Balaram Dey Variable Energy Cyclotron Centre 1/AF Bidhan Nagar, - - PowerPoint PPT Presentation

Giant Dipole Resonance with at very low temperatures and the critical behavior

Balaram Dey

Variable Energy Cyclotron Centre 1/AF Bidhan Nagar, Kolkata-700064, INDIA

Supervisor : Prof Sudhee Ranjan Banerjee

16 -09-2015 COMEX5-2015

Giant Resonance : Collective modes of vibration of nucleus

E (MeV) 5 10 15 20 25 30 GDR Strength Fn (a.u.) 0.00 0.05 0.10 0.15 0.20 0.25 EGDR GDR

 

2 2 GDR 2 2 2 2 2 GDR GDR

E Γ F E (Ε Ε ) E Γ

   

  

Centroid Energy : Inversely proportional to the linear dimension of the nucleus. Strength Function: Gives an idea about the nuclear shape degrees of freedom. Resonance Width : Related to the damping mechanism of the collective motion.

Giant Dipole Resonance

Temperature (MeV)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

GDR Width (MeV) 5 10

120Sn

(b) J = 15

Evolution of GDR width as a function of temperature

Experimental observation Mostly investigated Nucleus  120Sn Experimental systematic shows GDR width increases monotonically with temperatures (typically 6-10 MeV for change in ‘T’ of 1.5-2.5 MeV)

Why GDR width increases with increase in temperature ???

PLB 709 (2012) 9

Thermal Shape Fluctuation Model : (∆β vs T)

GDR Width (MeV)

5 10 5

120Sn

(b) J = 15

Temperature (MeV) GDR width (MeV)

3 2 1

!! At low temperatures (T<1.5 MeV), the picture is not clear !!

Critical Temperature Fluctuation Model : Including an important physics point

GDR vibration itself produce a quadrupole moment causing the nuclear shape to fluctuate even at T = 0 MeV ( GDR vibration induced intrinsic fluctuation) : βGDR

• At low T : βGDR > ∆β
• βGDR  Independent of T
• ∆β  Increases with temperature
• Competition between βGDR and ∆β

The effect of thermal fluctuation on GDR width will appear

• nly when it becomes greater than the intrinsic fluctuation.

GDR Width (MeV)

5 10 5

120Sn

(b) J = 15

Temperature (MeV) GDR width (MeV)

3 2 1

Critical behavior :

TSFM CTFM

 Study of GDR width at very low temperatures (T < 1.5 MeV).

We probed A=100 mass region at very low temperature (T ~ 0.8 to 1.5 MeV ) to understand exact nature of the damping mechanism inside the nucleus.

 Verify the critical behavior : The number of GDR width measurements at low T < 1 MeV are inadequate to conclude that GDR width remains same at below the critical point.  Mass dependence of the critical behavior. My WORK:

Experimental Details

High energy gamma photons are the main tools to study GDR characteristics  Need a detector system with high detection efficiency and very good time resolution.

Projectile : 4He Target : 93Nb E lab : 28, 35, 42, 50 MeV

4He + 93Nb  97Tc* E* : 29.3, 36.0, 43.0, 50.4 MeV J = 10 – 20 h

LAMBDA

Multiplicity Filter Liquid Scintillator

Experimental Setup Electronics Setup

ARRAY

TOP MULT

F.A DELAY (Attenuator) 100 ns

QDC 1 QDC 2 TDC 1 TDC 2

FAN IN (Linear) CFD 1 FAN IN (Logic) 20 ns

FOLD

5 m 5 m 5 m

E long (1/10) E short (9/10)

Long gate 2 s Short gate 50 ns

F.A F.A CFD 2 CFD 3

BOTT MULT

CFD 6 CFD 7

5 m

DISC FAN IN (Logic)

AND 2 DISC GDG

CFD 4 FA N IN (Lo gic)

QDC 3 QDC 4

DISC DISC 100 ns FAN IN (Logic) LED 3 LED 2 LED 1 VETO DISC FAN IN (Logic) AND 1

CFD 5 GDG

Schematic Electronics Circuit Diagram for LAMBDA and Multiplicity

Schematic Electronics Circuit Diagram for BC501A

TOF Spectrum PSD spectrum cluster summing Cosmic rejection

Channel Number

500 1000 1500 2000 2500 3000 3500

Counts

200 400 600 800 1000 1200

Extraction of GDR parameters

Experimental data compared with a theoretical model (CASCADE) to extract the GDR parameter !! The following steps are essential !!

5 10 15 20 25 Yield /(0.5 MeV) 100 101 102 103 104 105 E (MeV)

• Detector Response Function
• Measuring

angular momentum distribution

• Measuring Nuclear Level Density

parameter

(1) Detector simulation studies using GEANT4

Detector response function must be folded with CASCADE calculation. Only after that it can be compared with experimental spectrum

NIM A 582 (2007) 603

(2) Mapping of experimental Fold to Angular Momentum space with a very realistic technique

NIM A624 (2010) 148

Fold

2 3 4 5 6 7 8 9

Counts

100 101 102 103 104

Angular Momentum ( )

5 10 15 20 25 30

Counts x 103

5 10 15 20 25 30 Very essential  To determine average angular momentum  To determine average rotational energy  To construct initial population matrix for CASCADE calculation

J = C

     

m M M M M P 

   max

exp 1 1 2    

Incident distribution :

Geant4 Simulation

(3) Nuclear level density parameter from neutron evaporation spectrum

Crucial input for CASCADE Calculations & important for the proper estimation

• f nuclear temperature

Neutron detector (BC501A) is generally used to measure the neutron energy spectrum by TOF technique

100 101 102 103 104 105 106 107 5 10 15 20 25 101 102 103 104 105 106 107 108 10 15 20 25

Elab = 28 MeV Elab = 35 MeV Elab = 42 MeV Elab = 50 MeV

F > 2 F=2 F=3 F>4 F=2 F=3 F>4 F=2 F=3 F>4

Yield / 0.5 MeV Energy (MeV)

High energy gamma spectra along with CASCADE calculation

Final GDR spectra along with CASCADE calculation

Temperature (MeV)

0.0 0.5 1.0 1.5 2.0 2.5

GDR width (MeV)

4 6 8 10

GDR width (MeV)

4 6 8 10 12 TSFM CTFM PDM

(a) (b)

Results and Discussion

Tc = 1.08 MeV

Balaram Dey et al., Physics Letter B 731 (2014) 92

First experimental data at below and above the critical temperature

First exp data point in A ~ 100

GDR induced intrinsic fluctuation could play a decisive role in describing the increase of GDR width as a function of T. Intrinsic fluctuation due to GDR vibration should be incorporated in TSFM (macroscopically) to explain the behavior of GDR width at low T.

PDM : PRC 86 (2012) 044333

Summary and conclusion

 A systematic study of the Giant Dipole Resonance width at very low temperature (T = 0.8 – 1.5 MeV) in A ~ 100 mass region.  GDR widths have been compared with different theoretical calculations (TSFM, CTFM and PDM).  TSFM fails to explain the experimental data where as CTFM and PDM calculation nicely matches with the experimental data.  GDR induced intrinsic fluctuation plays an important role in describing the evolution GDR width as a function of temperature  First experimental data at below and above the critical temperature.  Microscopic PDM (with pairing fluctuation) also explain the data very well.  It would also be interesting if the pairing fluctuation can be included in the TSFM calculation.

THANKS

GDR group at VECC, kolkata, India

Channel Number

100 200 300 400 500 600 700

Energy (MeV)

4 8 12 16 20 24 28

Channel Number

100 120 140 160 180 200

Counts

200 400 600 800 1000 1200

Channel Number

160 200 240 280 100 200 300 400 500 Channel Number 200 400 600 800 1000 20 40 60 80 100 120 140 22Na 241Am - 9Be Cosmic

0.511 MeV 1.274 MeV 4.43 MeV 23.1 MeV

1/A

0.00 0.01 0.02 0.03

GDR

0.04 0.08 0.12 0.16

GDR = 0.04 + 4.13/A

Temperature (MeV) 0.0 0.5 1.0 1.5 2.0 2.5  0.00 0.05 0.10 0.15 GDR

Detector properties

• The time resolution of

35cm BaF2 detector = 960 ps 5cm BaF2 detector = 460 ps

• The energy resolution of

35cm BaF2 detector = 16/ √E MeV thus 20% at 0.662 MeV 5cm BaF2 detector = 12% at 0.662 MeV

• The intrinsic efficiency of single

35cm BaF2 detector = 95 % at 0.662 MeV, 93 % 10 MeV. 5cm BaF2 detector = 80 % at 0.662 MeV, 73% at 1 MeV.

• The photopeak efficiency of

cluster summing technique(3x3) 35cm BaF2 detector = ~ 50 % from energy range 10MeV

• The photopeak efficiency of

cluster summing technique(7x7) 35cm BaF2 detector = ~ 70 % from energy range 10 MeV What is the advantage of BaF2 detector over NaI detector?

• BaF2 are non-hygroscopic

where as NaI is highly hygroscopic.

• NaI detectors cannot be used

in modular(array) from since they have to be kept inside a air tight container.

• Energy resolution is

comparable.

• BaF2(600 ps) have better

timing resolution than NaI (250ns)

• Density of BaF2(4.88 g/cc) is

greater than NaI(3.67 g/cc) hence for efficient for high energy detection.

• BaF2 has high Z (56) than

NaI(53) which is required for high energy gamma

• detection. Also BaF2 has low

capture cross section for thermal neutrons due to the neutron magic number

1/A 0.005 0.010 0.015 Tc (MeV) 0.8 1.0 1.2 1.4 Tc = 0.7 + 37.5/A

208Pb 120Sn 63Cu