Pu(n,f) EAR2 · EAR2 floor. n_TOF Collaboration meeting Granada, 27th- 28th Nov 2018 Shockley-Ramo...

STEFF 239Pu(n,f)EAR2

(17th Sep - 11th Nov)

n_TOF Collaboration meetingGranada, 27th- 28th Nov 2018

P.J. Davies1, A.G. Smith1, T. Wright1, J. Billowes1,

A. Sekhar1, N.V. Sosnin1, S. Bennet1,

M.A. Millan-Callado2, A. McFarlane1, A. Smith1,

and the n_TOF collaboration

1. The University of Manchester2. University of Seville


Experimental campaign● Various improvements since 2016 – noise

reduction and addition of 6 small LaBr3

detectors → higher En possible cf. NaI

● Off-line setup in BLDG. 272 – replaced gas window, stop detector repairs, leak testing etc.

● 17th Sep - 11th Nov, 5.1x1018 protons on target

● Small collimator

● 24 μg/cm2 239Pu target on 38 μg/cm2 mylar support, 50 mm diameter active area – JRC

● Target at 22.5º wrt beam

● 2nd level (fission) trigger for DAQ

● ~ 3 fission master triggers per proton pulse of which ~ 45% are 2E-2v


Statistics & protons

(Proton data from: https://acc-stats.web.cern.ch/acc-stats/#ps/ps-ntof-run-data)

Total number of detected fission fragments: ~ 2.1x106

Interventions to fix stop detector: ‘2v’ statistics x2 as a result

Problem with 2nd level trg.

EAR1: collimator change - 16O


Preliminary data

n-flash + α + Fission fragments → n-flash + αα

Dedicated pulses only:

Probably two components for FiFI’s: inel. n scatering - see S. Urlass’s talk + n,cp


A problem: γ/n flash

Tγ≈ 19,750 ns

Anode pad (segmented anode) signals from Brgg ion. chamber :

Fission and ‘flash-charge’

pile up

Typical ff. signal


Tγ≈ 19,750 ns

Anode pad (segmented anode) signals from Brgg ion. chamber :

Fission and ‘flash-charge’

pile up

Typical ‘flash-charge’ signals

Typical ff. signal

‘flash-charge’ observed at ~T

γ for each anode segment



Implications for the data:● Fragment energies from

amplitude of sum of anode signals, used for mass yields

● Difficult to extract fragment energies for fissions at tof - T

γ < 4 μs

● Achievable ΔE/E ~ 1.5%, here increased to ~ 10%

● A,Z,E*,ν measurements compromised at E

n> 100 keV

Typical ‘flash-charge’ signals

Brgg anode pad signals



‘Flash-charge’ interpretation

From 2014 n_TOF technical meeting. V. Vlachoudis:

https://indico.cern.ch/event/302093/

Approximately to scale:

High energy background

neutrons

Handful of n,cp reactions on Al/Cu per p. pulse – large Al/Cu surface

areas

~3 m

~1.8 mn

EAR2 floor


Shockley-Ramo based calculations:Data:1.

γ/n flash diagnostics

Calculated signal shapes assuming

2 or 3 ionising cp’s entering the

chamber from edges with

random orientations


2. Filter tests: Assuming that the neutrons/gammas are scattered AFTER the filter box (~10 m below):

11cm Al and 3 cm Pb filters used – the ratios of filter/no-filter ‘flash-charge’ matches the expected ratios for NEUTRONS, not for γ.

3. Shielding test: Pb and borated polyethylene below STEFF: See the expected reduction of charge → n’s are coming from below

γ/n flash diagnostics Shockley-Ramo based calculations:Data:1.

Calculated signal shapes assuming

2 or 3 ionising cp’s entering the

chamber from edges with

random orientations


19,700 19,800 19,900Time (ns)

Am

plit

ud

e

γ/n flash diagnosticsTypical start detector trace:● Start detector is also sensitive

to n,cp reactions: contains copper collection anode next to an MCP (in vacuum)

● CP’s will produce e- upon collisions with MCP

● Fast (5 ns width), so can count individual n,cp reactions

Tγ


19,700 19,800 19,900Time (ns)

Am

plit

ud

e

γ/n flash diagnosticsTypical start detector trace:● Start detector is also sensitive

to n,cp reactions: contains copper collection anode next to an MCP (in vacuum)

● CP’s will produce e- upon collisions with MCP

● Fast (5 ns width), so can count individual n,cp reactions

● Calculate an expected tof spectrum using total 63Cu(n,cp) cross section and background fluence from previous Fluka simulation for EAR2 (see 2014 n_TOF technical meeting. V. Vlachoudis):

Tγ


5 MeV

CalculationStart det. data

γ/n flash diagnostics

Summary:● Threshold reactions, probably n,cp● One source of ionising particles in

the chambers → n,cp● Unexplained bump at the 1 MeV

hump in EAR2 flux. cf. μMGAS EAR2 ● Still need to work to quantify the

flux at the positions of each detector (if this possible) – some simulation work too


To do

● Finalise UI parameters, fission event building - now

● More work to properly understand n-flash issue – curiosity and useful for a solution for future EAR2 campaigns

● Subtraction of g-flash signals from anode signals for fissions at E

n > 100 keV

● Gamma array response matrices with Geant4 for deconvolution – A. Sekhar

● Reconstruction of anode signals with multiple corrections (Frisch-grid inefficiency, energy loss in foils/windows, e-/ion recombination, pre-amplifier gain matching) - ‘2E’ - me

● And more...


Thanks for your atention!

Somewhere in the Jura – 25/11/2018

Also, many thanks to everyone who helped the STEFF team over the last 4 months!


Extra: Preliminary data

Measured: Expected:

Direct comparison makes no sense at this stage – counting spectra contain ff. + alphas + g-flash etc..


Extra: STEFF - motivations and potentialNot only useful nuclear data, also fundamental fission process info:

Single particle motion

Spin and alignment

Fission barrier, spin,average moment of

inertia at scission

‘Energy sorting’ at scissionand two step (n,γf) process

Mass and charge yields

Gamma multiplicity (NEA HPRL)and angular distribution

Fission fragment angulardistributions as fn. of E

n

Neutron (and γ) multiplicityas fn. of E

n (and fn. of resonance,

NEA HPRL)

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Pu(n,f) EAR2 · EAR2 floor. n_TOF Collaboration meeting Granada, 27th- 28th Nov 2018 Shockley-Ramo...

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