KATRIN实验中的伽马射线诱导背景研究

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KATRIN（Karlsruhe Tritium Neutrino Experiment）是一个国际性的科研项目，其主要目标是通过精确测量电子反中微子的质量上限，特别是寻求$$ m _ {\overline{\nu_e}} $$ (电子反中微子质量)是否低于$$ 0.2 \, \text{eV/c}^2$$这一物理前沿问题。为了实现这样的高灵敏度，KATRIN实验采用了先进的MAC-E（迈克尔逊-列文森磁透镜系统和能量分析器）过滤技术，这是一种在低温和高真空环境下工作的气体原子源。文章《Gamma-induced background in the KATRIN main spectrometer》发表于《欧洲物理杂志C》(Eur.Phys.J.C) 2019年第79期，具有开放存取权限。研究关注的核心问题是伽马射线在KATRIN主光谱仪中的诱发背景，这可能对实验结果产生潜在的干扰。背景主要包括来自环境、仪器材料衰变以及实验过程中放射性同位素产生的伽马辐射。这些背景粒子可能会被误识别为电子反中微子信号，从而降低测量精度。伽马射线背景的控制是实验设计的关键部分，因为它们可能在能量分辨率或能量分布上对测量结果造成误导。研究团队列举了多位来自不同机构的科学家，他们在实验装置的设计、操作优化以及数据分析方法上进行了深入探讨，如探测器的选择、屏蔽技术、数据处理策略等，以减少伽马射线的影响。文章中详细讨论了各种减少伽马射线背景的技术手段，如采用高纯度材料、精心设计的屏蔽结构、以及实时的数据筛选和校准方法。同时，还可能探讨了对已知伽马射线源的精确测量和模拟，以便更好地理解并控制这些背景。总结来说，该研究论文揭示了KATRIN实验面临的挑战，即如何在追求极致敏感度的同时，有效抑制伽马射线引发的背景噪声。通过综合应用科学方法和精密技术，研究人员致力于提高KATRIN对电子反中微子质量下限测量的准确性，这在基础物理学领域具有重要的理论和实践意义。

Eur. Phys. J. C (2019) 79 :807 Page 3 of 12 807

Fig. 1 The KATRIN beamline. From left to right: the rear section

(RS), the windowless gaseous tritium source (WGTS), the differen-

tial pumping section (DPS), the cryogenic pumping section (CPS), the

pre-spectrometer (PS), the main spectrometer (MS) with surrounding

air-coils, and the focal-plane detector (FPD) system

polarity of the air-coil currents, it is possible to create non-

standard magnetic ﬁeld settings inside the vessel (see Sect. 4).

The magnetic ﬁeld inside the MS acts as a passive shield

against charged particles emitted from the vessel surface; the

particles are deﬂected by the Lorentz force back toward the

walls of the vessel, or they follow magnetic ﬁeld lines that

do not reach the detector. Additional shielding is provided by

a wire electrode system installed at the inner surface of the

MS [19]. The inner electrodes (IE) can be placed at an offset

potential ΔU

relative to the vessel (i.e. U

= U

+ ΔU

If ΔU

is set to a negative value, most low-energy electrons

emitted from the vessel walls will be repelled by the wires

back toward the surface.

2.2 Focal-plane detector

The FPD [20] is a monolithic, segmented silicon PIN diode

that detects particles emerging from the MS. These parti-

cles pass through an ultra-high vacuum system mated to the

exit of the MS. Charged particles can be further accelerated

using a post-acceleration electrode (PAE), with voltage U

PAE

immediately preceding the detector wafer; this acceleration

helps distinguish signal electrons from background origi-

nating within the FPD system. A superconducting solenoid

focuses charged particles onto the wafer, which has a sen-

sitive area with a diameter of 90 mm [20]. The dartboard-

segmentation pattern of the wafer into 148 equal-area pixels

provides sensitivity to the transverse spatial distribution of

the particles. The wafer is divided into 12 concentric rings,

each with 12 pixels, and a central bullseye with four pixels.

The PIN diode is biased with a voltage U

bias

; signals pass

through preampliﬁers (located in vacuum) before readout.

Energy and timing for each event are analyzed online in

the data-acquisition system using a cascaded pair of trape-

zoidal ﬁlters. Energy calibration is provided by gammas

from an

241

Am calibration source. The electron response is

characterized using a photo-electron source with adjustable

energy [20]. Each source can be inserted into the line of

sight of the FPD for a periodic, dedicated calibration run. An

energy resolution of 1.52 keV (full width at half-maximum,

FWHM) and timing resolution of 246 ns (FWHM) have been

achieved with the system using 18.6 keV electrons and a

6.4 µs shaping time [20].

3 Environmental gamma radiation in the spectrometer

hall

During standard KATRIN operation, β-particles that are

detected by the FPD must overcome the retarding poten-

tial applied to the MS. This potential reaches its largest value

near the middle of the vessel, approximately equidistant from

both ends of the MS. Because the value of the retarding poten-

tial will be scanned close to the endpoint energy, β-particles

traveling through the middle region of the MS will have low

kinetic energies, below about 30 eV [1]. Because of the rel-

atively poor energy resolution of the FPD compared to the

MS, any low-energy secondary electrons produced in this

region cannot be energetically distinguished from the signal

β-particles.

When passing through the MS steel, environmental

gamma radiation (i.e., primary radiation) can cause the

emission of secondary particles, including secondary elec-

trons. Of particular concern is the generation of “true-

secondary” electrons, which are deﬁned to have energies

below 50 eV [21]. The inner surface of the MS provides a

large area for electron emission: 690 m

for the steel hull and

532 m

for the wire electrode system (although the effective

surface area for emission from the latter is reduced due to

the two-layer structure of the IE) [15]. Due to imperfections

in the magnetic and electrostatic shielding, secondary elec-

trons emitted from these surfaces may have a small probabil-

ity to enter the sensitive magnetic ﬂux tube that connects to

the FPD. The validity of this background-generating mecha-

nism has been conﬁrmed with other MAC-E-ﬁlter spectrome-

ters [9,10]. Because of the increased size of the spectrometer,

there is the potential for a signiﬁcant background contribu-

tion from gamma-induced surface electrons in the MS.

3.1 Radioactivity in the spectrometer hall

The MS was constructed using low-radioactivity materials

in order to limit background production from environmental

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