DAMPE的氦核能譜以及高能天體物理研究

馬鵬雄

(中國科學(xué)院紫金山天文臺(tái) 南京 210023)

宇宙線的研究自19世紀(jì)末開始, 人們對其已進(jìn)行了深入廣泛的百年探索. 近些年人們發(fā)現(xiàn)宇宙線在幾千億電子伏特(幾百GeV)處存在明顯的能譜變硬特征, 這一發(fā)現(xiàn)跟此前預(yù)測的單冪律能譜存在明顯的沖突, 可能對理解宇宙線加速、傳播等物理過程有重要意義. 近些年空間宇宙線直接探測實(shí)驗(yàn)也觸及TeV能段, 但對于地面實(shí)驗(yàn)的百TeV能段, 空間實(shí)驗(yàn)數(shù)據(jù)仍然較少. 這主要是因?yàn)楦吣芏瘟Ｗ訑?shù)密度低且本身實(shí)驗(yàn)設(shè)備的有效觀測面積較小. 而這一能段恰好較好地銜接了目前空間和地面實(shí)驗(yàn)的能區(qū), 因此具有重要的科學(xué)意義和探索價(jià)值. 宇宙線觀測歷史和探測方式主要在本論文第1章介紹, 同時(shí), 我們也著重描述一些宇宙線探測實(shí)驗(yàn)以及直接和間接兩類探測方式的不同. 接下來的部分主要介紹本人在“悟空”號(hào)(Dark Matter Particle Explorer, DAMPE)電荷測量、氦核能譜分析等方面所開展的研究工作, 最后還將介紹我們在天體物理方面所開展的兩項(xiàng)理論研究.

第2章介紹我們對“悟空號(hào)”的塑閃探測器(PSD)位置刻度和熒光衰減修正以及電荷重建方面開展的研究.塑閃電荷分辨率的優(yōu)劣非常依賴PSD的準(zhǔn)確位置. 我們發(fā)展了一種計(jì)算塑閃位置變化的方法, 結(jié)果顯示全部PSD在探測器坐標(biāo)系的垂直方向出現(xiàn)較顯著的移動(dòng),第1、2層PSD的平均垂直移動(dòng)分別為-2 mm、-1 mm; 兩層PSD在其安裝的水平面內(nèi)出現(xiàn)逆時(shí)針的整體轉(zhuǎn)動(dòng), 平均轉(zhuǎn)動(dòng)角～0.0015 rad; 所有的位置變動(dòng)參數(shù)表現(xiàn)出隨時(shí)間的穩(wěn)定性. 應(yīng)用PSD位置標(biāo)定的參數(shù), 電荷分辨率可以提高至少4%. 此外同一種宇宙線核子因擊中點(diǎn)不同使電離能損測量出現(xiàn)最高達(dá)到2倍左右的偏差, 對此我們完成了全部PSD的熒光衰減修正. 另外, 對于重核的猝滅現(xiàn)象和PSD不同條的讀出響應(yīng)有差異, 我們提出了專門的修正方法. 基于上述系列修正, 我們完成了塑閃的電荷重建,其中質(zhì)子和氦核的電荷分辨率可達(dá)到0.137和0.238個(gè)單位電荷.

第3章中首先介紹了量能器在高能量讀出時(shí)的晶體飽和修正, 然后重點(diǎn)介紹我們對DAMPE數(shù)據(jù)的分析研究以及所獲的氦核能譜測量. DAMPE的氦核能譜覆蓋幾十GeV到幾百TeV近4個(gè)量級(jí). 當(dāng)量能器測量到幾十TeV時(shí),一些晶體會(huì)出現(xiàn)飽和, 我們基于模擬數(shù)據(jù)發(fā)展了高效修正晶體飽和的方法. 利用自2016年初起, 總共45個(gè)月在軌數(shù)據(jù), 得到20 GeV/n–27 TeV/n的氦核能譜, 初步結(jié)果顯示氦核能譜在TeV左右存在能譜變硬特征, 與阿爾法磁譜儀(Alpha Magnetic Spectrometer, AMS02)的發(fā)現(xiàn)基本一致. 此外, DAMPE氦核能譜在～10 TeV/n處存在非常顯著的變軟特征. 這是DAMPE首次在～10 TeV/n處發(fā)現(xiàn)高置信度的能譜變軟特征, 與之前高空氣球?qū)嶒?yàn)ATIC (Advanced Thin Ionization Calorimeter)、CREAM (Cosmic Ray Energetics and Mass)以及空間實(shí)驗(yàn)NUCLEON發(fā)現(xiàn)的跡象一致, 與一些地面實(shí)驗(yàn)發(fā)現(xiàn)的質(zhì)子與氦核混合能譜存在幾十TeV處的變軟特征一致. 在TeV到幾百TeV能段,目前各種地面實(shí)驗(yàn)的測量差別較大, DAMPE高精度的氦核能譜對宇宙線分成分的膝區(qū)特征能量測量具有重要意義.

第4章中介紹我們在高能天體物理領(lǐng)域的兩項(xiàng)理論研究. 首先基于DAMPE質(zhì)子宇宙線能譜和AMS02、ATIC、CREAM以及地面實(shí)驗(yàn)ARGO-YBJ (Astrophysical Radiation with Ground-based Observatory at YangBaJing)等的宇宙線能譜, 研究DAMPE質(zhì)子譜～14 TeV變軟結(jié)構(gòu)的可能物理起源. 我們發(fā)現(xiàn)單成分的宇宙線能譜很難同時(shí)擬合寬能段的宇宙線各組分以及全粒子能譜. 基于多成分的能譜, 我們討論了多個(gè)源模型和鄰近源加背景宇宙線模型. 這兩類模型均可擬合從TeV至PeV的宇宙線能譜.結(jié)合目前宇宙線各向異性的研究進(jìn)展, 我們認(rèn)為鄰近源加背景宇宙線模型更加可信. 基于第1例雙中子星并合引力波事件GW170817的數(shù)據(jù)我們對中子星最大引力質(zhì)量進(jìn)行了估計(jì). 我們發(fā)現(xiàn)如果該次事件的伽馬暴對應(yīng)體GRB 170817A中心引擎是一個(gè)黑洞的話, 那么中子星的最大引力質(zhì)量一般應(yīng)小于2.3M⊙.

The research of cosmic rays has been started since late in 1890s and lasted more than 100 years.People have done many works extensively and deeply.In recent years, some experiment found the hardening features at a few hundred GeV compared with the conventional scenario, just single power law model.The obvious inconsistency between new discovery and conventional one may indicate some hints on understanding of acceleration,propagation and some other topics of cosmic rays. However, the recent spacebased experiments measured spectra up to several TeV as high as possible, the upper bound of energy range is lower than the ones obtained from groundbased expriments. The range from～TeV to a few hundred TeV is almost unobserved and also limited by low particle number density of high energetic cosmic ray, as well the relatively small observation area of detector. Intriguingly, this range could act as the bridge between direct observation (balloon-borne or onboard satellite and international space station)and indirect observation (ground-based), which is meaningful and important in astroparticle physics over a wide energy range.

We introduce the PSD (Plastic Scintillator Detector) position alignment, fluorescence attenuation and charge reconstruction in chapter 2. The precise position of scintillator in the frame of DAMPE(Dark Matter Particle Explorer) is essential to path length within its volume as particle passes through, which plays an important role in charge measurement. We have finished position calibration of all plastic scintillators by using flight data. For all of scintillators,the movements along the width are relative small. The movements along height are dominated, the averages are-2 mm,-1 mm for the first layer and second layer,respectively. As a whole,the rotation of PSD detector is～0.0015 rad. All of the movement and rotation are stable over operation time, we found the alignment of position is meaningful to improve the charge resolution at least by 4%. We also corrected attenuation of fluorescence within scintillator and found that the attenuation would reduce the intensity of ionization energy by twice as much as possible when one particle passes through a scintillator at different positions along its length. On the other hand, we developed methods to correct the quenching effect and non-equalization of PSD bar. After applying these corrections, the charge resolution of proton and helium are 0.137 and 0.238 charge unit, respectively.

In the third chapter, firstly, we introduce the correction to saturation at a few ten TeV. Secondly,the data analysis and helium spectrum are presented. The BGO (Bismuth Germanate) calorimeter of DAMPE could measure helium spectra from dozens of GeV to a few hundred TeV over nearly four orders of magnitude. There are some saturated readouts within calorimeter as the deposited energy as high as a few ten TeV. We developed a method to correct the saturation based on Monte Carlo data effectively. We obtained the spectrum of helium with DAMPE by using flight data with period from January in 2016 to September in 2019. The preliminary spectrum shows the hardening feature at a few hundred GeV, which is consistent with AMS02 (Alpha Magnetic Spectrometer). DAMPE also found the significant softening feature at～10 TeV/n. DAMPE found the softening with high confidence level for the first time, which are consistent with results from previous experiment ATIC (Advanced Thin Ionization Calorimeter), CREAM (Cosmic Ray Energetics and Mass)and NUCLEON.The softening also favors the breaking of proton and helium mixed spectrum by ground-based experiment. At the current stage,there are relative large uncertainties from a few TeV to a few hundred TeV in ground-based experiment,DAMPE’s high precision spectrum could be essential and crucial to measurement of knee energies of each kind of nucleus.

In chapter 4, we report our theoretical works on high energy astrophysics. We have studied the possible reason about softening of DAMPE’s proton spectrum at～14 TeV by combination with spectra of AMS02, ATIC, CREAM and ground-based experiment, i.e. ARGO-YBJ (Astrophysical Radiation with Ground-based Observatory at YangBaJing). We found the single power law could not fit the softening feature and spectra measured by air shower experiment simultaneously. Based on the multiple components spectrum,we discussed the multi sources model and nearby source with background model. Both of the two models could fit the spectra from TeV to PeV.The nearby source model could be more concrete as we took into account progress of anisotropy in recent years. We also discussed the fate of remnants by binary neutron stars (NS) merge based on the first event,GW179817,and estimated the maximum gravitational mass of NS. We found that the maximum gravitational mass of NS would be less than 2.32M⊙if the central engine of GRB 170817A was a black hole.