Basic non-linear effects in silicon radiation detector in detection of highly ionizing particles:

registration of ultra rare events of super - heavy nuclei in the long-term experiments. Yu.S.Tsyganov

FLNR, JINR, 141980 Dubna, Moscow reg., Russia

Content1. Introduction

2. DGFRS, detection system (in brief)

3. EVR spectra formation in PIPS

4.4. Active correlation techniqueActive correlation technique

5. Applications in HI-induced reactions aimed to SHE synthesis

6. Summary

The Dubna heavy element research group

DETECTOR & NUCLEAR ELECTRONICS..In fact…Detection method

• Introduction

• field of interest: complete fusion reactions with heavy ions , synthesis of SHE (Z >110)

• 48 Са - chance for relatively higher cross sections of the product under investigation (it follows from Pb + Ca )

• Nevertheless, cross section ~units/parts of picobarn -> intense beams, sensitive detection system, perfect separation

• cyclotron U 400 + gas filled recoil separator (separation by the difference in magnetic rigidity. Gas<- smaller std. of equilibrium charge -> higher efficiency)

• Duration of experiments typically months – ½ year only units of events under interest to look for a “needle in a haystack” © <rate>~102 Hz totally event number N = 102 Hz ∙105 s ∙ 102 days = 109 events ! (> 800 KeV threshold)

• special high active materials for targets production (242,244Pu, 245,248Cm, 243Am, 249Cf )

• In collaboration with LLNL ( USA, CA)/ Dubna - Livermore

Schematic of experiments with Schematic of experiments with 4848Ca Ca beambeam

Data taking + “real-time” r-a search

spectrum of 48Ca ionsTOF/energy

Sep. + beam

Visualization+det.param. monitoring

Beamchopper

Recoil SeparatorDGFRS

detectors

Beam tuning

TOF, ns

DAYS

S= 0.033 ∙spectrum(i), from protocol file

i

(N max -6, Nmax+6),

Nmax – channel of the peak maximum TOF (U400)

Detecting Module of DGFRS

Veto detector: to suppress charge particles, coming from cyclotron, passing through the focal plane one and creating no signal in the gaseous TOF detector( ~ 320 μm n - Si )

Three 4x4 cm2 chips~320 μm depthTotally depleted

Gaseous TOF moduleSuppression factor≥80 MeV 48Ca ~ 0.99996

12 pos. sensitive strips9.9 x 40 mmSide detectors: Eight chips 4x4 cm2

Totally depleted

Small over depletion ratio

1.0

1.6

MKS system for H2

No any feed-back

( few weeks)

pent. flow VARIAN oil free pump

efficiency

Number of tested ions 48Ca (≥ 80 MeV)

Unfortunately,not for (8-12) MeV

Subsystems

• 1. data acquisition ( PC, CAMAC, CC012+CC202 ACC, 85/8*/8 µS dead time)

• 2. separator parameters monitoring, including beam associated (under upgrade, see poster by A.N.Polyakov- next session)

• 3. detecting module parameters monitoring

• 4. visualization ( ~ 100 histograms, Builder C++)

• 5. real-time search of recoil-alpha time - energy-position correlated sequences for beam-associated backgrounds suppression :

a) algorithm method b) EVR spectra simulations + empirics, taking into account large PHD

• initial idea of method – Yu.Tsyganov , Varna NEC’97 + Seoul, HPC ASIA’97 + Brighton - Lewis( UK) 1998 (SU3 jubilee conf.) + London (UK) PSD’4 , 1999

• Modern (modified version) Leicester, Liverpool (UK) 2002,2005 PSD’5,6+ TAN’2001(Swiss)

* Afternoon session: report of my colleague Alexey Voinov will follow to this item

Search for recoil-alpha correlation chains in a real-time mode

• Goal : because of the SHE decay is multi-chain, starting from the EVR signal, detecting in a real-time mode first correlation EVR- can be used to switch off the cyclotron to minimize radically the background, therefore, the forthcoming decays (α, SF) will be

detected in fact in background free condition ------------------

Method can be divided by the following components:• - simulation of recoil spectra as well as using empirical approximations

(registered EVR energy signal in PIPS detector against incoming one) taking into account large PHD (and it’s fluctuation) for SH EVR.

• - algorithm design development for quick search for pointer to the probable correlations (!) (+electronics)

• Execution unit design (beam chopper: ~5 µs linear growth of voltage at parallel deflection plates ) – to provide fast deflection of the beam from cyclotron

Interruption of the beam takes placeIn the cyclotron injection line(~20 kV), so, after chopper “yes” operation, some delay ~ 60 s occurs, due to the finite life - time of the already accelerated ions in cyclotron (~270-290 MeV).

Linear growth

DGFRS Life time

EVR flighttime

RSTR

Trivial case (not in use): single EVR provides a break point in the irradiation of target

(Yu.Tsyganov, JINR P7-2005-117)

Really < 10 ms, when tst~1 min and tRα~ 1 s, under condition Of real rate for “-like” signals in PIPS

Detector Flow chart of the process

bckg1bckg2

bckg2bckg1 !

Effect demonstration(in symbolical form)

Sometimes the nature of the phenomenon gives us some selective clue that simplifies the detection method in principle.

time

PHD in silicon detectors ( a few words of theory…

goal EVR spectra n PIPS)

Highly ionizing particle: pulse height defect as: = w + n + r

multiplication probability f (r, n, F, p-n..) f 0 for EVR (two reasons) Yu.Tsyganov , A.Polyakov and V.Kusniruk IEEE tr. Nucl. Sci.43 N5 (1996)-range, n – electron-hole density; p-n –junction type

Dead layer + nuclear stopping + recombination • In my calculations and simulations it used : PIPS typically ~11.6 g /cm2 of Si equivalent

as a dead layer (see e.g. S.Hofmann et al. Eur.Phys. J A 32(2007) 251)

• - Wilkins formula for nucl. Stopping + surface recomb. concept ( V.F.Kushniruk JINR P13-11933, 1973 + V.Kushniruk, Yu.Tsyganov Prib.Techn.ExperimentaN3(1998)30, Yu,Tsyganov &&A.Polyakov NIM A 363(1996)611)

• for r , of course IN EFFECTIVE FORM 1/τeff = 1/τv + 1/τs , where τeff – non equilibrium carriers life-time

• Therefore velocity of surface recombination considered as Seff τeff

• SEFF ~ 103 cm/s (see graph in upper right corner) see e.g. Yu.Tsyganov JINR P13-96-430 (1996)

• And Kushniruk equation λ = Seff∙TP/R, (relative recomb. Loss) ( Yu.Tsyganov JINR E13-2006-77 ) λ = g∙Seff∙TP/R,

• Where form factor g = 1 (cylinder) or ~ 0.5 (short track, ~sphere R ≤ 5 m in silicon)

• TP – plasma time according to Seibt et al., R - charged particle range in silicon:

FaD

O

AL

enQ

PT

13/1)2)(332

03

(

Computer simulation for Z=118 recoil spectrum. Measured events are shown by arrows.

No free parameters!

Error function: est-d. syst. err. against E in, MeV

Seff ≈ 103 cm/s

Ac, Rf, JINR

Hs, JINR112, Riken

112, GSI

Seff = 1166 cm/s

112, RikenClosedparameters

λ

TP, ns

09.28

09.28

14

)809.5(10252.3

,),(

),72.020.6/(464.4

2/13/24

AZ

Zk

EAZk

Si

in

n

TP=1.32∙10-10(N/RxE)1/3 / F, nsN – pair number, R – range, cmF – field V/cm

fluct.: δ=1.698 + 0.057-0.001352,

Yu.Tsyganov NIM A 378 (1996)356 )

FWHM; 4 ≤≤20 LSS

Low field: strip not depleted

Normalize:Ortec 419Interest overlap with Semiconductors

physics:(A.Sachenko, O.Snitko //Kiyev, Naukova dumka, (1984)55 “Photoeffects in near surface layers of semiconductors”)1, 2- different treatments of Si surface,Ys – surface potential in e/kT units(shift with resp. to Fermi level if to compare with bulky Si) < 0 depletion, > 0 enrichment n-Si

Note, whereas ~103 close to PIPS, 104 cm/s is close to Si(Au) SBD.

0 – flat bands condition~ it corresponds to Seibt SCLC model (~equipotentiality track and top electrode)

~6000 -- 14000

SBD(HI) // V.Kushniruk && Yu.Tsyganov, JINR E7-91-75, 1991

PIPS ( α )

Our dataFor 102

Spectra simulation + empirics

Dependence of EVR registered energy signal against it’s incoming energy

E_meas ( Z,A, F, Sacteff)= E_meas ( 252No) – k1x( A – 252 ) – k2x( Z – 102 ) - r 0[ (s-s0)/s0 – (F-F0)/F0]

k1=0.0169; k2 = 0.058; // calculated over depletion ratio for PIPS detector close to~ 1-1.2And implantation depth about ~ 4 μm (it corresponds to 206Pb + 48Ca 252No + 2n reaction)

F0 ~ 0.2 Volt/µm ( F= 2<F>, <F> - mean field )

S0=1.16∙103 cm/s r0=1.1 MeV

?? *

To determine a first approximation for α-particle pre-set energy the following systematic is used which is in fact a fit for 65 even-even nuclei

• lgT1/2=(a Z+b)Q-1/2+c Z+d, a=1.78722, b=-21.398, c=-0.25488, d=-28.423• (N=65; 99<Z<109.

Measured FF events(in brief : post-analysis)

R(implantation to Si), µm

??

k = Eside/ (Eside + E focal )

(Will be probably closed in the nearest future!)

( Pentagons – simulation.Yu.Tsyganov , in Proc. NEC’2003 )

Etot meas=<TKE> - 20(5), MeV(GSI detector: - 25(~2) MeV)

Dependence of dimensionless energy against Nuclide implantation depth

EVR implantation direction

PIPS depletion layer

Implantationdepth

Top electrode

Residueof 2nd FF

Bottom electrode(Ohmic)

FF1+part.FF2

]})1([int{ ,,00max

ERescai

i

i

i

i

ii

yiyyi

R

R

R

R

bNa

bNaNj

)(,, elapsedtt EVRji

dt = (dt1<dt2) ? Dt1 : dt2; dt = (dt<dt3) ? dt : dt3; dt = (dt<dt4) ? dt : dt4; dt = (dt<dt5) ? dt : dt5; dt = (dt<dt6) ? dt : dt6; dt = (dt<dt7) ? dt : dt7;

. δ ≡ 0 . i = strip number1..12

a, b, ay,by, R0, Ri –calibration coefficients. Nmax = 170 .Calibr. coefficients are extracted fromtest nuclear reacions, like:206Pb+48Ca 252No + 2n, natYt+48Ca221Th+3n, etc…

T1<TOF<T2E1 < E <E2.m = 10001 b

TOF = 0E11<E<E22.m=1 b

(Borland’s)

if (( dt >= 0 && dt < EPSILEN && correlation==0 && e_total>EAMIN)||( corr_f==1) )

{ correlation = 1;

cnt_cor++;… code prolongation…

Dead time ~ 160 мкс

One strip equivalent input circuit

Only PIPS equiv. circuit// no preamp’s, bias, ADC’s .. etc. …

CAMAC

Suppression factor 9 – 12 MeV . (totally: “veto” || Suppression factor 9 – 12 MeV . (totally: “veto” || TOF)TOF)

In the vicinity of region under interest : only ~ 5 factor (typically)

Depends onNot only reaction type,Target type ..etc…..but on cyclotron beam tuning,..sometimes greater than tens of percents,..factors from case to case!

TOF = true if:Or tof amplitude >0Or 1 bit info“start” or “stop”operation

“veto” (ADC) > 0

Region of interestFor SHE’s -decays

Suppression factorTof || veto = true

E, MeV

demonstration of background events suppression ( right demonstration of background events suppression ( right

upper)upper)

( 9 ( 9 , 12) , 12) ММeV – in fact,eV – in fact, no background events!no background events!

~ 5.4 МэV peakIs not from reaction,(calib.-monitor)

After suppression byTOF-VETO detectors

0 5000 10000 150000

200

400

600

800

1000

strip #2

E2,

ch

an

.

E1, KeV

Reasonable scenarios for beam ass. backgrounds:Charged particles, beam- and target-like, others.. – suppressed by TOF module

1) Neutron induced signals +2) Long path particles (if no signal in TOF or in VETO )-------------------------------------------------

Focal planeDetector (#2)

“veto” detector against - focal-plane

mn

mn

K

C

n – number of alpha particles in chain, m – number of alpha switching OFF the beam , K – suppression parameter for alphas out of beam, usually 1/100 – 1/1000

)!(!

!

mnm

nCmn

If n =4, m=1 (the first alpha particle stops cyclotron beam) and K~1000, η= 4/(1000)4-1 =4/1.0e+09= 4x10-9

Recoil- α-α-α-α-SFchain

Typical losses in the experiment efficiency against incoming beam

intensity

EVR – alpha1,2

20% ~ 45 μcA// ~ 2.5 pμcA

Fragment of pauses protocol for reaction with 239Cf 9:00 to 24:00 28/02/2005. pause 1 min, Summary loss time for 15 h was 29 min. Or, 29 x 100/15*60 = 3 %.

09:23 24/ 2 2048 10:40 24/ 2 2048 11:17 24/ 2 2048 11:19 24/ 2 2048 12:38 24/ 2 2048 13:31 24/ 2 2048 13:48 24/ 2 2048 14:20 24/ 2 2048 17:19 24/ 2 2048 17:27 24/ 2 2048 18:07 24/ 2 2048 19:29 24/ 2 2048 19:33 24/ 2 2048 20:27 24/ 2 2048 20:29 24/ 2 2048 20:31 24/ 2 2048 20:46 24/ 2 2048 21:18 24/ 2 2048 21:26 24/ 2 2048 21:33 24/ 2 2048 21:37 24/ 2 2048 21:40 24/ 2 2048 22:23 24/ 2 2048 22:39 24/ 2 2048 22:43 24/ 2 2048 22:45 24/ 2 2048 23:22 24/ 2 2048 23:29 24/ 2 2048

Advantage factor = p(std

.err)/p(err, with beam off)

Of course: for given detecting module, Size, number strips..

Application in the experiment aimed to the synthesis of elementApplication in the experiment aimed to the synthesis of element Z=115Z=115( Phys. Rev. C 69, 021601®, (2004); C 72 (034611 (2005) )( Phys. Rev. C 69, 021601®, (2004); C 72 (034611 (2005) )

• ReactionReaction 243243Am + Am + 4848Ca ->115 + 3,4nCa ->115 + 3,4n……

• Average intensity of Average intensity of 4848Ca ions at the targetCa ions at the target

~ 1 pmcA ( 18~ 1 pmcA ( 18++))

• ЕЕnergy nergy 248, 253 MeV,248, 253 MeV, beam dose ~ 9.0E+18beam dose ~ 9.0E+18

• active correlation technique was appliedactive correlation technique was applied

• Four decay chains were obtainedFour decay chains were obtained

( 3 ( 3 -- 3n de - excitation channel 3n de - excitation channel + + one was attributed toone was attributed to 4n4n))

• Recently, results were confirmed in chemical experiment with long-lived isotopeRecently, results were confirmed in chemical experiment with long-lived isotope 226868Db Db extraction)extraction)

The paper is capital!

R.Casten, Editor Phys.Rev.

Application of the active correlation technique in 48Ca + 249Cf 118 reaction

For detection of expected sequential decays of daughter nuclides in the absence of beam associated background, the beam was switched off after a recoil signal

was detected with parameters of Implantation energy E=7-16 MeV expected for complete-fusion EVR, followed by an α-like signal with an energy 9.9≤E≤12.0 MeV or 9.9 ≤E≤11.3 for Z=118 and 116 recoils, respectively in the sameStrip, within 1.8-2.5 mm wide position window and a time interval of t ≤ 1 s. If, during the first 1-min beam off interval, an α particle with E2 =9.5 – 11.15 MeV was registered in any position in theSame strip, the beam-off interval was automatically extended to 12 min.

The example to demonstrate “α-like”

backgrounds suppression

(Z=115 Phys. Rev. C 69, 021601(2004) )

Reaction Integral background suppression

factor(9 – 11 МeV)

EVR - 1

correlationenergy interval,

MeV

EVR - αpre-setting time

interval, s

Beam OFFpause, min

238U+ 48Ca 112* 9,5 е+03 9,43 – 9,63/10,3-11,8

12/0,3 1

242Pu+48Ca114* 4 е +03 9,9 – 10,35 4 1

245Cm+48Ca116* 1,5 е+04 9,9 - 11 1 1

243Am+48Ca115* 2.0 е+04 9,6 - 11 8 2

249Cf+ 48Ca118* 1,1 е+04 9,9 - 12 1 1

Table 1Integral suppression factors

For different HI inducedNuclear reactions

Def.: K = (all events without method application)/

(ones out of beam, when method is applied)

Z=115

• based on:

- theoretical models, EVR spectra simulations, empirical relations and formulae obtained from test nuclear reactions ,- Real-time matrix algorithm to search for pointer for potential forthcoming correlated sequence ,- DGFRS detection system, on the base of 12 strip position sensitive PIPS detector and low pressure TOF gaseous detector,- HI U-400 cyclotron complex and, especially, “beam chopper” device, locating in the injection line, after ECR ion source,.

..new radical technique of ” active correlations” is designed, tested and successfully applied in the HI induced nuclear reactions during last five years

--------------------------------------------------------------------------------------------------------------------------------------------------------

• Detection a recoil-alpha correlated sequences in a real-time mode allow to provide a deep suppression of beam associated background events, when ultra rare alpha decays are detected. It provides more clear event detection and identification in long-term experiments aimed to the synthesis of SHE with Z=112-118

• The mentioned detection system allows to detect events are originated in the reactions with cross sections down to units of 10-37 см2.

• Losses in the whole experiment efficiency are in fact only units of percents, when additional integral cleaning factor about ~104 in the vicinity of ( 9 - 12) MeV interval has been achieved

• Some of the results already have the independentindependent confirmations in GSI, PSI experiments (112, 114, 113, 115) (both physical and radiochemistry experiments. //See R.Eichler et al., Nature 447/3(2007)72 ; S.Hofmann et al. EPJ A32(2007)251 ; S.Dmitriyev et al. Mendeleyev Comm. V.1(2005)5 )

Yu.S.TsyganovYu.S.Tsyganov

118118

From Memorandum of Peter Armbruster

..Y.Oganessian and his team discovered at least four new elementsand about 30 new isotopes. The independent confirming experiments destroyed my former standing doubts in their work, which I formulated also in retracted paper. In this note I want to conveymy congratulations for their outstanding discoveries of the last years,

late, but by full heart.Liber spät als nie! (Better late than never)113113

114114115115

116116

ГНС

112112

Number of observed decay chainsElement 118 3Element 116 25Element 115 4Element 114 43Element 113 6(from -decay of 287, 288115)Element 112 8

Shell 162Charge states systematics

267Hs265265SgSg

237,238237,238CfCf

e-h recombination model

e-h recombination model

Bimodal multiplication

Bimodal multiplication

EVR &

FF spectra simulation

EVR &

FF spectra simulation

RT m

atrix algorithm

The Dubna Gas-Filled Recoil Separator groupThe Dubna Gas-Filled Recoil Separator group