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Experimental problems in RIB experiments

A. Di Pietro

ISOLDE Nuclear Reaction and Nuclear Structure Course

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Outline of the lecture:

ÞRIB production methods

ÞResolutions.

Þ Background problems in RIB experiments.

Þ Experimental developments.

Þ Summary and conclusion.

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RIB production methods

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 ….)

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.

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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.

EbeamECoulomb barrier

e2

e1e3

e4=1-50%

i= 101215pps

i=103-109pps

s=?

e5=n. decays

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RIB from High Resolution Separator

Low resolution mass separatorDM/M ~150

Beam contaminated by elements present in the ionisation source: buffer gas, cathode, etc…

Ionisation Xn+

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Independence on the chemical properties of the secondary beam.

Very short separation time ( 0.1-1 ms)

Poor beam quality (longitudinal and transverse emittance, large beam spot).Large energy spread.Contamination with particles having similar m/q values.

Possibility to measure simultaneously nuclear properties of several species.

Ebeam ~ 30MeV/u 500 MeV/u

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In-Flight production.

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.

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Production7Li(d,p)8Li

TandemIon Source

14°

n-det

90°

by-pass

MCP2

MCP1

CD2 target

QD1

QD2

QD3

QD4Selection (8Li)

4He Gas cell

7Li 1 mA

7Li3+ 20-25 MeV

8Li “in-flight” production in Catania

Backward solution for 8Li chosenBetter separation 7Li-8Li

7Li

8Li10

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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.

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Acceleratorhigh resolution separatorbeam puritybeam emittance:1. transverse emittance angular resolution2. longitudinal emittance beam time & energy resolution beam attenuators (x102-105) in-beam diagnostics

Diagnostics:collimatorsFCscintillator + CCTVSi PIN PDother detectors

An Ideal Facility?

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Limits in angular resolutionTransverse 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 ~1emittance = 0.3 / 0.113 = 2.66p mm.mrad

r: target-detector distanced: optimal beam spot sizedq : optimal angular resolution (@ optimal beam spot size)W: detector strip width

Transverse emittance limits angular resolution of the experiment

From T. Davinson lectures at SPES school

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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 experimentIt is not defined for a DC beam

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Transverse emittance can be reduced if a proper collimation system is used

Two collimators are requered:

1) to reduce the size of the beam 2) to reduce the angular divergency of the beam

An antiscattering is 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.

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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:

1) Energy straggling of beam and detected particles 2) Energy loss (E.L.) + Kinematics

e.g. detector placed at forward angles

DE=E1-E2=beam E.L. in the target

target

E1 E2

E’1

E’2

no beam E.L. E.L. in the whole target of

detected particle produced with E given by kinematics

E’1

beam E.L. in the whole target

no E.L. of detected particle produced with E given by kinematics

E’2

e.g. detectors placed at backward angles

target

E1 E2

E’1

E’2

no beam E.L. no E.L. in the target of

detected particle produced with E given by kinematics

E’1

beam E.L. in the whole target

E.L. in the whole target of detected particle produced with E given by kinematics

E’2

DE’=E’1-E’2= energy defference of detected particles

DE’ forwad angles << DE’ backward angles

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Other limits to the Energy Resolution

Silicon detector energy resolution:DE2 = DEs

2 + DEstats2 + DEcoll

2 +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. Davinson lectures at SPES school

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Experimental problems in experiments with RIBs

RNBs have low intensities (i≈ 103-109 pps) compared to stable beams (i≈1010 -1011 pps) 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.

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Background sources and related problems.

Background from the decay of the beam:

p rich:

b+ typical energy 1-3 MeV

0.511 MeV g from e+e-

g from de-excitation after b+ decay

n rich:

b- typical energy 1-3 MeV

g from de-excitation after b- decay

Probability of in-flight decay of the beam small (not a real problem).

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

Faraday cup.

Beam collimators along the beam line.

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Additional background problems: the 8,9Li and 8B cases

Background a spectrum from 8Be(2+) decay

50% branching b-,n to 8Be* (2+)

~1.5 MeV ~5 MeV

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How b interact in a solid-state detector?

a

Range

Range of 1-2 MeV b ~ 4-5 mm in SiHow much is the energy loss in eg.

300mm Si?

b

for 90° scattering can release all energy

aI/I0

t

t

-HV

b p+ n+

Exponential range

bI/I0

e ba

cksc

atte

ring

pro

babi

lity

0.1

0.2

0.3

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Annihilation radiation from b+ decay

How 511 keV gs interact in the detector?

Most probable hn energy ~ 400keVMost probable e- energy ~110 keV

Eg. only 7% of 511keV g interacts on 300 mm Si

More severe problems on Ge g-ray detectors.

e-

hn ´⇝511keV

⇝e-•

-HV

p+ n+

Compton scattering dominant

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In designing detection systems one MUST consider all problems related to background and low counting rate.

Particle detectors:Ö Detector thickness as small as possible (according to the experiment needs).Ö High segmentationÖ Large solid angle

Ref. NIM A262(1987)353, NIM A288(1990) 245

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Double Sided Silicon Strip Detectors independent p+ and n+ strips

p+ srtips

n+ strips

Particle into front face activate one p+ and one n+ strip.bs scattered at large angles activate one p+ strips and more n+ strips.

b

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Ö High granularityÖ Geometrical flexibilityÖ 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

80 mm

Examples of detection systems developed for RIB experiments

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Flexibility of the apparatus according to the experimental needs!

SHARC to be coupled to g-array TIGRESS

TUDA @ TRIUMFMUST 2

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Efficiency of DSSSD detectorsAL SiO2

p+n+n

z

xIt is already known from the literature that the segmentation of the electrodes is responsible for:

1. charge sharing between neighboring strips.2. opposite polarity signals in 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.

Yorkston, NIM A262 (1987) 353;Blumenfeld NIM A421 (1999) 471.

For front strips this is no more valid. By summing two coincidence events the full energy is not recovered.

In the back side interstrip events give a charge sharing between the two neighboring strips.

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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

EFront = EBack

Event selection

Efront = Eback

Independently on the origin of these phenomena a procedure is required to select the full energy events.

Geometrical efficiency

Efront = Eback_i+ Eback_i

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Proton m-beam measurement

The m-beam probes the interstrip region.

B/D : ratio between bias and full deplation voltage

Dim

ensi

on o

f stri

p

eff

ectiv

e st

rip si

ze /

gom

etric

al si

ze

Interstrip region extends in the strip region.The higher is the Bias the smaller is the interstrip.

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Other type of detectors

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Connection between q - z

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Helios

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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.

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.

Good angular resolution is needed.

In-Beam g-ray spectroscopy

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The Doppler-shift problem.

detector solidangledW=singdgdjg

y

z

xg-detector

beamg

jg

Dg

v

Eg, g, jg, dWg in Lab. system Eg0, qg, fg, dWg0 in rest system

Assume: orientation of the coordinate system such that z-axis // v.

Fully relativistic:

γδγβcos1

γβsin

0Eγ0δEγ

2

0EγEγ

γd0dΩ

γΦγγβcos1βγcos

0cosθ

2β1

γβcos1Eγ

0Eγ

g

j

-=

=

W

=-

-=

-

-=

For a detector opening angle Dg=20°

Doppler broadening after correction: b ~ 0.05: dEg0~20 keV

b ~ 0.5: dEg0 ~ 200 keV Typical HpGe detectors resolution ~2 keV

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The MINIBALL array at REX-ISOLDE.

24 encapsulated HpGe detectors Cluster of 3 detectors

6-fold segmentedNo BGO shields

Coupling with highly segmented Double-Sided-Si-Detector

http://isolde.web.cern.ch/ISOLDE/

Prog.Part.Nucl.Phys. Vol.38 (1997) 29

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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

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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.

Azimuthal position:if one defines the Asymmetry: A=(Ql-Qr)/(Ql+Qr) Where:Ql charge induced on the left neighbour segmentQr charge induced on the right neighbour segmentAt first order A depends only on the distance of the main interaction from the two neighbour segments.

Pulse shape analysis

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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 systems the quality of the experimental results are getting better and better.