of satellite fragmentation products and the background of undiscriminated primary-beam particles was attained in our experiment. We have also studied the yield of Z = 2–11 isotopes in the fragmentation of 20 Ne ions with an energy of 52 MeV/A on a 9 Ве target. A 60% degree of purification from admixtures of satellite products of primary-beam fragmentation was obtained for 18 Ne neutron-deficient nuclei, which are promising for studying direct two-proton decay. The purification degree reached in our experiment for isotopes produced in the reactions of stripping of one proton off a 40 Ar projectile ion and two neutrons off a 20 Ne projectile ion makes it possible to form nearly monoisotope 39 Cl and 18 Ne secondary beams of intensity sufficient for spectroscopic investigations. On the basis of known data on the fragmentation of 40 Ar primary ions at various energies, we have shown that the cross sections for fragment production depend only slightly on the projectile energy over broad ranges of Z and А. PHYSICS OF ATOMIC NUCLEI Vol. 77

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ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (project no. 13-02-00533) and was also funded by target-oriented grants from the plenipotentiaries of Czech Republic and Poland at the Joint Institute for Nuclear Research (Dubna). REFERENCES 1. A. G. Artukh, G. F. Gridnev, M. Grushezki, et al., Nucl. Instrum. Methods Phys. Res. A 426, 605 (1999); A. G. Artyukh, Yu. M. Sereda, S. A. Klygin, et al., Instrum. Exp. Tech. 54, 668 (2011).

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2. R. Anne, D. Bazin, A. C. Mueller, et al., Nucl. Instrum. Methods Phys. Res. A 257, 215 (1987). 3. B. M. Sherrill, D. J. Morrissey, J. A. Nolen, Jr., and J. A. Winger, Nucl. Instrum. Methods Phys. Res. B 56/57, 1106 (1991). 4. T. Kubo, M. Ishihara, N. Inabe, et al., Nucl. Instrum. Methods Phys. Res. B 70, 309 (1992). 5. H. Geissel, P. Armbruster, K. H. Behr, et al., Nucl. Instrum. Methods Phys. Res. B 70, 286 (1992). 6. A. Ozawa et al., Nucl. Phys. A 673, 411 (2000). 7. X. H. Zhang et al., Phys. Rev. C 85, 024621 (2012). 8. E. Kwan et al., Phys. Rev. C 86, 014612 (2012). 9. S. Momota et al., Nucl. Phys. A 701, 150c (2002). 10. S. Lukyanov et al., J. Phys. G 37, 105111 (2010).