Experimental problems in RIB experiments

1. ISOLDE Nuclear Reaction and Nuclear Structure Course Experimental problems in RIB experiments A. Di Pietro

2. Outline of the lecture: • RIB production methods • Resolutions. •  Background problems in RIB experiments. •  Experimental developments. •  Summary and conclusion.

3. Isotope Separation On Line (ISOL) ( ISOLDE, SPIRAL, TRIUMF) Projectile Fragmentation (PF) (GSI, GANIL, Catania, MSU, RIKEN) In-Flight production (San Paolo, Notre Dame, Legnaro, ….) Batch-mode production (the oldest approach used to produce RIBs like 7Be, 14C ….) RIB production methods The various approaches are often complementary and it is doubtful that one production method will satisfy all the experimental needs. The beam characteristics will depend upon production method.

4. Excellent beam quality (energy resolution, beam emittance etc.. depending upon Post-Accelerator). Reasonable intensities (up to 108-109 pps). High purity (not in all cases, depends on the resolution of the Isotope/Isobar Separator and on the facility design). Limits on the half-life of the beam particle due on the time needed to diffuse out of the target (T1/2≈1s). Dependence from the chemistry of the element. e1 e3 e2 i= 101215pps s=? e4=1-50% i=103-109pps e5=n. decays EbeamECoulomb barrier

5. Ionisation Xn+ RIB from High Resolution Separator Low resolution mass separator DM/M ~150 Beam contaminated by elements present in the ionisation source: buffer gas, cathode, etc…

6. Poor beam quality (longitudinal and transverse emittance, large beam spot). Large energy spread. Contamination with particles having similar m/q values. Independence on the chemical properties of the secondary beam. Very short separation time ( 0.1-1 ms) Possibility to measure simultaneously nuclear properties of several species. Ebeam~ 30MeV/u  500 MeV/u

7. This technique can be considered a low-energy version of the fragmentation method. The facility layout is similar to a fragmentation facility. The radioactive species are produced by transfer reaction in inverse kinematics between primary beam particles and thin solid or gas targets. The choice of the inverse kinematics results in a forward focusing of the secondary beam particles. Advantages and limitations are similar to the fragmentation method. In-Flight production.

8. 7Li3+ 20-25 MeV Tandem Ion Source 90° 7Li 1 mA 14° 8Li “in-flight” production in Catania QD1 7Li CD2 target QD2 8Li10 Production 7Li(d,p)8Li QD3 by-pass QD4 Selection (8Li) n-det MCP1 4He Gas cell MCP2 Backward solution for 8Li chosen Better separation 7Li-8Li

9. Batch Mode production. • Suitable only for beams of long-lived nuclei (eg. 3H T1/2=12.3 y,7Be T1/2 =53d). • Nuclei of interest produced , converted into a suitable chemical form. • Specific quantities or “batches” of these materials are introduced in the ion source of an accelerator for beam production.

10. An Ideal Facility? • Accelerator • high resolution separatorbeam purity • beam emittance: • transverse emittance  angular resolution • longitudinal emittance beam time & energy resolution • beam attenuators (x102-105) • in-beam diagnostics • Diagnostics: • collimators • FC • scintillator + CCTV • Si PIN PD • other detectors

11. Limits in angular resolution Transverse emittance: size of beam  beam angular divergency (normalised transverse emittance) bgexy :p mm.mrad e.g. 6MeV/u 26Al at ISACIIb = 0.113, g ~1 emittance = 0.3 / 0.113 = 2.66p mm.mrad From T. Davinson lectures at SPES school r: target-detector distance d: optimal beam spot size dq : optimal angular resolution (@ optimal beam spot size) W: detector strip width Transverse emittance limits angular resolution of the experiment

12. Longitudinal Emittance (for a bunched beam) Momentum dispersion (i.e. beam energy resolution)  time resolution Longitudinal emittance limits the energy and time resolution of the experiment It is not defined for a DC beam

13. Transverse emittance can be reduced if a proper collimation system is used • Two collimators are requered: • to reduce the size of the beam • to reduce the angular divergency of the beam • Anantiscatteringis also required (beam scattered on the collimators do not reach the detectors placed at small angles) dq collimator 1 collimator 2 antiscattering The farer (or narrower) are the two collimators the smaller is the beam divergency.

14. Effect of target thickness on energy resolution The choice of the target thickness is a compromise between counting rate and required energy resolution. There are two effects that affects the energy resolution: Energy straggling of beam and detected particles Energy loss (E.L.) + Kinematics e.g. detector placed at forward angles • beam E.L. in the whole target • E.L. in the whole target of detected particle produced with E given by kinematics • no beam E.L. • no E.L. in the target of detected particle produced with E given by kinematics • beam E.L. in the whole target • no E.L. of detected particle produced with E given by kinematics • no beam E.L. • E.L. in the whole target of detected particle produced with E given by kinematics E’1 E’2 E’1 E’2 E’1 E’2 E2 E2 E1 E1 E’1 E’2 target target e.g. detectors placed at backward angles DE=E1-E2=beam E.L. in the target DE’=E’1-E’2= energy defference of detected particles DE’ forwad angles << DE’ backward angles

15. Other limits to the Energy Resolution Silicon detector energy resolution: DE2 = DEs2 + DEstats2 + DEcoll2 +DEe2 DEs (keV FWHM Si) = 10.07 z x1/2 energy straggling (Bohr estimate[1]) DEstats (keV FWHM Si) = 1.519 E1/2 statistics DEcoll (keV FWHM) = 0.7 z1/2A4/3 [2] nuclear collisions DEe (keV FWHM) electronic noise z - ion atomic number, A - ion atomic mass number, x - detector dead layer (μm) and E - particle energy (MeV) Assume DEe= 10keV FWHM, x = 0.05μm (~0.7μm is more typical) [1] alternatively use SRIM/SSSM (http://www.srim.org) [2] J.Lindhard & V.Neilson, Phys. Lett. 2 (1962) 209 From T. Davinsonlectures at SPES school

16. Experimental problems in experiments with RIBs RNBs have low intensities (i≈103-109pps) compared to stable beams (i≈1010 -1011pps) and to background (principally b-particles) coming from the decay of the elastically scattered beam. To perform measurement in a reasonable period of time is necessary to use high efficiency (large solid angle) detection systems and in some cases “smart” experimental techniques. • Necessary to manage the rate and the background to minimise the pile-up. • Necessary to reduce the low energy background events coming from the elastically scattered beam. • Necessary to increase the granularity of the detection system. Even “simple” experiments ( eg. elastic scattering measurements) can be extremely difficult.

17. b+typical energy 1-3 MeV 0.511 MeV g from e+e- p rich: g from de-excitation after b+ decay Background from the decay of the beam: b- typical energy 1-3 MeV n rich: • from de-excitation after b- decay Elastically scattered beam particles  scattering chamber walls active source of background. Eg. 13N @ 50 MeV i~ 1  108 pps on 12C target q=5° ~ 1 104 ions  cm2/s at 10 cm q=10° ~ 7 103 ions  cm2/s at 10 cm q=20° ~ 5 ions  cm2/s at 10 cm Background sources and related problems.  Probability of in-flight decay of the beam  small (not a real problem).  Faraday cup.  Beam collimators along the beam line.

18. Additional background problems: the 8,9Liand8B cases Background a spectrum from 8Be(2+) decay ~1.5 MeV ~5 MeV 50% branching b-,n to 8Be* (2+)

19. a a b I/I0 t b I/I0 Exponential range b 0.3 p+ n+ 0.2 e backscattering probability 0.1 -HV How binteract in asolid-state detector? Range t Range of 1-2 MeVb ~ 4-5 mm in Si How much is the energy loss in eg. 300mm Si? for 90° scattering can release all energy

20. Compton scattering dominant p+ n+ e- 511keV ⇝ hn ´  ⇝ e- • -HV Annihilation radiation from b+ decay How 511 keVgs interact in the detector? Most probable hn energy ~ 400keV Most probable e- energy ~110 keV Eg. only 7% of 511keV g interacts on 300 mm Si More severe problems on Ge g-ray detectors.

21. 19Ne n+ strips b p+ srtips In designing detection systems one MUST consider all problems related to background and low counting rate. Particle detectors: • Detector thickness as small as possible (according to the experiment needs). • High segmentation • Large solid angle Double Sided Silicon Strip Detectors independent p+ and n+ strips Particle into front face activate one p+ and one n+ strip. bsscattered at large angles activate one p+ strips and more n+ strips. Ref. NIM A262(1987)353, NIM A288(1990) 245

22. 80 mm Examples of detection systems developed for RIB experiments  High granularity  Geometrical flexibility  Very large solid angle possible Many experiment performed with this type of detector. Nuclear Astrophysics, Reaction studies, spectroscopy etc.. 16 strips in q 8 sectors in f T.Davinson et al. NIM A 454(2000) 350

23. Flexibility of the apparatus according to the experimental needs! TUDA @ TRIUMF MUST 2 SHARC to be coupled to g-array TIGRESS

24. Efficiency of DSSSD detectors AL SiO2 z n+ p+ It is already known from the literature that the segmentation of the electrodes is responsible for: charge sharing between neighboring strips. opposite polarity signalsin coincidence with normal polarity signals. It has been observed that not all events are detected with full energy but part of the events are detected with lower energy i.e. full energy detection efficiency < geometrical efficiency. n For front strips this is no more valid. By summing two coincidence events thefull energyis not recovered. Yorkston, NIM A262 (1987) 353; Blumenfeld NIM A421 (1999) 471. x In the back side interstrip events give a charge sharingbetween the two neighboring strips.

25. Event selection Independently on the origin of these phenomena a procedure is required to select the full energy events. EFront= EBack Geometrical efficiency Efront= Eback_i+ Eback_i Efront= Eback With the event selection E(Front) = E(Back) we get an efficiency which is dependent on energy and bias! By selecting events with E(Front) = E(Back_i)+E(Back i±1) the efficiency is higher and the energy and bias dependence is removed

26. Proton m-beam measurement The m-beam probes the interstrip region. effective strip size / gometrical size Dimension of strip B/D : ratio between bias and full deplation voltage Interstrip region extends in the strip region. The higher is the Bias the smaller is the interstrip.

27. Other type of detectors

30. Connection between q - z

31. Helios

32. In-Beam g-ray spectroscopy Good angular resolution is needed. High efficiency of g-ray detectors prerequisite to overcome the low counting rate of experiments with RIBs. Geometrical efficiency limited by the need of Compton shielding to enhance the peak-to-total ratio. Need to measure the recoil velocity to reduce the peak broadening due to kinematical spread. In the case of high energy beam but also, low energy beam in inverse kinematics reaction, recoil velocities v > 5% c large Doppler broadening. Solutions? Segmentation, pulse shape analysis and coupling with particle detector arrays.

33. Assume: orientation of the coordinate system such that z-axis // v. Eg, g, jg, dWg in Lab. system Eg0, qg, fg, dWg0 in rest system ^ y Dg g ^ x g-detector beam g   v ^ z The Doppler-shift problem. detector solidangle dW=singdgdjg Fully relativistic: For a detector opening angle Dg=20° Doppler broadening after correction: b ~0.05: dEg0~20 keV b ~0.5: dEg0 ~200 keV Typical HpGe detectors resolution ~2 keV

34. The MINIBALL array at REX-ISOLDE. http://isolde.web.cern.ch/ISOLDE/ 24 encapsulated HpGe detectors Cluster of 3 detectors 6-fold segmented No BGO shields Coupling with highly segmented Double-Sided-Si-Detector Prog.Part.Nucl.Phys. Vol.38 (1997) 29

35. The MINIBALL array at REX-ISOLDE. The position sensitivity achieved by segmentation of the outer contact and by analysing the charge drift times within a segment (radial information) and the mirror charges induced in the neighbouring segments (azimuthal information) . Prog.Part.Nucl.Phys. Vol.46 (2001) 389

36. Pulse shape analysis Azimuthal position: if one defines the Asymmetry:A=(Ql-Qr)/(Ql+Qr) Where: Ql charge induced on the left neighbour segment Qrcharge induced on the right neighbour segment At first orderAdepends only on the distance of the main interaction from the two neighbour segments. Radial Position : from charge collection time of electrons measured from the core signal. The current signal (derivative of the charge) has its steepest slope at the time when all electrons are collected.

39. Summary and conclusions • Experiments with RIBs are very challenging due to the low intensity and the high background. • Large solid angle and high granularity detection systems developed along with the associated electronics and pulse processing techniques to overcome the problem of the low statistics and b background. • The performances of the detection system depend upon the quality of the combination of beam and target that one is using. • With this new and more performing detection systemsthe quality of the experimental results are getting better and better.