新型無(wú)機(jī)閃爍體的能量分辨率

1、成都理工大學(xué)學(xué)生畢業(yè)設(shè)計(jì)（論文）外文譯文學(xué)生姓名：劉勇學(xué)號(hào)：2006xxxxxxxx專(zhuān)業(yè)名稱(chēng)：核工程與核技術(shù)譯文標(biāo)題（中英文）：新型無(wú)機(jī)閃爍體的能量分辨率（New inorganic scintillatorsaspects ofenergy resolution）譯文出處：荷蘭代爾夫特理工大學(xué)，國(guó)際共和研究所，核輻射集團(tuán)指導(dǎo)教師審閱簽名：曾國(guó)強(qiáng)譯文正文：新型無(wú)機(jī)閃爍體的能量分辨率摘要：通過(guò)對(duì)Y射線探測(cè)器的能量分辨率的討論。實(shí)驗(yàn)表明，該能量分辨率是可以顯著改善的，對(duì)于新的閃爍體LaCl3（Ce），在能量為662Kev是的能量分辨率為3%?？茖W(xué)家B.V.保留所有 權(quán)。PACS 系統(tǒng)：07.85.

2、Nc; 78.55.Hx; 78.90 +t關(guān)鍵詞：無(wú)機(jī)閃爍；LaC（Ce），能量分辨率，伽瑪射線探測(cè)器介紹：3無(wú)機(jī)閃爍體被廣泛應(yīng)用于伽瑪射線檢測(cè)。探測(cè)器的選擇主要根據(jù)有關(guān)探測(cè)要求的基礎(chǔ) 而定。例如：效率，精力和時(shí)間分辨率，死時(shí)間，位置分辨，增長(zhǎng)的可能性較大的晶體，品體 質(zhì)量（輻射硬度，力學(xué)性能等）和成本。例如見(jiàn)文件在1-4。在許多情況下，能量分辨率是最重要的。然而，通常的一個(gè)半導(dǎo)體探測(cè)器，例如：Ge， 只是基于對(duì)無(wú)機(jī)閃爍探測(cè)器的應(yīng)用。我們處理這個(gè)問(wèn)題，是否可以提高無(wú)機(jī)閃爍探測(cè)器的探測(cè) 的能量分辨率，因此，在閃爍體的適用性可以拓展，我們將討論研究一種新的閃爍體。無(wú)機(jī)閃爍體的基礎(chǔ)要素：在制定有效

3、的伽瑪射線檢測(cè)新閃爍體，我們選擇的材料一般都具有較高的密度R和高原 子序數(shù)。此外，該材料閃爍光傳輸率該較高，因此，我們依靠離子晶體或者某種共價(jià)品體。但 與導(dǎo)帶和化合價(jià)之間的禁止能量，E大到足夠可以傳輸。另一方面，良好的能量，時(shí)間和位置分辨率，我們需要大量的閃爍光Nph （相對(duì)變異數(shù)81/Nph），禁差距竟可能的小。、=（E/EQ SQ（1）在右邊第項(xiàng)代表的生成電子空穴對(duì)的數(shù)目Neh吸收伽瑪射線所產(chǎn)生的能量。平均生成個(gè)電 子空穴對(duì)所需要的能量為：Eeh2.5Egapo S是閃爍體中心的能量發(fā)光轉(zhuǎn)化效率。Q是閃爍體受 激發(fā)后的光子發(fā)光效率。在這三個(gè)階段S是至少可以預(yù)測(cè)的。目前這在很大程度依賴(lài)于閃爍

4、體 的缺陷，不同于閃爍體中心，可以同時(shí)俘獲電子或者電子空穴。這些缺陷可能來(lái)自晶體本身結(jié) 品時(shí)侯的，也可能來(lái)自某些雜質(zhì)的原因。接下來(lái)我們考慮發(fā)光中心我們將只討論了鑭系金屬離子Ce3+，該離子在第四層有一個(gè)殼 層電子，受激發(fā)吼躍遷到底五層。隨后的退激將發(fā)生在第4層和第5層之間的電偶極子，伴隨的 衰減時(shí)間tN30ns。作為前提說(shuō)明和必要條件的描述，該Ce3+必須是一種合乎規(guī)格的材料來(lái)融入閃爍體中心。在大多數(shù)情況下，發(fā)射光譜與光傳感器的靈敏度曲線擬合當(dāng)好。我們觀察到每 Mev能量的伽馬射線的光子產(chǎn)量2000?，F(xiàn)在我們來(lái)討論能量分辨率。分辨率是：R= AE (FWHM) /E在一個(gè)光電峰能量為E的伽馬脈

5、沖幅度譜可表示為：R2=Rsci2+Riid2+Rnise2( 2 )Rsci表示由于光源的光檢測(cè)器不是理想的原因，即不服從泊松分布的統(tǒng)計(jì)數(shù)據(jù)時(shí)對(duì)光的貢獻(xiàn)。 由于材料的不均勻性，在伽馬射線的吸收和光子數(shù)量的收集依賴(lài)于伽馬射線的入射位置和閃爍 光檢測(cè)器耦合的不完善，所以并沒(méi)有跟入射伽馬射線的能量成正比的響應(yīng)(不相稱(chēng)的響應(yīng))。 Rlid表示在理想的光源檢測(cè)機(jī)制和光源檢測(cè)器下的結(jié)果，后者的理想偏差也包括在Rsic里面， 這個(gè)我們以后再討論。Rnise表示電子噪聲。理想的閃爍體通過(guò)理想的光電倍增管(PMT)可以完全的傳輸光子。所以Rsci=Rnise=0 因此R2=Rlid2假設(shè)經(jīng)Y-射線吸收(a)

6、Nph的閃爍光子生產(chǎn)和到達(dá)的光電倍增管陰極，(b)光 電子是后來(lái)n Nph，(c)這些=n Nph電子在第一倍增極和到達(dá)(d)倍增極的k (k = 1, 2.) 放大后為。k并且我們假設(shè) j 2= 3= k=的，并且 /產(chǎn)1的。我們可以 得出：R2=Rlid2=5.56 /n ( -1) 5.56/、(3)Nel表示第一次到達(dá)光電倍增管的數(shù)目。在試驗(yàn)中， 110 2= 3= k，因此，在實(shí)際情 況下，我們可以通過(guò)(3)看出R2的值比實(shí)際測(cè)得大。請(qǐng)注意，對(duì)于一個(gè)半導(dǎo)體二極管(不倍增極結(jié)構(gòu))(3)也適用。那么Nel就是是在二極管產(chǎn) 生電子空穴對(duì)的數(shù)目。在物質(zhì)不均勻，光收集不完整，不相稱(chēng)和偏差的影響

7、從光電子生產(chǎn)過(guò)程中的二項(xiàng)式分布及電子收集在第一倍增極不理想的情況下，例如由于陰極不均勻性和不 完善的重點(diǎn)，我們有：|R2=Rsci2+Riid2a5.56(v N-1/Nei)+1/Nei(4)v N光子的產(chǎn)生包括所有非理想情況下的收集和1/Nel的理想情況。為了說(shuō)明，我們?cè)趫D上顯示，如圖1所示。 E/E的作為伽瑪射線能量E的函數(shù)，為碘 化鈉：鉈閃爍耦合到光電倍增管1。20 4U 1002(K)400 tOOOE （keV）圖。1。對(duì)AE/E的示意圖（全曲線）作為伽瑪射線能量E功能的碘化鈉：鉈晶體耦合到光電倍增管。虛 線/虛線代表了主要貢獻(xiàn)。例如見(jiàn)9,10。對(duì)于Rsci除了 1/（Nel）i/

8、2的組成部分，我們看到有兩個(gè)組成部分，代表在0-4% 的不均勻性，不完整的光收集水平線，等等，并與在0-400代表非相稱(chēng)keV的最大曲線。表 1給出了 E=662Kev時(shí)的數(shù)值（137Cs） 在傳統(tǒng)的閃爍體資料可見(jiàn)。從圖一我們可以清楚的看到在低能量EV100Kev，如果Nel，也就是N h增大的話，是 可以提高能量分辨率的。這是很難達(dá)到的，因?yàn)楣忸~產(chǎn)量已經(jīng)很高了（見(jiàn)表1）在能量E 300Kev時(shí)，Rsci主要由能量支配其能量分辨率，這是沒(méi)辦法減小Rsci的。然而，在下一節(jié)我 們將會(huì)講到，可以用閃爍體在高能量一樣有高的分辨率。3。新的閃爍和能量分辨率在表1中顯示的是能量為662Kev是的光電峰的

9、分辨率，在代爾夫特理工大學(xué)和伯爾尼大學(xué)的 合作下開(kāi)發(fā)的傳統(tǒng)閃爍探測(cè)器探測(cè)應(yīng)用新的閃爍體記錄下來(lái)的數(shù)據(jù)，在第1列表示Ce摻雜濃 度為mol%。第二列給出N，即每兆電子伏特，產(chǎn)生的光子數(shù)。第三列給出Nel，電子的或電 子空穴在每吸收662 keV的伽馬射線探測(cè)器產(chǎn)生的光量子對(duì)數(shù)。從對(duì)時(shí)間的積分看所示的數(shù)據(jù)， 第四列給出了 662Kev照片峰實(shí)驗(yàn)R值。Rlid的計(jì)算是通過(guò)Nel，包括一個(gè)忽略倍增極統(tǒng)計(jì)5% 的盤(pán)整（第五列）。為準(zhǔn)則和APD的它代表了探測(cè)器（過(guò)量）和電子噪音，光電倍增管的Rnoise 被認(rèn)為是可以忽略的。從4-6列的值Rsci計(jì)算公式是（2）。Table 1EriGTgy lesulu

10、tiun data at 662keV foi sume old and new scintillatuis; fui d-sfinitiurLS s睥 te?:tCrystalN 103/MeV網(wǎng)直蜘keVJi%玲孩Size (mm )Light d-stectuiRef.NaI:Tl4060006.73.209PMTtypicalCsI:Tl6560006.63工05.SdisLini x 7.5PMT XP2254BPhilips17CsI :T16526,0004.31.53.SdiairL2.& x 5SDD風(fēng)YAlQj: Ce21iyoo4.32.32.62.53 x-3 x 10

11、APD 6307073500 AdvPhutlTLG/RbGd3Bi7:56SSOO4.12.603.2PMT R1791Hamamatsu1gLad3：0.57%.Ce40600073.206.2PMT R1791HamamatsuLaa3: 10%Ce4973003.12.S01.4disiinE x 5PMT R1791HamamatsuIW6LaQ3: LOCe493.651.7L.85.2.64diam8 x 5APD 6307073510UAdvPhotlrLC表1表示的是在能量為662Kev的一些傳統(tǒng)閃爍體的能量分辨率的數(shù)據(jù)，其定義見(jiàn)文中。正如第二節(jié)提及到的NaI（Tl）的結(jié)果符

12、合圖1。這是他們這種材料的特點(diǎn)。Cs（Tl）也有 類(lèi)似的特征，例如見(jiàn)17。使用硅探測(cè)器（SDD）探測(cè)出了一個(gè)很好的能量分辨率R=4.3%， SDD有比較高的光電倍增管其中從0到8%-16%的種類(lèi)而定，所以從0到60%的閃爍體發(fā)光 （探測(cè)效率最高為565nm）。然而，這并不能解釋R值變小，顯然對(duì)于使用Cs（Tl）探測(cè)時(shí) Rsci=3.8%，即遠(yuǎn)小于上述的晶體的值。另外一個(gè)好的結(jié)果是，最近公布了 YALO3（Tl）。采 用雪崩二極管（APD），R=4.3%。而且，R的值不能通過(guò)高量子效率來(lái)解釋?zhuān)谶@種情況下， Rs技 2.5%。energy keV圖。2。LaCl3 （Tl） 662千電子伏的脈沖

13、y射線光譜測(cè)定高度在（直徑85平方毫米）耦合到光電倍增管（R1791,形成時(shí) 間為10毫秒）。SAUnoo在代爾夫特，伯爾尼的方案中，我們選擇Ce摻雜閃爍的要求，并在第1和第2條所述原 則的基礎(chǔ)材料。我們專(zhuān)注于鹵化物，特別是漠化物和氯化物，目標(biāo)針對(duì)探測(cè)效率高于或等于 NaI（Tl）的，至少相當(dāng)于光子產(chǎn)量產(chǎn)量，更快速的反應(yīng)和更好的能量分辨率。我們最新介紹 的閃爍體RbGd2Br7（Ce）12.13。我們得到的R=4.1%。通過(guò)式2我們計(jì)算得出Rsci=3.2% 這個(gè)R和R .的值明顯比NaI（Tl）的值小。RbGd2Br7（Ce）有了一個(gè)小部分的改善，使得 Rd有較高的光子產(chǎn)量。相對(duì)于NaI（T

14、l）和Cs（Tl）而言與光電倍增管的靈敏度曲線和閃爍發(fā) 光光譜更好的匹配，CsI的情況一樣。另外一種新的閃爍體是LaCl3（Ce）。起初這種材料的性能并不樂(lè)觀，與0.57%的Ce混 合后，具有較高的光子產(chǎn)額，使用光電倍增管可以讀出，但是分辨率只有7%。但是，提高Ce 的摻雜度是可以得到更好的分辨率，如Ce10%時(shí)，R=3.1%。看表格2La（+Ce）KX射線的逃逸封以及脫離光峰。在這種情況下，Rsci=1.4%，也就是說(shuō)這項(xiàng)閃爍體的貢獻(xiàn)非常小。只要不 改變的Rsci價(jià)值與APD的讀出應(yīng)有的更大的Nel，并考慮到電子噪音，可以預(yù)算的分辨率V 2.9%。正如文章的第一節(jié)R=3.65%，表明，Rsc

15、i從1.4%增長(zhǎng)到了 2.64%。這可能是由于APD 的入口處窗口不均勻響應(yīng)。對(duì)于能讀出YAlO3（Ce）較高的值的APD已經(jīng)非常好了。4.結(jié)論：在上一節(jié)，我們了解到，由于LaCl3 （Ce）的良好分辨率我們可以觀察到較小的Rsci （見(jiàn)表1）這部分可能是由非相稱(chēng)的影響小的貢獻(xiàn)Rsci解釋。這是RbGd2Br7 （Ce）在此情 況下觀測(cè)到的跡象，在662Kev的光子產(chǎn)額（光子/兆電子伏特）是在0到5%和0，能量 50-1400Kev之間的幅度變化。同樣對(duì)于YA1O3 (Ce)所占的比例還是比較小為0到7%。 對(duì)于NaI(T1)和NaI(T1)，卻相反它的范圍卻是0到15%。對(duì)于LaC13(Ce

16、)不相稱(chēng)的是， 目前還沒(méi)有測(cè)量，是無(wú)法預(yù)測(cè)的晶體類(lèi)型顯示的最小的非均衡影響。目前還不清楚什么影響晶體的光收集和不均勻的。LaC13容易潮解，我們是通過(guò)一個(gè)石 英晶體與光電倍增管耦合后才能用LaC13的測(cè)量，將這個(gè)晶體和光電被怎管一起耦合在一個(gè)防 潮的盒子里面。硅潤(rùn)滑脂(通用電氣公司，粘度60,000cst)負(fù)責(zé)耦合和樣品所用的聚四氟乙 烯覆蓋。觀察到的光子的產(chǎn)量和能量分辨率是一樣的。Rsci值較小的原因，集中在第一倍增 極的影響是相當(dāng)重要的，如陰極不均勻性和不完善。在LaC13(Ce)的情況下對(duì)APD的應(yīng)用 也同樣說(shuō)明了。其結(jié)果比預(yù)期的結(jié)果要差。下一個(gè)步驟將是越來(lái)越多的大晶體。那是光電效應(yīng)的不

17、勻稱(chēng)的影響將會(huì)更大。顯然，我們需要更多關(guān)注的是閃爍探測(cè)器的優(yōu)化和研 究。按照閃爍體對(duì)溫度的依賴(lài)性研究可能要好一點(diǎn)，冷卻的準(zhǔn)則可能需要更好的結(jié)果，但前提 是閃爍的響應(yīng)沒(méi)有變得更糟。我們都應(yīng)該牢記，相對(duì)較小的R值只能在較高的能量實(shí)現(xiàn)。(見(jiàn)圖1)在能量V100Kev 他的本質(zhì)是光產(chǎn)額的原因，至于產(chǎn)量方面見(jiàn)圖5。更加值得注意的是在能量為662Kev時(shí)的能 量分辨率是3.1%，這相當(dāng)于市場(chǎng)上可見(jiàn)的CdZnTe閃爍體。至于其他的性能，在LaCl3 (Tl) Ce含量為10%的情況下，在26nm內(nèi)衰減發(fā)射出20000個(gè)光子，其中90%只在1ms內(nèi)發(fā)射。 LaCl3 (Tl)的檢測(cè)效率和NaI (Tl)差不多

18、，想了解更多信息(見(jiàn)16)。最后，通過(guò)上述的結(jié)論我們可以明顯知道，在探測(cè)效率和能量分辨率的提高，我們可以 通過(guò)新的閃爍體來(lái)實(shí)現(xiàn)，這將比傳統(tǒng)的閃爍體更好。特別是對(duì)于LaCl3 (Tl)而言，其他的材 料將繼續(xù)發(fā)展下去。5.參考文獻(xiàn)：南韋布，醫(yī)療影像，亞當(dāng)希爾杰，布里斯托爾，1990年物理。頁(yè)Dorenbos，C.W.E.Eijk，關(guān)于無(wú)機(jī)閃爍體及其應(yīng)用，SCINT95，代爾夫特大學(xué)出版社國(guó)際 會(huì)議論文集，代爾夫特，荷蘭，1996年。殷之文，F(xiàn)engXiqi，黎呸軍，薛志林法律程序無(wú)機(jī)閃爍的國(guó)際會(huì)議及其應(yīng)用，SCINT97，中 科院上海分行出版社，上海，中國(guó)，1997年。巴頓伯克斯的理論和實(shí)踐的閃

19、爍計(jì)數(shù)，帕加馬，紐約，1967年。頁(yè) Dorenbos，等。，IEEE 期刊。Nucl?？萍肌?2 (6)(1995) 2190。JD 瓦倫丁節(jié)，等。，IEEE 期刊。Nucl?？萍肌?5 (3)(1998) 512。卜蜂阿列等。，讀出了鑭(III)閃爍晶體的一大面積雪崩光電二極管，發(fā)表于2000年電 機(jī)及電子學(xué)工程師聯(lián)合會(huì)高中，麥克風(fēng)，里昂，法國(guó)，16-202000年10月。頁(yè) Dorenbos，等。，Nucl。Instr。和冰毒。乙 132 (1997) 728。澳吉洛-No.el，等。，IEEE 期刊。Nucl?？萍肌?6 (5)(1999) 1274。澳吉洛-No.el，等。，JD 魯

20、閩。85 (1999) 21。16 16 E.V.D.van Loef，等。，閃爍性能鑭：Ce3 +的品體：快速，高效和高能量分辨率閃爍體，在2000年電機(jī)及電子學(xué)工程師聯(lián)合會(huì)NSSMIC提出，法國(guó)里昂，10月16-20日2000。卜蜂阿列等。，IEEE 期刊。Nucl。科技。45 (3)(1998) 576。長(zhǎng)菲奧里尼，樓佩蒂，Nucl。Instr。和冰毒。401 (1997) 104。米 Moszynski，等。，Nucl。Instr。和冰毒。一個(gè) 442 (2000) 230。電動(dòng)汽車(chē)產(chǎn)品，產(chǎn)品目錄。外文正文：.NewinorganicscintillatorsFaspectsofene

21、rgyresolutionCarel W.E. van Eijk* Radiation Technology Group, IRI, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The NetherlandsAbstractEnergy-resolution of inorganic-scintillator gamma-ray detectors is discussed. Experiment shows that the resolution can be significantly improved. For

22、the new scintillator LaCl3 : Ce, an energy resolution of 3.1% was observed at 662 keV. r 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85.Nc; 78.55.Hx; 78.90+tKeywords: Inorganic scintillators; LaCl3:Ce; Energy resolution; Gamma-ray detectorsIntroductionInorganic scintillators are widely

23、applied for gamma-ray detection. Detector selection occurs on the basis of requirements concerninge.g . efficiency,energy and time resolution, dead time, position resolution, the possibility to grow large crystals, crystal quality (radiation hardness, mechanical properties, etc.) and cost. E.g. see

24、papersin 1-4. In a number of cases energy resolution is allimportant. Then, in general a semi-conductor detector, e.g. Ge, is applied instead of a detector based on an inorganic scintillator. We address the question whether the energy resolution of an inorganic scintillator can be improved and, cons

25、equently, the scintillator applicability can be extended. Amongothers a new scintillator will be discussed.Inorganic-scintillator basicsIn developingnew scintillators for efficient gamma-ray detection, we select in general a material with a relatively high density r, and high atomic number Z. Furthe

26、rmore, the material should transmit scintillation light efficiently. Consequently we rely on ionic crystals or crystals with some degree of covalency, but with a forbiddengap energy between valence and conduction band, Egap, large enough to transmit the light. On the other hand, for good energy, tim

27、e and position resolution we need a large number of scintillation photons, Nph, (relative variancep1=Nph), and consequently the forbidden gap should be as small as possible as 5Nph= (E/Eeh)SQ(1)The first term on the right side represents the number of thermalized electron-hole pairs Ne2h produced in

28、 absorbing gamma-ray energy E. The average energy required to produce one thermalized electron-hole pair: Ee2hE2:5Egap. S is the transport/transfer efficiency of the e2h pair/ energy to the luminescence centre (LC) of the scintillator, and Q is the efficiency for photon emission once the LC is excit

29、ed (quantum efficiency). Of the three stages S is the least predictable. It depends very much on defects present in the scintillator, other than the LC, that may capture electrons or holes or both. These defects can arise from the interaction itself, from crystal growing, or be due to impurities 3,4

30、. Next we consider the luminescence centre. We will confine ourselves to the Ce3+ lanthanide ion 6,7. This ion has one electron in the 4f state which is lifted to the empty 5d shell upon excitation. Subsequent de-excitation will occur by an allowed 5d-4f electric dipole transition with a decay time

31、tX30 ns. The Ce3+ ion has to be incorporated as LC in a material with specifications as described in the previous paragraph and the requirement that it can act as a host for the 3+ ion. In most cases the emission spectrum matches the light sensor sensitivity curve rather well. Light yields 20,000 ph

32、otons per MeV of absorbed gamma-ray energy are observed 3,4,6. We now turn to energy resolution. The resolution, R DEeFWHMT=E, of a photopeak atenergy E in a gamma-ray pulse height spectrum can be expressed as, e.g. see 8:R2=R 2+R 2+R . 2(2)sci lid niseR2 sci represents the contributions from the sc

33、intillator due to the fact that it is not a perfect light source, i.e. at the light detector it does not deliver a number of photons obeyingPoi sson statistics due to material inhomogeneities, light collection dependence on the gamma-ray-absorption position and imperfect scintillator-light detector

34、coupling, and it does not have a response proportional to the gamma-ray energy (non-proportionality effect).R2 lid represents the light detection mechanism for a perfect light source and an ideal light detector. Deviations from ideality of the latter are usually also included in Rsci. We will come b

35、ack to this.R2 noise represents the electronic noise. For an ideal scintillator read out by an ideal photomultiplier tube (PMT) Rsci Rnoise 0 and (2) becomes R2 R2 lid. Assumingth at upon gamma-ray absorption (a) Nph scintillation photons are produced and arrive at the photocathode of the PMT, (b) s

36、ubsequently ZNph photoelectrons are produced in the photocathode, (c) of these aZNph electrons arrive at the first dynode and (d) at dynode k (k 1; 2;y) the amplification is dk, and furthermore assuming d1 d2 d3 dk d and d=d21E1, one can derive 8,9R2=Rlid2=5.568/8nNph (8-1) 5.56/Nel(3)with Nel the n

37、umber of photoelectrons arrivingat the first dynode. In practice d1E10 d2 d3 dk. Consequently under practical circumstances R2 is B10% larger than the value obtained from (3). Note that for a semiconductor diode (no dynode structure) (3) is also applicable. Then Nel is the number of electron-hole pa

38、irs produced in the diode.In the non-ideal case of material inhomogeneities, incomplete light collection, non-proportionality effects and deviations from the binomial distributions of the photoelectron-production process and the electron collection at the first dynode,e.g. due to photocathode inhomo

39、geneity and imperfect focusing, we have R2=Rsci2+Rlid25.56(vN-1/Nel)+1/Nel(4)with vN the variance of the light production includingthe effect of all the non-ideal processes and 1=Nph the variance in the ideal case. To illustrate the above we show in Fig. 1 schematically DE=E as a function of the1004

40、0AE/Eao(%)1041=020401(H)200400E (keV)Fig. 1. Schematic of DE=E (full curve) as a function of gammarayenergy E for a NaI :Tl crystal coupled to a PMT. The dotted/dashed lines represent the main contributions. gamma-ray energy E, for a NaI : Tl scintillator coupled to a PMT, e.g. see 9,10. In addition

41、 to the 1=ONel component we observe two componentsof Rsci, the horizontal line at B4% representing inhomogeneity, incomplete light collection, etc., and the curve with a maximum at B400 keV representingnon- proportionality. In Table 1 numerical values are presented at E 662 keV (137Cs). For informat

42、ion on energy resolution of more traditional scintillators e.g. see. It is clear from Fig. 1 that at low energies, Eo100 keV, significant energy-resolution improvement can only be obtained if Nel, i.e. Nph, is increased. This is not easy as light yields are already high (Table 1). At E 300 keV the R

43、sci components are dominatingthe energy resolution. There is no recipe to decrease Rsci. Yet, in the next section we show that scintillators can be found with a better energy resolution at the higher energies.New scintillators and energy resolutionIn Table 1 we show data relevant for the energy reso

44、lution of the 662 keV photopeak, recorded by means of a few traditional scintillators and some new scintillators developed in a collaboration of Delft University of Technology and University ofBern 11-16. In column 1 Ce-dopingcon centrations reindicated in mol%. The second column gives N, the light

45、yield in photons per MeV, the third column gives Nel, the number of electrons or electron-hole pairs produced in the light detectorper absorbed 662 keV gamma quantum. For integration times see the mentioned papers. The fourth column gives the experimental R values of the 662 keV photopeaks. Rlid is

46、calculated from Nel using(3), includinga 5% correction for neglecting dynode statistics (column 5). For PMTs Rnoise is considered to be negligible, for SDDs and APDs it represents the detector (excess) and electronic noise. From the values of columns 4-6 Rsci is calculated using(2).As already mentio

47、ned in Section 2, the NaI : Tl results correspond with Fig. 1. They are characteristic of this material. CsI : Tl, has a similar characteristic R-value. E.g. see 17. A very good energy resolution of R 4:3%, obtained usinga silicon drift detector (SDD), was reported in18. The SDD has an efficiency of

48、 B60% fordetection of the scintillation light (maximum at 565 nm) which is high compared to that of a PMT, B8-18%dependingon the type. Yet, this does not explain the small R-value. Apparently Rsci 3:8% for the used CsI : Tl crystal, i.e. much smaller thanfor the crystal of the row above. Another ver

49、y good result was recently reported for the scintillator YAlO3 : Ce. Usingan avalanche photodiodeTable LEmTgy lesulutiun data at 662 keV feu sume old and new scintillatuis; fui d-sfinitiurLS s睥 textCrystalN 103/MeV虬的皿胡職玲孩Size (mm )Light d-stectuiRef.NaI:Tl4060006.73.209PMTtypicalCsI:Tl6560006.63工05.

50、SdisLini x 7.5PMT XP2254BPhilips17CsI Tl6526,0004.31.53.SdiairL2.& x 5SDD風(fēng)YAlQj: Ce21iyoo4.32.32.62.53 x-3 x 10APD 6307073500 AdvPhutlTLG/RbGd3Bi7: 9用.Ce56ssoo4.12.603.2PMT R1791Hamamatsu1gLad3：0.57%.Ce40600073.206.2PMT R1791HamamatsuLaa3: 10%Ce4973003.12.S01.4disiinE x 5PMT R1791HamamatsuLaQ3: LOCe

51、493.651.7L倍.2.64diam8 x 5APD 6307073510AdvPhotlTLCIHTable 1Energy resolution data at 662 keV for some old and new scintillators; for definitions see text (APD), R 4:3% 19. Again this R-value cannot be explained by the high quantum efficiency of B70%. In this case RsciE2:5%. In the Delft-Bern program

52、me we selected Cedoped scintillator materials based on requirements and principles mentioned in Sections 1 and 2. We focused on halides, in particular bromides and chlorides, aimingat detection efficiencies equal to or better than that of NaI : Tl, at least equal light yield, a faster response and a

53、 better energy resolution. Recently we introduced the new scintillator RbGd2Br7 :Ce 12,13. We obtained R 4:1%. Using(2) we calculate Rsci 3:2%. These values of R and Rsci are significantly smaller than the correspondingvalues obtained with a PMT for NaI : Tl and Cs : Tl. Part of the improvement is a

54、 consequence of the smaller Rlid due the high light yield of RbGd2Br7 :Ce compared to that of NaI : Tl and the better matchingof the scintillation-emission spectrum with the PMT sensitivity curve in comparison with the CsI : Tl case. Another new scintillator is LaCl3 : Ce. At first this material did

55、 not appear to be very promising.Doped with 0.57% Ce it has a high light yield butthe energy resolution is 7% using PMT readout 14. However, at a higher doping concentration the resolution improves dramatically, e.g. at 10%Ce R 3:1% 15,16. See Fig. 2. The La(+Ce) K X-ray escape peak is well separate

56、d from the photopeak. In this case Rsci 1:4%, i.e. the scintillator contribution is very small. Provided that the value of Rsci does not change, one would expect a resolution of o2.9% with APD readout due to the much larger Nel and takinginto account the electronic noise. As indicated in Table 1, R

57、3:65% 11. It appears that Rsci increased from 1.4% to 2.64%. This may be due to inhomogeneous response of the APD entrance window. For APD readout of YAlO3 :Ce a higher Rsci value was reported as well 19.SAUnooi 200180。-160014001200-LaCI :10%Ceo o o o o 8- . - 皿Do 4J2600800energy keVFig. 2. Pulse he

58、ight spectrum of 662 keV gamma rays detected in a LaCl3 : 10%Ce crystal (diam 8_5mm2) coupled to a PMT (R1791, shapingtime 10 ms).DiscussionIn the previous section we learned that the very good energy resolution observed for LaCl3 : Ce is due to the small Rsci value (see Table 1). Part of this may b

59、e explained by a small contribution of the non-proportionality effect to Rsci. An indication of this was observed in the case of RbGd2Br7 :Ce 13. The relative light yield (photons/MeV), normalized to that at 662 keV, is constant within B5% in the range B50 -400 keV. Also for YAlO3 :Ce a relatively s

60、mall nonproportionalitywas measured of B7% for the same energy range 9. For NaI : Tl and Cs : Tl, on the contrary, the spread is B15%. For LaCl3 : Ce the non-proportionality has yet to be measured. At present it is not possible to predict which type of crystals shows the smallest non-proportionality