What is a breakdown of continuous component hidden under 4.4-MeV gamma-ray peakfrom the AmBe source?
Kohei Wada(Yokohama National University (YNU))

Akihiro Minamino (YNU), Hiroshi Ito , Masaki ishitsuka , Yuga Ommura (Tokyo University of Science) , Takatomi Yano (ICRR)

Americium-Beryllium(AmBe)n1

Purpose・To understand the radiation from AmBe detected by inorganic scintillator.
・To Measure the energy spectrum of neutron tagged with 4.4 MeV gamma ray.

・ AmBe is a radioactive neutron source,commonly used for the calibration ofthe neutron detectors.
!"# Am → !$% Np + α

5.5 MeV&#!
∗α + Be → C (1st excitd state) + 9(#! C ∗ → #! C + γ (4.4 MeV)

n0AmBe neutron energy spectrum[MeV]

ISO 8529:1989(E)・Neutron emission can betagged with 4.4 MeV gamma-ray.
・Another important reaction is
α + & Be → #! C (ground state) + 9)

Purpose of this study

Research background・ Neutron tagging technique with pure-water isstudied in Super-Kamiokande (SK), an undergroundlarge water Cherenkov neutrino detector.
- AmBe source was used for evaluating the neutron
tagging efficiency.
- AmBe was surrounded by BGO scintillator, toincrease the photons from 4.4 MeV gamma-ray.
Measured tagging efficiencies are relatively
10% smaller than ones of the MC simulation.

4.4 MeV peakHigh energycomponents

Scintillation emission spectrum of BGO in SK(p.e)

High energy components above 4.4 MeV exist inthe BGO spectra and are suspected as the cause.

Isao Murata, J Nucl Sci Technol. 4 (2014) 345

Ryosuke Akutsu. University of Tokyo,
2019, Ph.D. thesis.

High energy components above 4.4 MeV gamma-ray peakPurposeTo clarify the breakdown of high energy components above 4.4 MeV found in inorganic scintillators.
・We measured radiation from AmBe using NaI(Tl) scintillation detector.
→ 5-10 MeV high energy components also exist as did in BGO measurement in SK.

NaI(Tl) scintillation detector
and PMT・The cause of this components is considered due to n0 but not yet fully understood.
・Part of this components may be misidentified as 4.4 MeV gamma ray in measurement.

AmBe source
Intensity : 3.6 MBq

It is important to understand above 4.4 MeV gamma-ray, for the precision study withthe gamma-ray tagged neutron.

↓ 4.4 MeV peakPurpose5-10 MeV high energy
components ( E > 4750keV )↓・To investigate the characteristic of 5-10 MeV

components, we measured the ratio (defined as bellow) atdifferent distances between AmBe and the detector (= r).

The ratio =Purpose

For better understanding of our AmBe source, energy of neutron taggedwith 4.4 MeV gamma ray was also measured.
Total : PMT charge in 0 ~ 350 ns

Set upLiquid scintillation detector

5~10 MeV high energy counts
4.4 MeV peak counts

r

[keV]

60cm

4.4 MeV γ

n1- We suspect if both gamma ray and n1 simultaneously
react in NaI, such event makes 5-10 MeV component.

detection probability ∝ 1# 2r

① Constant ( due to n0 )② Component that decreases by 1/r2 ( due to n1+ gamma ).

①→ The neutron’s kinetic energy
was Calculated by its Time of Flightn0#r 2

probabilityr = 13.1 cm[cunt /sec][keV]

r = 23.1 cm

Summary[keV]

NaI deposit energy at each r[keV]・There's no large discrepancy
between our result and the expected
spectrum.

- 2-6 MeV spectrum was expected to be obtained.

n1∝ 1K 4r

・Time resolution is not considered in MC.

② component ③ component ・In r < 25 cm region, both the expected ①
and ② components are confirmed.

[count /sec]

r = 3.1 cm[count /sec][count /sec]

Measurements were done for several r.

J. Scherzinger, App. Rad. Iso. 127 (2017) 98.

Resultdetection probability ∝ 1# 2r②simultaneous detection

Total charge [nc]① Neutron event are selected at
Liquid scintillation detector

detection probability ∝ 1The ratior = 13.1 cm

PSD at liquid scintillator② 4.4 MeV gamma-ray are selecte at
NaI(Tl) scintillation detector

4.4 MeV γ
r = 3.1 cm

AmBen1

4.4 MeV γ

The ratio should have below two components

Set upSlow : PMT charge in 30 ~ 350 ns
Select asneutron event 0 is the rise time of waveform

NaI(Tl) scintillation detector

If 5-10 MeV component is made by only n0 , the ratio
should be constant for any r.

ResultTagged neutron energy spectrum
NaI deposit energy

Slow/Total

Ratio of 5-10 MeV events to 4.4 MeV peak

AmBe energy spectrum:NaI(Tl) [keV]① component

The ratio vs r

・Energy spectrum obtained by MC showedsimilar behavior to our measurement result.
- Absolute value doesn’t match with our result.
・There is an increasing component in r > 25 ,shown as ③ component in the figure, and it
is still under investigation.
Effect of the surrounding material?

○ We studied about the components above 4.4 MeV found in the measurement of AmBe neutron source using NaI(Tl) scintillation detector.

・The Ratio has a constant component at ~0.2 and a continuous component that decreases as the distance increases.(r<25 cm)→ This implies that detection of n0 and simultaneous detection of n1 and gamma rays make up part of the 5-10 MeV region.
・The energy spectrum of neutron tagged with 4.4 MeV gamma-ray calculated by its Time of Flight was roughly in agreement with previous studies.
Future issue
・ To understand the effect of the surrounding environment by MC simulation and additional experiments.
・ To perform analysis taking detection efficiency into account .